Add circular WAL decoding buffer, take II.

[pgsql.git] / src / backend / access / transam / xlog.c

/*-------------------------------------------------------------------------
 *
 * xlog.c
 *              PostgreSQL write-ahead log manager
 *
 * The Write-Ahead Log (WAL) functionality is split into several source
 * files, in addition to this one:
 *
 * xloginsert.c - Functions for constructing WAL records
 * xlogrecovery.c - WAL recovery and standby code
 * xlogreader.c - Facility for reading WAL files and parsing WAL records
 * xlogutils.c - Helper functions for WAL redo routines
 *
 * This file contains functions for coordinating database startup and
 * checkpointing, and managing the write-ahead log buffers when the
 * system is running.
 *
 * StartupXLOG() is the main entry point of the startup process.  It
 * coordinates database startup, performing WAL recovery, and the
 * transition from WAL recovery into normal operations.
 *
 * XLogInsertRecord() inserts a WAL record into the WAL buffers.  Most
 * callers should not call this directly, but use the functions in
 * xloginsert.c to construct the WAL record.  XLogFlush() can be used
 * to force the WAL to disk.
 *
 * In addition to those, there are many other functions for interrogating
 * the current system state, and for starting/stopping backups.
 *
 *
 * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/xlog.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <ctype.h>
#include <math.h>
#include <time.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>

#include "access/clog.h"
#include "access/commit_ts.h"
#include "access/heaptoast.h"
#include "access/multixact.h"
#include "access/rewriteheap.h"
#include "access/subtrans.h"
#include "access/timeline.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogarchive.h"
#include "access/xloginsert.h"
#include "access/xlogreader.h"
#include "access/xlogrecovery.h"
#include "access/xlogutils.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_database.h"
#include "common/controldata_utils.h"
#include "common/file_utils.h"
#include "executor/instrument.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "port/pg_iovec.h"
#include "postmaster/bgwriter.h"
#include "postmaster/startup.h"
#include "postmaster/walwriter.h"
#include "replication/basebackup.h"
#include "replication/logical.h"
#include "replication/origin.h"
#include "replication/slot.h"
#include "replication/snapbuild.h"
#include "replication/walreceiver.h"
#include "replication/walsender.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/large_object.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/reinit.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "storage/sync.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/relmapper.h"
#include "utils/pg_rusage.h"
#include "utils/snapmgr.h"
#include "utils/timeout.h"
#include "utils/timestamp.h"

extern uint32 bootstrap_data_checksum_version;

/* timeline ID to be used when bootstrapping */
#define BootstrapTimeLineID             1

/* User-settable parameters */
int                     max_wal_size_mb = 1024; /* 1 GB */
int                     min_wal_size_mb = 80;   /* 80 MB */
int                     wal_keep_size_mb = 0;
int                     XLOGbuffers = -1;
int                     XLogArchiveTimeout = 0;
int                     XLogArchiveMode = ARCHIVE_MODE_OFF;
char       *XLogArchiveCommand = NULL;
bool            EnableHotStandby = false;
bool            fullPageWrites = true;
bool            wal_log_hints = false;
int                     wal_compression = WAL_COMPRESSION_NONE;
char       *wal_consistency_checking_string = NULL;
bool       *wal_consistency_checking = NULL;
bool            wal_init_zero = true;
bool            wal_recycle = true;
bool            log_checkpoints = true;
int                     sync_method = DEFAULT_SYNC_METHOD;
int                     wal_level = WAL_LEVEL_MINIMAL;
int                     CommitDelay = 0;        /* precommit delay in microseconds */
int                     CommitSiblings = 5; /* # concurrent xacts needed to sleep */
int                     wal_retrieve_retry_interval = 5000;
int                     max_slot_wal_keep_size_mb = -1;
bool            track_wal_io_timing = false;

#ifdef WAL_DEBUG
bool            XLOG_DEBUG = false;
#endif

int                     wal_segment_size = DEFAULT_XLOG_SEG_SIZE;

/*
 * Number of WAL insertion locks to use. A higher value allows more insertions
 * to happen concurrently, but adds some CPU overhead to flushing the WAL,
 * which needs to iterate all the locks.
 */
#define NUM_XLOGINSERT_LOCKS  8

/*
 * Max distance from last checkpoint, before triggering a new xlog-based
 * checkpoint.
 */
int                     CheckPointSegments;

/* Estimated distance between checkpoints, in bytes */
static double CheckPointDistanceEstimate = 0;
static double PrevCheckPointDistance = 0;

/*
 * GUC support
 */
const struct config_enum_entry sync_method_options[] = {
        {"fsync", SYNC_METHOD_FSYNC, false},
#ifdef HAVE_FSYNC_WRITETHROUGH
        {"fsync_writethrough", SYNC_METHOD_FSYNC_WRITETHROUGH, false},
#endif
#ifdef HAVE_FDATASYNC
        {"fdatasync", SYNC_METHOD_FDATASYNC, false},
#endif
#ifdef OPEN_SYNC_FLAG
        {"open_sync", SYNC_METHOD_OPEN, false},
#endif
#ifdef OPEN_DATASYNC_FLAG
        {"open_datasync", SYNC_METHOD_OPEN_DSYNC, false},
#endif
        {NULL, 0, false}
};


/*
 * Although only "on", "off", and "always" are documented,
 * we accept all the likely variants of "on" and "off".
 */
const struct config_enum_entry archive_mode_options[] = {
        {"always", ARCHIVE_MODE_ALWAYS, false},
        {"on", ARCHIVE_MODE_ON, false},
        {"off", ARCHIVE_MODE_OFF, false},
        {"true", ARCHIVE_MODE_ON, true},
        {"false", ARCHIVE_MODE_OFF, true},
        {"yes", ARCHIVE_MODE_ON, true},
        {"no", ARCHIVE_MODE_OFF, true},
        {"1", ARCHIVE_MODE_ON, true},
        {"0", ARCHIVE_MODE_OFF, true},
        {NULL, 0, false}
};

/*
 * Statistics for current checkpoint are collected in this global struct.
 * Because only the checkpointer or a stand-alone backend can perform
 * checkpoints, this will be unused in normal backends.
 */
CheckpointStatsData CheckpointStats;

/*
 * During recovery, lastFullPageWrites keeps track of full_page_writes that
 * the replayed WAL records indicate. It's initialized with full_page_writes
 * that the recovery starting checkpoint record indicates, and then updated
 * each time XLOG_FPW_CHANGE record is replayed.
 */
static bool lastFullPageWrites;

/*
 * Local copy of the state tracked by SharedRecoveryState in shared memory,
 * It is false if SharedRecoveryState is RECOVERY_STATE_DONE.  True actually
 * means "not known, need to check the shared state".
 */
static bool LocalRecoveryInProgress = true;

/*
 * Local state for XLogInsertAllowed():
 *              1: unconditionally allowed to insert XLOG
 *              0: unconditionally not allowed to insert XLOG
 *              -1: must check RecoveryInProgress(); disallow until it is false
 * Most processes start with -1 and transition to 1 after seeing that recovery
 * is not in progress.  But we can also force the value for special cases.
 * The coding in XLogInsertAllowed() depends on the first two of these states
 * being numerically the same as bool true and false.
 */
static int      LocalXLogInsertAllowed = -1;

/*
 * ProcLastRecPtr points to the start of the last XLOG record inserted by the
 * current backend.  It is updated for all inserts.  XactLastRecEnd points to
 * end+1 of the last record, and is reset when we end a top-level transaction,
 * or start a new one; so it can be used to tell if the current transaction has
 * created any XLOG records.
 *
 * While in parallel mode, this may not be fully up to date.  When committing,
 * a transaction can assume this covers all xlog records written either by the
 * user backend or by any parallel worker which was present at any point during
 * the transaction.  But when aborting, or when still in parallel mode, other
 * parallel backends may have written WAL records at later LSNs than the value
 * stored here.  The parallel leader advances its own copy, when necessary,
 * in WaitForParallelWorkersToFinish.
 */
XLogRecPtr      ProcLastRecPtr = InvalidXLogRecPtr;
XLogRecPtr      XactLastRecEnd = InvalidXLogRecPtr;
XLogRecPtr      XactLastCommitEnd = InvalidXLogRecPtr;

/*
 * RedoRecPtr is this backend's local copy of the REDO record pointer
 * (which is almost but not quite the same as a pointer to the most recent
 * CHECKPOINT record).  We update this from the shared-memory copy,
 * XLogCtl->Insert.RedoRecPtr, whenever we can safely do so (ie, when we
 * hold an insertion lock).  See XLogInsertRecord for details.  We are also
 * allowed to update from XLogCtl->RedoRecPtr if we hold the info_lck;
 * see GetRedoRecPtr.
 *
 * NB: Code that uses this variable must be prepared not only for the
 * possibility that it may be arbitrarily out of date, but also for the
 * possibility that it might be set to InvalidXLogRecPtr. We used to
 * initialize it as a side effect of the first call to RecoveryInProgress(),
 * which meant that most code that might use it could assume that it had a
 * real if perhaps stale value. That's no longer the case.
 */
static XLogRecPtr RedoRecPtr;

/*
 * doPageWrites is this backend's local copy of (forcePageWrites ||
 * fullPageWrites).  It is used together with RedoRecPtr to decide whether
 * a full-page image of a page need to be taken.
 *
 * NB: Initially this is false, and there's no guarantee that it will be
 * initialized to any other value before it is first used. Any code that
 * makes use of it must recheck the value after obtaining a WALInsertLock,
 * and respond appropriately if it turns out that the previous value wasn't
 * accurate.
 */
static bool doPageWrites;

/*----------
 * Shared-memory data structures for XLOG control
 *
 * LogwrtRqst indicates a byte position that we need to write and/or fsync
 * the log up to (all records before that point must be written or fsynced).
 * LogwrtResult indicates the byte positions we have already written/fsynced.
 * These structs are identical but are declared separately to indicate their
 * slightly different functions.
 *
 * To read XLogCtl->LogwrtResult, you must hold either info_lck or
 * WALWriteLock.  To update it, you need to hold both locks.  The point of
 * this arrangement is that the value can be examined by code that already
 * holds WALWriteLock without needing to grab info_lck as well.  In addition
 * to the shared variable, each backend has a private copy of LogwrtResult,
 * which is updated when convenient.
 *
 * The request bookkeeping is simpler: there is a shared XLogCtl->LogwrtRqst
 * (protected by info_lck), but we don't need to cache any copies of it.
 *
 * info_lck is only held long enough to read/update the protected variables,
 * so it's a plain spinlock.  The other locks are held longer (potentially
 * over I/O operations), so we use LWLocks for them.  These locks are:
 *
 * WALBufMappingLock: must be held to replace a page in the WAL buffer cache.
 * It is only held while initializing and changing the mapping.  If the
 * contents of the buffer being replaced haven't been written yet, the mapping
 * lock is released while the write is done, and reacquired afterwards.
 *
 * WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
 * XLogFlush).
 *
 * ControlFileLock: must be held to read/update control file or create
 * new log file.
 *
 *----------
 */

typedef struct XLogwrtRqst
{
        XLogRecPtr      Write;                  /* last byte + 1 to write out */
        XLogRecPtr      Flush;                  /* last byte + 1 to flush */
} XLogwrtRqst;

typedef struct XLogwrtResult
{
        XLogRecPtr      Write;                  /* last byte + 1 written out */
        XLogRecPtr      Flush;                  /* last byte + 1 flushed */
} XLogwrtResult;

/*
 * Inserting to WAL is protected by a small fixed number of WAL insertion
 * locks. To insert to the WAL, you must hold one of the locks - it doesn't
 * matter which one. To lock out other concurrent insertions, you must hold
 * of them. Each WAL insertion lock consists of a lightweight lock, plus an
 * indicator of how far the insertion has progressed (insertingAt).
 *
 * The insertingAt values are read when a process wants to flush WAL from
 * the in-memory buffers to disk, to check that all the insertions to the
 * region the process is about to write out have finished. You could simply
 * wait for all currently in-progress insertions to finish, but the
 * insertingAt indicator allows you to ignore insertions to later in the WAL,
 * so that you only wait for the insertions that are modifying the buffers
 * you're about to write out.
 *
 * This isn't just an optimization. If all the WAL buffers are dirty, an
 * inserter that's holding a WAL insert lock might need to evict an old WAL
 * buffer, which requires flushing the WAL. If it's possible for an inserter
 * to block on another inserter unnecessarily, deadlock can arise when two
 * inserters holding a WAL insert lock wait for each other to finish their
 * insertion.
 *
 * Small WAL records that don't cross a page boundary never update the value,
 * the WAL record is just copied to the page and the lock is released. But
 * to avoid the deadlock-scenario explained above, the indicator is always
 * updated before sleeping while holding an insertion lock.
 *
 * lastImportantAt contains the LSN of the last important WAL record inserted
 * using a given lock. This value is used to detect if there has been
 * important WAL activity since the last time some action, like a checkpoint,
 * was performed - allowing to not repeat the action if not. The LSN is
 * updated for all insertions, unless the XLOG_MARK_UNIMPORTANT flag was
 * set. lastImportantAt is never cleared, only overwritten by the LSN of newer
 * records.  Tracking the WAL activity directly in WALInsertLock has the
 * advantage of not needing any additional locks to update the value.
 */
typedef struct
{
        LWLock          lock;
        XLogRecPtr      insertingAt;
        XLogRecPtr      lastImportantAt;
} WALInsertLock;

/*
 * All the WAL insertion locks are allocated as an array in shared memory. We
 * force the array stride to be a power of 2, which saves a few cycles in
 * indexing, but more importantly also ensures that individual slots don't
 * cross cache line boundaries. (Of course, we have to also ensure that the
 * array start address is suitably aligned.)
 */
typedef union WALInsertLockPadded
{
        WALInsertLock l;
        char            pad[PG_CACHE_LINE_SIZE];
} WALInsertLockPadded;

/*
 * State of an exclusive backup, necessary to control concurrent activities
 * across sessions when working on exclusive backups.
 *
 * EXCLUSIVE_BACKUP_NONE means that there is no exclusive backup actually
 * running, to be more precise pg_start_backup() is not being executed for
 * an exclusive backup and there is no exclusive backup in progress.
 * EXCLUSIVE_BACKUP_STARTING means that pg_start_backup() is starting an
 * exclusive backup.
 * EXCLUSIVE_BACKUP_IN_PROGRESS means that pg_start_backup() has finished
 * running and an exclusive backup is in progress. pg_stop_backup() is
 * needed to finish it.
 * EXCLUSIVE_BACKUP_STOPPING means that pg_stop_backup() is stopping an
 * exclusive backup.
 */
typedef enum ExclusiveBackupState
{
        EXCLUSIVE_BACKUP_NONE = 0,
        EXCLUSIVE_BACKUP_STARTING,
        EXCLUSIVE_BACKUP_IN_PROGRESS,
        EXCLUSIVE_BACKUP_STOPPING
} ExclusiveBackupState;

/*
 * Session status of running backup, used for sanity checks in SQL-callable
 * functions to start and stop backups.
 */
static SessionBackupState sessionBackupState = SESSION_BACKUP_NONE;

/*
 * Shared state data for WAL insertion.
 */
typedef struct XLogCtlInsert
{
        slock_t         insertpos_lck;  /* protects CurrBytePos and PrevBytePos */

        /*
         * CurrBytePos is the end of reserved WAL. The next record will be
         * inserted at that position. PrevBytePos is the start position of the
         * previously inserted (or rather, reserved) record - it is copied to the
         * prev-link of the next record. These are stored as "usable byte
         * positions" rather than XLogRecPtrs (see XLogBytePosToRecPtr()).
         */
        uint64          CurrBytePos;
        uint64          PrevBytePos;

        /*
         * Make sure the above heavily-contended spinlock and byte positions are
         * on their own cache line. In particular, the RedoRecPtr and full page
         * write variables below should be on a different cache line. They are
         * read on every WAL insertion, but updated rarely, and we don't want
         * those reads to steal the cache line containing Curr/PrevBytePos.
         */
        char            pad[PG_CACHE_LINE_SIZE];

        /*
         * fullPageWrites is the authoritative value used by all backends to
         * determine whether to write full-page image to WAL. This shared value,
         * instead of the process-local fullPageWrites, is required because, when
         * full_page_writes is changed by SIGHUP, we must WAL-log it before it
         * actually affects WAL-logging by backends.  Checkpointer sets at startup
         * or after SIGHUP.
         *
         * To read these fields, you must hold an insertion lock. To modify them,
         * you must hold ALL the locks.
         */
        XLogRecPtr      RedoRecPtr;             /* current redo point for insertions */
        bool            forcePageWrites;        /* forcing full-page writes for PITR? */
        bool            fullPageWrites;

        /*
         * exclusiveBackupState indicates the state of an exclusive backup (see
         * comments of ExclusiveBackupState for more details). nonExclusiveBackups
         * is a counter indicating the number of streaming base backups currently
         * in progress. forcePageWrites is set to true when either of these is
         * non-zero. lastBackupStart is the latest checkpoint redo location used
         * as a starting point for an online backup.
         */
        ExclusiveBackupState exclusiveBackupState;
        int                     nonExclusiveBackups;
        XLogRecPtr      lastBackupStart;

        /*
         * WAL insertion locks.
         */
        WALInsertLockPadded *WALInsertLocks;
} XLogCtlInsert;

/*
 * Total shared-memory state for XLOG.
 */
typedef struct XLogCtlData
{
        XLogCtlInsert Insert;

        /* Protected by info_lck: */
        XLogwrtRqst LogwrtRqst;
        XLogRecPtr      RedoRecPtr;             /* a recent copy of Insert->RedoRecPtr */
        FullTransactionId ckptFullXid;  /* nextXid of latest checkpoint */
        XLogRecPtr      asyncXactLSN;   /* LSN of newest async commit/abort */
        XLogRecPtr      replicationSlotMinLSN;  /* oldest LSN needed by any slot */

        XLogSegNo       lastRemovedSegNo;       /* latest removed/recycled XLOG segment */

        /* Fake LSN counter, for unlogged relations. Protected by ulsn_lck. */
        XLogRecPtr      unloggedLSN;
        slock_t         ulsn_lck;

        /* Time and LSN of last xlog segment switch. Protected by WALWriteLock. */
        pg_time_t       lastSegSwitchTime;
        XLogRecPtr      lastSegSwitchLSN;

        /*
         * Protected by info_lck and WALWriteLock (you must hold either lock to
         * read it, but both to update)
         */
        XLogwrtResult LogwrtResult;

        /*
         * Latest initialized page in the cache (last byte position + 1).
         *
         * To change the identity of a buffer (and InitializedUpTo), you need to
         * hold WALBufMappingLock.  To change the identity of a buffer that's
         * still dirty, the old page needs to be written out first, and for that
         * you need WALWriteLock, and you need to ensure that there are no
         * in-progress insertions to the page by calling
         * WaitXLogInsertionsToFinish().
         */
        XLogRecPtr      InitializedUpTo;

        /*
         * These values do not change after startup, although the pointed-to pages
         * and xlblocks values certainly do.  xlblocks values are protected by
         * WALBufMappingLock.
         */
        char       *pages;                      /* buffers for unwritten XLOG pages */
        XLogRecPtr *xlblocks;           /* 1st byte ptr-s + XLOG_BLCKSZ */
        int                     XLogCacheBlck;  /* highest allocated xlog buffer index */

        /*
         * InsertTimeLineID is the timeline into which new WAL is being inserted
         * and flushed. It is zero during recovery, and does not change once set.
         *
         * If we create a new timeline when the system was started up,
         * PrevTimeLineID is the old timeline's ID that we forked off from.
         * Otherwise it's equal to InsertTimeLineID.
         */
        TimeLineID      InsertTimeLineID;
        TimeLineID      PrevTimeLineID;

        /*
         * SharedRecoveryState indicates if we're still in crash or archive
         * recovery.  Protected by info_lck.
         */
        RecoveryState SharedRecoveryState;

        /*
         * InstallXLogFileSegmentActive indicates whether the checkpointer should
         * arrange for future segments by recycling and/or PreallocXlogFiles().
         * Protected by ControlFileLock.  Only the startup process changes it.  If
         * true, anyone can use InstallXLogFileSegment().  If false, the startup
         * process owns the exclusive right to install segments, by reading from
         * the archive and possibly replacing existing files.
         */
        bool            InstallXLogFileSegmentActive;

        /*
         * WalWriterSleeping indicates whether the WAL writer is currently in
         * low-power mode (and hence should be nudged if an async commit occurs).
         * Protected by info_lck.
         */
        bool            WalWriterSleeping;

        /*
         * During recovery, we keep a copy of the latest checkpoint record here.
         * lastCheckPointRecPtr points to start of checkpoint record and
         * lastCheckPointEndPtr points to end+1 of checkpoint record.  Used by the
         * checkpointer when it wants to create a restartpoint.
         *
         * Protected by info_lck.
         */
        XLogRecPtr      lastCheckPointRecPtr;
        XLogRecPtr      lastCheckPointEndPtr;
        CheckPoint      lastCheckPoint;

        /*
         * lastFpwDisableRecPtr points to the start of the last replayed
         * XLOG_FPW_CHANGE record that instructs full_page_writes is disabled.
         */
        XLogRecPtr      lastFpwDisableRecPtr;

        slock_t         info_lck;               /* locks shared variables shown above */
} XLogCtlData;

static XLogCtlData *XLogCtl = NULL;

/* a private copy of XLogCtl->Insert.WALInsertLocks, for convenience */
static WALInsertLockPadded *WALInsertLocks = NULL;

/*
 * We maintain an image of pg_control in shared memory.
 */
static ControlFileData *ControlFile = NULL;

/*
 * Calculate the amount of space left on the page after 'endptr'. Beware
 * multiple evaluation!
 */
#define INSERT_FREESPACE(endptr)        \
        (((endptr) % XLOG_BLCKSZ == 0) ? 0 : (XLOG_BLCKSZ - (endptr) % XLOG_BLCKSZ))

/* Macro to advance to next buffer index. */
#define NextBufIdx(idx)         \
                (((idx) == XLogCtl->XLogCacheBlck) ? 0 : ((idx) + 1))

/*
 * XLogRecPtrToBufIdx returns the index of the WAL buffer that holds, or
 * would hold if it was in cache, the page containing 'recptr'.
 */
#define XLogRecPtrToBufIdx(recptr)      \
        (((recptr) / XLOG_BLCKSZ) % (XLogCtl->XLogCacheBlck + 1))

/*
 * These are the number of bytes in a WAL page usable for WAL data.
 */
#define UsableBytesInPage (XLOG_BLCKSZ - SizeOfXLogShortPHD)

/*
 * Convert values of GUCs measured in megabytes to equiv. segment count.
 * Rounds down.
 */
#define ConvertToXSegs(x, segsize)      XLogMBVarToSegs((x), (segsize))

/* The number of bytes in a WAL segment usable for WAL data. */
static int      UsableBytesInSegment;

/*
 * Private, possibly out-of-date copy of shared LogwrtResult.
 * See discussion above.
 */
static XLogwrtResult LogwrtResult = {0, 0};

/*
 * openLogFile is -1 or a kernel FD for an open log file segment.
 * openLogSegNo identifies the segment, and openLogTLI the corresponding TLI.
 * These variables are only used to write the XLOG, and so will normally refer
 * to the active segment.
 *
 * Note: call Reserve/ReleaseExternalFD to track consumption of this FD.
 */
static int      openLogFile = -1;
static XLogSegNo openLogSegNo = 0;
static TimeLineID openLogTLI = 0;

/*
 * Local copies of equivalent fields in the control file.  When running
 * crash recovery, LocalMinRecoveryPoint is set to InvalidXLogRecPtr as we
 * expect to replay all the WAL available, and updateMinRecoveryPoint is
 * switched to false to prevent any updates while replaying records.
 * Those values are kept consistent as long as crash recovery runs.
 */
static XLogRecPtr LocalMinRecoveryPoint;
static TimeLineID LocalMinRecoveryPointTLI;
static bool updateMinRecoveryPoint = true;

/* For WALInsertLockAcquire/Release functions */
static int      MyLockNo = 0;
static bool holdingAllLocks = false;

#ifdef WAL_DEBUG
static MemoryContext walDebugCxt = NULL;
#endif

static void CleanupAfterArchiveRecovery(TimeLineID EndOfLogTLI,
                                                                                XLogRecPtr EndOfLog,
                                                                                TimeLineID newTLI);
static void CheckRequiredParameterValues(void);
static void XLogReportParameters(void);
static int      LocalSetXLogInsertAllowed(void);
static void CreateEndOfRecoveryRecord(void);
static XLogRecPtr CreateOverwriteContrecordRecord(XLogRecPtr aborted_lsn,
                                                                                                  XLogRecPtr missingContrecPtr,
                                                                                                  TimeLineID newTLI);
static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);
static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo);
static XLogRecPtr XLogGetReplicationSlotMinimumLSN(void);

static void AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli,
                                                                  bool opportunistic);
static void XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible);
static bool InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
                                                                   bool find_free, XLogSegNo max_segno,
                                                                   TimeLineID tli);
static void XLogFileClose(void);
static void PreallocXlogFiles(XLogRecPtr endptr, TimeLineID tli);
static void RemoveTempXlogFiles(void);
static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr lastredoptr,
                                                           XLogRecPtr endptr, TimeLineID insertTLI);
static void RemoveXlogFile(const char *segname, XLogSegNo recycleSegNo,
                                                   XLogSegNo *endlogSegNo, TimeLineID insertTLI);
static void UpdateLastRemovedPtr(char *filename);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
static void UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force);
static bool PerformRecoveryXLogAction(void);
static void InitControlFile(uint64 sysidentifier);
static void WriteControlFile(void);
static void ReadControlFile(void);
static void UpdateControlFile(void);
static char *str_time(pg_time_t tnow);

static void pg_start_backup_callback(int code, Datum arg);
static void pg_stop_backup_callback(int code, Datum arg);

static int      get_sync_bit(int method);

static void CopyXLogRecordToWAL(int write_len, bool isLogSwitch,
                                                                XLogRecData *rdata,
                                                                XLogRecPtr StartPos, XLogRecPtr EndPos,
                                                                TimeLineID tli);
static void ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos,
                                                                          XLogRecPtr *EndPos, XLogRecPtr *PrevPtr);
static bool ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos,
                                                          XLogRecPtr *PrevPtr);
static XLogRecPtr WaitXLogInsertionsToFinish(XLogRecPtr upto);
static char *GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli);
static XLogRecPtr XLogBytePosToRecPtr(uint64 bytepos);
static XLogRecPtr XLogBytePosToEndRecPtr(uint64 bytepos);
static uint64 XLogRecPtrToBytePos(XLogRecPtr ptr);

static void WALInsertLockAcquire(void);
static void WALInsertLockAcquireExclusive(void);
static void WALInsertLockRelease(void);
static void WALInsertLockUpdateInsertingAt(XLogRecPtr insertingAt);

/*
 * Insert an XLOG record represented by an already-constructed chain of data
 * chunks.  This is a low-level routine; to construct the WAL record header
 * and data, use the higher-level routines in xloginsert.c.
 *
 * If 'fpw_lsn' is valid, it is the oldest LSN among the pages that this
 * WAL record applies to, that were not included in the record as full page
 * images.  If fpw_lsn <= RedoRecPtr, the function does not perform the
 * insertion and returns InvalidXLogRecPtr.  The caller can then recalculate
 * which pages need a full-page image, and retry.  If fpw_lsn is invalid, the
 * record is always inserted.
 *
 * 'flags' gives more in-depth control on the record being inserted. See
 * XLogSetRecordFlags() for details.
 *
 * 'topxid_included' tells whether the top-transaction id is logged along with
 * current subtransaction. See XLogRecordAssemble().
 *
 * The first XLogRecData in the chain must be for the record header, and its
 * data must be MAXALIGNed.  XLogInsertRecord fills in the xl_prev and
 * xl_crc fields in the header, the rest of the header must already be filled
 * by the caller.
 *
 * Returns XLOG pointer to end of record (beginning of next record).
 * This can be used as LSN for data pages affected by the logged action.
 * (LSN is the XLOG point up to which the XLOG must be flushed to disk
 * before the data page can be written out.  This implements the basic
 * WAL rule "write the log before the data".)
 */
XLogRecPtr
XLogInsertRecord(XLogRecData *rdata,
                                 XLogRecPtr fpw_lsn,
                                 uint8 flags,
                                 int num_fpi,
                                 bool topxid_included)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        pg_crc32c       rdata_crc;
        bool            inserted;
        XLogRecord *rechdr = (XLogRecord *) rdata->data;
        uint8           info = rechdr->xl_info & ~XLR_INFO_MASK;
        bool            isLogSwitch = (rechdr->xl_rmid == RM_XLOG_ID &&
                                                           info == XLOG_SWITCH);
        XLogRecPtr      StartPos;
        XLogRecPtr      EndPos;
        bool            prevDoPageWrites = doPageWrites;
        TimeLineID      insertTLI;

        /* we assume that all of the record header is in the first chunk */
        Assert(rdata->len >= SizeOfXLogRecord);

        /* cross-check on whether we should be here or not */
        if (!XLogInsertAllowed())
                elog(ERROR, "cannot make new WAL entries during recovery");

        /*
         * Given that we're not in recovery, InsertTimeLineID is set and can't
         * change, so we can read it without a lock.
         */
        insertTLI = XLogCtl->InsertTimeLineID;

        /*----------
         *
         * We have now done all the preparatory work we can without holding a
         * lock or modifying shared state. From here on, inserting the new WAL
         * record to the shared WAL buffer cache is a two-step process:
         *
         * 1. Reserve the right amount of space from the WAL. The current head of
         *        reserved space is kept in Insert->CurrBytePos, and is protected by
         *        insertpos_lck.
         *
         * 2. Copy the record to the reserved WAL space. This involves finding the
         *        correct WAL buffer containing the reserved space, and copying the
         *        record in place. This can be done concurrently in multiple processes.
         *
         * To keep track of which insertions are still in-progress, each concurrent
         * inserter acquires an insertion lock. In addition to just indicating that
         * an insertion is in progress, the lock tells others how far the inserter
         * has progressed. There is a small fixed number of insertion locks,
         * determined by NUM_XLOGINSERT_LOCKS. When an inserter crosses a page
         * boundary, it updates the value stored in the lock to the how far it has
         * inserted, to allow the previous buffer to be flushed.
         *
         * Holding onto an insertion lock also protects RedoRecPtr and
         * fullPageWrites from changing until the insertion is finished.
         *
         * Step 2 can usually be done completely in parallel. If the required WAL
         * page is not initialized yet, you have to grab WALBufMappingLock to
         * initialize it, but the WAL writer tries to do that ahead of insertions
         * to avoid that from happening in the critical path.
         *
         *----------
         */
        START_CRIT_SECTION();
        if (isLogSwitch)
                WALInsertLockAcquireExclusive();
        else
                WALInsertLockAcquire();

        /*
         * Check to see if my copy of RedoRecPtr is out of date. If so, may have
         * to go back and have the caller recompute everything. This can only
         * happen just after a checkpoint, so it's better to be slow in this case
         * and fast otherwise.
         *
         * Also check to see if fullPageWrites or forcePageWrites was just turned
         * on; if we weren't already doing full-page writes then go back and
         * recompute.
         *
         * If we aren't doing full-page writes then RedoRecPtr doesn't actually
         * affect the contents of the XLOG record, so we'll update our local copy
         * but not force a recomputation.  (If doPageWrites was just turned off,
         * we could recompute the record without full pages, but we choose not to
         * bother.)
         */
        if (RedoRecPtr != Insert->RedoRecPtr)
        {
                Assert(RedoRecPtr < Insert->RedoRecPtr);
                RedoRecPtr = Insert->RedoRecPtr;
        }
        doPageWrites = (Insert->fullPageWrites || Insert->forcePageWrites);

        if (doPageWrites &&
                (!prevDoPageWrites ||
                 (fpw_lsn != InvalidXLogRecPtr && fpw_lsn <= RedoRecPtr)))
        {
                /*
                 * Oops, some buffer now needs to be backed up that the caller didn't
                 * back up.  Start over.
                 */
                WALInsertLockRelease();
                END_CRIT_SECTION();
                return InvalidXLogRecPtr;
        }

        /*
         * Reserve space for the record in the WAL. This also sets the xl_prev
         * pointer.
         */
        if (isLogSwitch)
                inserted = ReserveXLogSwitch(&StartPos, &EndPos, &rechdr->xl_prev);
        else
        {
                ReserveXLogInsertLocation(rechdr->xl_tot_len, &StartPos, &EndPos,
                                                                  &rechdr->xl_prev);
                inserted = true;
        }

        if (inserted)
        {
                /*
                 * Now that xl_prev has been filled in, calculate CRC of the record
                 * header.
                 */
                rdata_crc = rechdr->xl_crc;
                COMP_CRC32C(rdata_crc, rechdr, offsetof(XLogRecord, xl_crc));
                FIN_CRC32C(rdata_crc);
                rechdr->xl_crc = rdata_crc;

                /*
                 * All the record data, including the header, is now ready to be
                 * inserted. Copy the record in the space reserved.
                 */
                CopyXLogRecordToWAL(rechdr->xl_tot_len, isLogSwitch, rdata,
                                                        StartPos, EndPos, insertTLI);

                /*
                 * Unless record is flagged as not important, update LSN of last
                 * important record in the current slot. When holding all locks, just
                 * update the first one.
                 */
                if ((flags & XLOG_MARK_UNIMPORTANT) == 0)
                {
                        int                     lockno = holdingAllLocks ? 0 : MyLockNo;

                        WALInsertLocks[lockno].l.lastImportantAt = StartPos;
                }
        }
        else
        {
                /*
                 * This was an xlog-switch record, but the current insert location was
                 * already exactly at the beginning of a segment, so there was no need
                 * to do anything.
                 */
        }

        /*
         * Done! Let others know that we're finished.
         */
        WALInsertLockRelease();

        END_CRIT_SECTION();

        MarkCurrentTransactionIdLoggedIfAny();

        /*
         * Mark top transaction id is logged (if needed) so that we should not try
         * to log it again with the next WAL record in the current subtransaction.
         */
        if (topxid_included)
                MarkSubxactTopXidLogged();

        /*
         * Update shared LogwrtRqst.Write, if we crossed page boundary.
         */
        if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
        {
                SpinLockAcquire(&XLogCtl->info_lck);
                /* advance global request to include new block(s) */
                if (XLogCtl->LogwrtRqst.Write < EndPos)
                        XLogCtl->LogwrtRqst.Write = EndPos;
                /* update local result copy while I have the chance */
                LogwrtResult = XLogCtl->LogwrtResult;
                SpinLockRelease(&XLogCtl->info_lck);
        }

        /*
         * If this was an XLOG_SWITCH record, flush the record and the empty
         * padding space that fills the rest of the segment, and perform
         * end-of-segment actions (eg, notifying archiver).
         */
        if (isLogSwitch)
        {
                TRACE_POSTGRESQL_WAL_SWITCH();
                XLogFlush(EndPos);

                /*
                 * Even though we reserved the rest of the segment for us, which is
                 * reflected in EndPos, we return a pointer to just the end of the
                 * xlog-switch record.
                 */
                if (inserted)
                {
                        EndPos = StartPos + SizeOfXLogRecord;
                        if (StartPos / XLOG_BLCKSZ != EndPos / XLOG_BLCKSZ)
                        {
                                uint64          offset = XLogSegmentOffset(EndPos, wal_segment_size);

                                if (offset == EndPos % XLOG_BLCKSZ)
                                        EndPos += SizeOfXLogLongPHD;
                                else
                                        EndPos += SizeOfXLogShortPHD;
                        }
                }
        }

#ifdef WAL_DEBUG
        if (XLOG_DEBUG)
        {
                static XLogReaderState *debug_reader = NULL;
                XLogRecord *record;
                DecodedXLogRecord *decoded;
                StringInfoData buf;
                StringInfoData recordBuf;
                char       *errormsg = NULL;
                MemoryContext oldCxt;

                oldCxt = MemoryContextSwitchTo(walDebugCxt);

                initStringInfo(&buf);
                appendStringInfo(&buf, "INSERT @ %X/%X: ", LSN_FORMAT_ARGS(EndPos));

                /*
                 * We have to piece together the WAL record data from the XLogRecData
                 * entries, so that we can pass it to the rm_desc function as one
                 * contiguous chunk.
                 */
                initStringInfo(&recordBuf);
                for (; rdata != NULL; rdata = rdata->next)
                        appendBinaryStringInfo(&recordBuf, rdata->data, rdata->len);

                /* We also need temporary space to decode the record. */
                record = (XLogRecord *) recordBuf.data;
                decoded = (DecodedXLogRecord *)
                        palloc(DecodeXLogRecordRequiredSpace(record->xl_tot_len));

                if (!debug_reader)
                        debug_reader = XLogReaderAllocate(wal_segment_size, NULL,
                                                                                          XL_ROUTINE(), NULL);

                if (!debug_reader)
                {
                        appendStringInfoString(&buf, "error decoding record: out of memory while allocating a WAL reading processor");
                }
                else if (!DecodeXLogRecord(debug_reader,
                                                                   decoded,
                                                                   record,
                                                                   EndPos,
                                                                   &errormsg))
                {
                        appendStringInfo(&buf, "error decoding record: %s",
                                                         errormsg ? errormsg : "no error message");
                }
                else
                {
                        appendStringInfoString(&buf, " - ");

                        debug_reader->record = decoded;
                        xlog_outdesc(&buf, debug_reader);
                        debug_reader->record = NULL;
                }
                elog(LOG, "%s", buf.data);

                pfree(decoded);
                pfree(buf.data);
                pfree(recordBuf.data);
                MemoryContextSwitchTo(oldCxt);
        }
#endif

        /*
         * Update our global variables
         */
        ProcLastRecPtr = StartPos;
        XactLastRecEnd = EndPos;

        /* Report WAL traffic to the instrumentation. */
        if (inserted)
        {
                pgWalUsage.wal_bytes += rechdr->xl_tot_len;
                pgWalUsage.wal_records++;
                pgWalUsage.wal_fpi += num_fpi;
        }

        return EndPos;
}

/*
 * Reserves the right amount of space for a record of given size from the WAL.
 * *StartPos is set to the beginning of the reserved section, *EndPos to
 * its end+1. *PrevPtr is set to the beginning of the previous record; it is
 * used to set the xl_prev of this record.
 *
 * This is the performance critical part of XLogInsert that must be serialized
 * across backends. The rest can happen mostly in parallel. Try to keep this
 * section as short as possible, insertpos_lck can be heavily contended on a
 * busy system.
 *
 * NB: The space calculation here must match the code in CopyXLogRecordToWAL,
 * where we actually copy the record to the reserved space.
 */
static void
ReserveXLogInsertLocation(int size, XLogRecPtr *StartPos, XLogRecPtr *EndPos,
                                                  XLogRecPtr *PrevPtr)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        uint64          startbytepos;
        uint64          endbytepos;
        uint64          prevbytepos;

        size = MAXALIGN(size);

        /* All (non xlog-switch) records should contain data. */
        Assert(size > SizeOfXLogRecord);

        /*
         * The duration the spinlock needs to be held is minimized by minimizing
         * the calculations that have to be done while holding the lock. The
         * current tip of reserved WAL is kept in CurrBytePos, as a byte position
         * that only counts "usable" bytes in WAL, that is, it excludes all WAL
         * page headers. The mapping between "usable" byte positions and physical
         * positions (XLogRecPtrs) can be done outside the locked region, and
         * because the usable byte position doesn't include any headers, reserving
         * X bytes from WAL is almost as simple as "CurrBytePos += X".
         */
        SpinLockAcquire(&Insert->insertpos_lck);

        startbytepos = Insert->CurrBytePos;
        endbytepos = startbytepos + size;
        prevbytepos = Insert->PrevBytePos;
        Insert->CurrBytePos = endbytepos;
        Insert->PrevBytePos = startbytepos;

        SpinLockRelease(&Insert->insertpos_lck);

        *StartPos = XLogBytePosToRecPtr(startbytepos);
        *EndPos = XLogBytePosToEndRecPtr(endbytepos);
        *PrevPtr = XLogBytePosToRecPtr(prevbytepos);

        /*
         * Check that the conversions between "usable byte positions" and
         * XLogRecPtrs work consistently in both directions.
         */
        Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
        Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
        Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);
}

/*
 * Like ReserveXLogInsertLocation(), but for an xlog-switch record.
 *
 * A log-switch record is handled slightly differently. The rest of the
 * segment will be reserved for this insertion, as indicated by the returned
 * *EndPos value. However, if we are already at the beginning of the current
 * segment, *StartPos and *EndPos are set to the current location without
 * reserving any space, and the function returns false.
*/
static bool
ReserveXLogSwitch(XLogRecPtr *StartPos, XLogRecPtr *EndPos, XLogRecPtr *PrevPtr)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        uint64          startbytepos;
        uint64          endbytepos;
        uint64          prevbytepos;
        uint32          size = MAXALIGN(SizeOfXLogRecord);
        XLogRecPtr      ptr;
        uint32          segleft;

        /*
         * These calculations are a bit heavy-weight to be done while holding a
         * spinlock, but since we're holding all the WAL insertion locks, there
         * are no other inserters competing for it. GetXLogInsertRecPtr() does
         * compete for it, but that's not called very frequently.
         */
        SpinLockAcquire(&Insert->insertpos_lck);

        startbytepos = Insert->CurrBytePos;

        ptr = XLogBytePosToEndRecPtr(startbytepos);
        if (XLogSegmentOffset(ptr, wal_segment_size) == 0)
        {
                SpinLockRelease(&Insert->insertpos_lck);
                *EndPos = *StartPos = ptr;
                return false;
        }

        endbytepos = startbytepos + size;
        prevbytepos = Insert->PrevBytePos;

        *StartPos = XLogBytePosToRecPtr(startbytepos);
        *EndPos = XLogBytePosToEndRecPtr(endbytepos);

        segleft = wal_segment_size - XLogSegmentOffset(*EndPos, wal_segment_size);
        if (segleft != wal_segment_size)
        {
                /* consume the rest of the segment */
                *EndPos += segleft;
                endbytepos = XLogRecPtrToBytePos(*EndPos);
        }
        Insert->CurrBytePos = endbytepos;
        Insert->PrevBytePos = startbytepos;

        SpinLockRelease(&Insert->insertpos_lck);

        *PrevPtr = XLogBytePosToRecPtr(prevbytepos);

        Assert(XLogSegmentOffset(*EndPos, wal_segment_size) == 0);
        Assert(XLogRecPtrToBytePos(*EndPos) == endbytepos);
        Assert(XLogRecPtrToBytePos(*StartPos) == startbytepos);
        Assert(XLogRecPtrToBytePos(*PrevPtr) == prevbytepos);

        return true;
}

/*
 * Subroutine of XLogInsertRecord.  Copies a WAL record to an already-reserved
 * area in the WAL.
 */
static void
CopyXLogRecordToWAL(int write_len, bool isLogSwitch, XLogRecData *rdata,
                                        XLogRecPtr StartPos, XLogRecPtr EndPos, TimeLineID tli)
{
        char       *currpos;
        int                     freespace;
        int                     written;
        XLogRecPtr      CurrPos;
        XLogPageHeader pagehdr;

        /*
         * Get a pointer to the right place in the right WAL buffer to start
         * inserting to.
         */
        CurrPos = StartPos;
        currpos = GetXLogBuffer(CurrPos, tli);
        freespace = INSERT_FREESPACE(CurrPos);

        /*
         * there should be enough space for at least the first field (xl_tot_len)
         * on this page.
         */
        Assert(freespace >= sizeof(uint32));

        /* Copy record data */
        written = 0;
        while (rdata != NULL)
        {
                char       *rdata_data = rdata->data;
                int                     rdata_len = rdata->len;

                while (rdata_len > freespace)
                {
                        /*
                         * Write what fits on this page, and continue on the next page.
                         */
                        Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || freespace == 0);
                        memcpy(currpos, rdata_data, freespace);
                        rdata_data += freespace;
                        rdata_len -= freespace;
                        written += freespace;
                        CurrPos += freespace;

                        /*
                         * Get pointer to beginning of next page, and set the xlp_rem_len
                         * in the page header. Set XLP_FIRST_IS_CONTRECORD.
                         *
                         * It's safe to set the contrecord flag and xlp_rem_len without a
                         * lock on the page. All the other flags were already set when the
                         * page was initialized, in AdvanceXLInsertBuffer, and we're the
                         * only backend that needs to set the contrecord flag.
                         */
                        currpos = GetXLogBuffer(CurrPos, tli);
                        pagehdr = (XLogPageHeader) currpos;
                        pagehdr->xlp_rem_len = write_len - written;
                        pagehdr->xlp_info |= XLP_FIRST_IS_CONTRECORD;

                        /* skip over the page header */
                        if (XLogSegmentOffset(CurrPos, wal_segment_size) == 0)
                        {
                                CurrPos += SizeOfXLogLongPHD;
                                currpos += SizeOfXLogLongPHD;
                        }
                        else
                        {
                                CurrPos += SizeOfXLogShortPHD;
                                currpos += SizeOfXLogShortPHD;
                        }
                        freespace = INSERT_FREESPACE(CurrPos);
                }

                Assert(CurrPos % XLOG_BLCKSZ >= SizeOfXLogShortPHD || rdata_len == 0);
                memcpy(currpos, rdata_data, rdata_len);
                currpos += rdata_len;
                CurrPos += rdata_len;
                freespace -= rdata_len;
                written += rdata_len;

                rdata = rdata->next;
        }
        Assert(written == write_len);

        /*
         * If this was an xlog-switch, it's not enough to write the switch record,
         * we also have to consume all the remaining space in the WAL segment.  We
         * have already reserved that space, but we need to actually fill it.
         */
        if (isLogSwitch && XLogSegmentOffset(CurrPos, wal_segment_size) != 0)
        {
                /* An xlog-switch record doesn't contain any data besides the header */
                Assert(write_len == SizeOfXLogRecord);

                /* Assert that we did reserve the right amount of space */
                Assert(XLogSegmentOffset(EndPos, wal_segment_size) == 0);

                /* Use up all the remaining space on the current page */
                CurrPos += freespace;

                /*
                 * Cause all remaining pages in the segment to be flushed, leaving the
                 * XLog position where it should be, at the start of the next segment.
                 * We do this one page at a time, to make sure we don't deadlock
                 * against ourselves if wal_buffers < wal_segment_size.
                 */
                while (CurrPos < EndPos)
                {
                        /*
                         * The minimal action to flush the page would be to call
                         * WALInsertLockUpdateInsertingAt(CurrPos) followed by
                         * AdvanceXLInsertBuffer(...).  The page would be left initialized
                         * mostly to zeros, except for the page header (always the short
                         * variant, as this is never a segment's first page).
                         *
                         * The large vistas of zeros are good for compressibility, but the
                         * headers interrupting them every XLOG_BLCKSZ (with values that
                         * differ from page to page) are not.  The effect varies with
                         * compression tool, but bzip2 for instance compresses about an
                         * order of magnitude worse if those headers are left in place.
                         *
                         * Rather than complicating AdvanceXLInsertBuffer itself (which is
                         * called in heavily-loaded circumstances as well as this lightly-
                         * loaded one) with variant behavior, we just use GetXLogBuffer
                         * (which itself calls the two methods we need) to get the pointer
                         * and zero most of the page.  Then we just zero the page header.
                         */
                        currpos = GetXLogBuffer(CurrPos, tli);
                        MemSet(currpos, 0, SizeOfXLogShortPHD);

                        CurrPos += XLOG_BLCKSZ;
                }
        }
        else
        {
                /* Align the end position, so that the next record starts aligned */
                CurrPos = MAXALIGN64(CurrPos);
        }

        if (CurrPos != EndPos)
                elog(PANIC, "space reserved for WAL record does not match what was written");
}

/*
 * Acquire a WAL insertion lock, for inserting to WAL.
 */
static void
WALInsertLockAcquire(void)
{
        bool            immed;

        /*
         * It doesn't matter which of the WAL insertion locks we acquire, so try
         * the one we used last time.  If the system isn't particularly busy, it's
         * a good bet that it's still available, and it's good to have some
         * affinity to a particular lock so that you don't unnecessarily bounce
         * cache lines between processes when there's no contention.
         *
         * If this is the first time through in this backend, pick a lock
         * (semi-)randomly.  This allows the locks to be used evenly if you have a
         * lot of very short connections.
         */
        static int      lockToTry = -1;

        if (lockToTry == -1)
                lockToTry = MyProc->pgprocno % NUM_XLOGINSERT_LOCKS;
        MyLockNo = lockToTry;

        /*
         * The insertingAt value is initially set to 0, as we don't know our
         * insert location yet.
         */
        immed = LWLockAcquire(&WALInsertLocks[MyLockNo].l.lock, LW_EXCLUSIVE);
        if (!immed)
        {
                /*
                 * If we couldn't get the lock immediately, try another lock next
                 * time.  On a system with more insertion locks than concurrent
                 * inserters, this causes all the inserters to eventually migrate to a
                 * lock that no-one else is using.  On a system with more inserters
                 * than locks, it still helps to distribute the inserters evenly
                 * across the locks.
                 */
                lockToTry = (lockToTry + 1) % NUM_XLOGINSERT_LOCKS;
        }
}

/*
 * Acquire all WAL insertion locks, to prevent other backends from inserting
 * to WAL.
 */
static void
WALInsertLockAcquireExclusive(void)
{
        int                     i;

        /*
         * When holding all the locks, all but the last lock's insertingAt
         * indicator is set to 0xFFFFFFFFFFFFFFFF, which is higher than any real
         * XLogRecPtr value, to make sure that no-one blocks waiting on those.
         */
        for (i = 0; i < NUM_XLOGINSERT_LOCKS - 1; i++)
        {
                LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);
                LWLockUpdateVar(&WALInsertLocks[i].l.lock,
                                                &WALInsertLocks[i].l.insertingAt,
                                                PG_UINT64_MAX);
        }
        /* Variable value reset to 0 at release */
        LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);

        holdingAllLocks = true;
}

/*
 * Release our insertion lock (or locks, if we're holding them all).
 *
 * NB: Reset all variables to 0, so they cause LWLockWaitForVar to block the
 * next time the lock is acquired.
 */
static void
WALInsertLockRelease(void)
{
        if (holdingAllLocks)
        {
                int                     i;

                for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
                        LWLockReleaseClearVar(&WALInsertLocks[i].l.lock,
                                                                  &WALInsertLocks[i].l.insertingAt,
                                                                  0);

                holdingAllLocks = false;
        }
        else
        {
                LWLockReleaseClearVar(&WALInsertLocks[MyLockNo].l.lock,
                                                          &WALInsertLocks[MyLockNo].l.insertingAt,
                                                          0);
        }
}

/*
 * Update our insertingAt value, to let others know that we've finished
 * inserting up to that point.
 */
static void
WALInsertLockUpdateInsertingAt(XLogRecPtr insertingAt)
{
        if (holdingAllLocks)
        {
                /*
                 * We use the last lock to mark our actual position, see comments in
                 * WALInsertLockAcquireExclusive.
                 */
                LWLockUpdateVar(&WALInsertLocks[NUM_XLOGINSERT_LOCKS - 1].l.lock,
                                                &WALInsertLocks[NUM_XLOGINSERT_LOCKS - 1].l.insertingAt,
                                                insertingAt);
        }
        else
                LWLockUpdateVar(&WALInsertLocks[MyLockNo].l.lock,
                                                &WALInsertLocks[MyLockNo].l.insertingAt,
                                                insertingAt);
}

/*
 * Wait for any WAL insertions < upto to finish.
 *
 * Returns the location of the oldest insertion that is still in-progress.
 * Any WAL prior to that point has been fully copied into WAL buffers, and
 * can be flushed out to disk. Because this waits for any insertions older
 * than 'upto' to finish, the return value is always >= 'upto'.
 *
 * Note: When you are about to write out WAL, you must call this function
 * *before* acquiring WALWriteLock, to avoid deadlocks. This function might
 * need to wait for an insertion to finish (or at least advance to next
 * uninitialized page), and the inserter might need to evict an old WAL buffer
 * to make room for a new one, which in turn requires WALWriteLock.
 */
static XLogRecPtr
WaitXLogInsertionsToFinish(XLogRecPtr upto)
{
        uint64          bytepos;
        XLogRecPtr      reservedUpto;
        XLogRecPtr      finishedUpto;
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        int                     i;

        if (MyProc == NULL)
                elog(PANIC, "cannot wait without a PGPROC structure");

        /* Read the current insert position */
        SpinLockAcquire(&Insert->insertpos_lck);
        bytepos = Insert->CurrBytePos;
        SpinLockRelease(&Insert->insertpos_lck);
        reservedUpto = XLogBytePosToEndRecPtr(bytepos);

        /*
         * No-one should request to flush a piece of WAL that hasn't even been
         * reserved yet. However, it can happen if there is a block with a bogus
         * LSN on disk, for example. XLogFlush checks for that situation and
         * complains, but only after the flush. Here we just assume that to mean
         * that all WAL that has been reserved needs to be finished. In this
         * corner-case, the return value can be smaller than 'upto' argument.
         */
        if (upto > reservedUpto)
        {
                ereport(LOG,
                                (errmsg("request to flush past end of generated WAL; request %X/%X, current position %X/%X",
                                                LSN_FORMAT_ARGS(upto), LSN_FORMAT_ARGS(reservedUpto))));
                upto = reservedUpto;
        }

        /*
         * Loop through all the locks, sleeping on any in-progress insert older
         * than 'upto'.
         *
         * finishedUpto is our return value, indicating the point upto which all
         * the WAL insertions have been finished. Initialize it to the head of
         * reserved WAL, and as we iterate through the insertion locks, back it
         * out for any insertion that's still in progress.
         */
        finishedUpto = reservedUpto;
        for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
        {
                XLogRecPtr      insertingat = InvalidXLogRecPtr;

                do
                {
                        /*
                         * See if this insertion is in progress.  LWLockWaitForVar will
                         * wait for the lock to be released, or for the 'value' to be set
                         * by a LWLockUpdateVar call.  When a lock is initially acquired,
                         * its value is 0 (InvalidXLogRecPtr), which means that we don't
                         * know where it's inserting yet.  We will have to wait for it. If
                         * it's a small insertion, the record will most likely fit on the
                         * same page and the inserter will release the lock without ever
                         * calling LWLockUpdateVar.  But if it has to sleep, it will
                         * advertise the insertion point with LWLockUpdateVar before
                         * sleeping.
                         */
                        if (LWLockWaitForVar(&WALInsertLocks[i].l.lock,
                                                                 &WALInsertLocks[i].l.insertingAt,
                                                                 insertingat, &insertingat))
                        {
                                /* the lock was free, so no insertion in progress */
                                insertingat = InvalidXLogRecPtr;
                                break;
                        }

                        /*
                         * This insertion is still in progress. Have to wait, unless the
                         * inserter has proceeded past 'upto'.
                         */
                } while (insertingat < upto);

                if (insertingat != InvalidXLogRecPtr && insertingat < finishedUpto)
                        finishedUpto = insertingat;
        }
        return finishedUpto;
}

/*
 * Get a pointer to the right location in the WAL buffer containing the
 * given XLogRecPtr.
 *
 * If the page is not initialized yet, it is initialized. That might require
 * evicting an old dirty buffer from the buffer cache, which means I/O.
 *
 * The caller must ensure that the page containing the requested location
 * isn't evicted yet, and won't be evicted. The way to ensure that is to
 * hold onto a WAL insertion lock with the insertingAt position set to
 * something <= ptr. GetXLogBuffer() will update insertingAt if it needs
 * to evict an old page from the buffer. (This means that once you call
 * GetXLogBuffer() with a given 'ptr', you must not access anything before
 * that point anymore, and must not call GetXLogBuffer() with an older 'ptr'
 * later, because older buffers might be recycled already)
 */
static char *
GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli)
{
        int                     idx;
        XLogRecPtr      endptr;
        static uint64 cachedPage = 0;
        static char *cachedPos = NULL;
        XLogRecPtr      expectedEndPtr;

        /*
         * Fast path for the common case that we need to access again the same
         * page as last time.
         */
        if (ptr / XLOG_BLCKSZ == cachedPage)
        {
                Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
                Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));
                return cachedPos + ptr % XLOG_BLCKSZ;
        }

        /*
         * The XLog buffer cache is organized so that a page is always loaded to a
         * particular buffer.  That way we can easily calculate the buffer a given
         * page must be loaded into, from the XLogRecPtr alone.
         */
        idx = XLogRecPtrToBufIdx(ptr);

        /*
         * See what page is loaded in the buffer at the moment. It could be the
         * page we're looking for, or something older. It can't be anything newer
         * - that would imply the page we're looking for has already been written
         * out to disk and evicted, and the caller is responsible for making sure
         * that doesn't happen.
         *
         * However, we don't hold a lock while we read the value. If someone has
         * just initialized the page, it's possible that we get a "torn read" of
         * the XLogRecPtr if 64-bit fetches are not atomic on this platform. In
         * that case we will see a bogus value. That's ok, we'll grab the mapping
         * lock (in AdvanceXLInsertBuffer) and retry if we see anything else than
         * the page we're looking for. But it means that when we do this unlocked
         * read, we might see a value that appears to be ahead of the page we're
         * looking for. Don't PANIC on that, until we've verified the value while
         * holding the lock.
         */
        expectedEndPtr = ptr;
        expectedEndPtr += XLOG_BLCKSZ - ptr % XLOG_BLCKSZ;

        endptr = XLogCtl->xlblocks[idx];
        if (expectedEndPtr != endptr)
        {
                XLogRecPtr      initializedUpto;

                /*
                 * Before calling AdvanceXLInsertBuffer(), which can block, let others
                 * know how far we're finished with inserting the record.
                 *
                 * NB: If 'ptr' points to just after the page header, advertise a
                 * position at the beginning of the page rather than 'ptr' itself. If
                 * there are no other insertions running, someone might try to flush
                 * up to our advertised location. If we advertised a position after
                 * the page header, someone might try to flush the page header, even
                 * though page might actually not be initialized yet. As the first
                 * inserter on the page, we are effectively responsible for making
                 * sure that it's initialized, before we let insertingAt to move past
                 * the page header.
                 */
                if (ptr % XLOG_BLCKSZ == SizeOfXLogShortPHD &&
                        XLogSegmentOffset(ptr, wal_segment_size) > XLOG_BLCKSZ)
                        initializedUpto = ptr - SizeOfXLogShortPHD;
                else if (ptr % XLOG_BLCKSZ == SizeOfXLogLongPHD &&
                                 XLogSegmentOffset(ptr, wal_segment_size) < XLOG_BLCKSZ)
                        initializedUpto = ptr - SizeOfXLogLongPHD;
                else
                        initializedUpto = ptr;

                WALInsertLockUpdateInsertingAt(initializedUpto);

                AdvanceXLInsertBuffer(ptr, tli, false);
                endptr = XLogCtl->xlblocks[idx];

                if (expectedEndPtr != endptr)
                        elog(PANIC, "could not find WAL buffer for %X/%X",
                                 LSN_FORMAT_ARGS(ptr));
        }
        else
        {
                /*
                 * Make sure the initialization of the page is visible to us, and
                 * won't arrive later to overwrite the WAL data we write on the page.
                 */
                pg_memory_barrier();
        }

        /*
         * Found the buffer holding this page. Return a pointer to the right
         * offset within the page.
         */
        cachedPage = ptr / XLOG_BLCKSZ;
        cachedPos = XLogCtl->pages + idx * (Size) XLOG_BLCKSZ;

        Assert(((XLogPageHeader) cachedPos)->xlp_magic == XLOG_PAGE_MAGIC);
        Assert(((XLogPageHeader) cachedPos)->xlp_pageaddr == ptr - (ptr % XLOG_BLCKSZ));

        return cachedPos + ptr % XLOG_BLCKSZ;
}

/*
 * Converts a "usable byte position" to XLogRecPtr. A usable byte position
 * is the position starting from the beginning of WAL, excluding all WAL
 * page headers.
 */
static XLogRecPtr
XLogBytePosToRecPtr(uint64 bytepos)
{
        uint64          fullsegs;
        uint64          fullpages;
        uint64          bytesleft;
        uint32          seg_offset;
        XLogRecPtr      result;

        fullsegs = bytepos / UsableBytesInSegment;
        bytesleft = bytepos % UsableBytesInSegment;

        if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
        {
                /* fits on first page of segment */
                seg_offset = bytesleft + SizeOfXLogLongPHD;
        }
        else
        {
                /* account for the first page on segment with long header */
                seg_offset = XLOG_BLCKSZ;
                bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

                fullpages = bytesleft / UsableBytesInPage;
                bytesleft = bytesleft % UsableBytesInPage;

                seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
        }

        XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, wal_segment_size, result);

        return result;
}

/*
 * Like XLogBytePosToRecPtr, but if the position is at a page boundary,
 * returns a pointer to the beginning of the page (ie. before page header),
 * not to where the first xlog record on that page would go to. This is used
 * when converting a pointer to the end of a record.
 */
static XLogRecPtr
XLogBytePosToEndRecPtr(uint64 bytepos)
{
        uint64          fullsegs;
        uint64          fullpages;
        uint64          bytesleft;
        uint32          seg_offset;
        XLogRecPtr      result;

        fullsegs = bytepos / UsableBytesInSegment;
        bytesleft = bytepos % UsableBytesInSegment;

        if (bytesleft < XLOG_BLCKSZ - SizeOfXLogLongPHD)
        {
                /* fits on first page of segment */
                if (bytesleft == 0)
                        seg_offset = 0;
                else
                        seg_offset = bytesleft + SizeOfXLogLongPHD;
        }
        else
        {
                /* account for the first page on segment with long header */
                seg_offset = XLOG_BLCKSZ;
                bytesleft -= XLOG_BLCKSZ - SizeOfXLogLongPHD;

                fullpages = bytesleft / UsableBytesInPage;
                bytesleft = bytesleft % UsableBytesInPage;

                if (bytesleft == 0)
                        seg_offset += fullpages * XLOG_BLCKSZ + bytesleft;
                else
                        seg_offset += fullpages * XLOG_BLCKSZ + bytesleft + SizeOfXLogShortPHD;
        }

        XLogSegNoOffsetToRecPtr(fullsegs, seg_offset, wal_segment_size, result);

        return result;
}

/*
 * Convert an XLogRecPtr to a "usable byte position".
 */
static uint64
XLogRecPtrToBytePos(XLogRecPtr ptr)
{
        uint64          fullsegs;
        uint32          fullpages;
        uint32          offset;
        uint64          result;

        XLByteToSeg(ptr, fullsegs, wal_segment_size);

        fullpages = (XLogSegmentOffset(ptr, wal_segment_size)) / XLOG_BLCKSZ;
        offset = ptr % XLOG_BLCKSZ;

        if (fullpages == 0)
        {
                result = fullsegs * UsableBytesInSegment;
                if (offset > 0)
                {
                        Assert(offset >= SizeOfXLogLongPHD);
                        result += offset - SizeOfXLogLongPHD;
                }
        }
        else
        {
                result = fullsegs * UsableBytesInSegment +
                        (XLOG_BLCKSZ - SizeOfXLogLongPHD) + /* account for first page */
                        (fullpages - 1) * UsableBytesInPage;    /* full pages */
                if (offset > 0)
                {
                        Assert(offset >= SizeOfXLogShortPHD);
                        result += offset - SizeOfXLogShortPHD;
                }
        }

        return result;
}

/*
 * Initialize XLOG buffers, writing out old buffers if they still contain
 * unwritten data, upto the page containing 'upto'. Or if 'opportunistic' is
 * true, initialize as many pages as we can without having to write out
 * unwritten data. Any new pages are initialized to zeros, with pages headers
 * initialized properly.
 */
static void
AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        int                     nextidx;
        XLogRecPtr      OldPageRqstPtr;
        XLogwrtRqst WriteRqst;
        XLogRecPtr      NewPageEndPtr = InvalidXLogRecPtr;
        XLogRecPtr      NewPageBeginPtr;
        XLogPageHeader NewPage;
        int                     npages = 0;

        LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);

        /*
         * Now that we have the lock, check if someone initialized the page
         * already.
         */
        while (upto >= XLogCtl->InitializedUpTo || opportunistic)
        {
                nextidx = XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);

                /*
                 * Get ending-offset of the buffer page we need to replace (this may
                 * be zero if the buffer hasn't been used yet).  Fall through if it's
                 * already written out.
                 */
                OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
                if (LogwrtResult.Write < OldPageRqstPtr)
                {
                        /*
                         * Nope, got work to do. If we just want to pre-initialize as much
                         * as we can without flushing, give up now.
                         */
                        if (opportunistic)
                                break;

                        /* Before waiting, get info_lck and update LogwrtResult */
                        SpinLockAcquire(&XLogCtl->info_lck);
                        if (XLogCtl->LogwrtRqst.Write < OldPageRqstPtr)
                                XLogCtl->LogwrtRqst.Write = OldPageRqstPtr;
                        LogwrtResult = XLogCtl->LogwrtResult;
                        SpinLockRelease(&XLogCtl->info_lck);

                        /*
                         * Now that we have an up-to-date LogwrtResult value, see if we
                         * still need to write it or if someone else already did.
                         */
                        if (LogwrtResult.Write < OldPageRqstPtr)
                        {
                                /*
                                 * Must acquire write lock. Release WALBufMappingLock first,
                                 * to make sure that all insertions that we need to wait for
                                 * can finish (up to this same position). Otherwise we risk
                                 * deadlock.
                                 */
                                LWLockRelease(WALBufMappingLock);

                                WaitXLogInsertionsToFinish(OldPageRqstPtr);

                                LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);

                                LogwrtResult = XLogCtl->LogwrtResult;
                                if (LogwrtResult.Write >= OldPageRqstPtr)
                                {
                                        /* OK, someone wrote it already */
                                        LWLockRelease(WALWriteLock);
                                }
                                else
                                {
                                        /* Have to write it ourselves */
                                        TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_START();
                                        WriteRqst.Write = OldPageRqstPtr;
                                        WriteRqst.Flush = 0;
                                        XLogWrite(WriteRqst, tli, false);
                                        LWLockRelease(WALWriteLock);
                                        WalStats.m_wal_buffers_full++;
                                        TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
                                }
                                /* Re-acquire WALBufMappingLock and retry */
                                LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
                                continue;
                        }
                }

                /*
                 * Now the next buffer slot is free and we can set it up to be the
                 * next output page.
                 */
                NewPageBeginPtr = XLogCtl->InitializedUpTo;
                NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;

                Assert(XLogRecPtrToBufIdx(NewPageBeginPtr) == nextidx);

                NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);

                /*
                 * Be sure to re-zero the buffer so that bytes beyond what we've
                 * written will look like zeroes and not valid XLOG records...
                 */
                MemSet((char *) NewPage, 0, XLOG_BLCKSZ);

                /*
                 * Fill the new page's header
                 */
                NewPage->xlp_magic = XLOG_PAGE_MAGIC;

                /* NewPage->xlp_info = 0; */    /* done by memset */
                NewPage->xlp_tli = tli;
                NewPage->xlp_pageaddr = NewPageBeginPtr;

                /* NewPage->xlp_rem_len = 0; */ /* done by memset */

                /*
                 * If online backup is not in progress, mark the header to indicate
                 * that WAL records beginning in this page have removable backup
                 * blocks.  This allows the WAL archiver to know whether it is safe to
                 * compress archived WAL data by transforming full-block records into
                 * the non-full-block format.  It is sufficient to record this at the
                 * page level because we force a page switch (in fact a segment
                 * switch) when starting a backup, so the flag will be off before any
                 * records can be written during the backup.  At the end of a backup,
                 * the last page will be marked as all unsafe when perhaps only part
                 * is unsafe, but at worst the archiver would miss the opportunity to
                 * compress a few records.
                 */
                if (!Insert->forcePageWrites)
                        NewPage->xlp_info |= XLP_BKP_REMOVABLE;

                /*
                 * If first page of an XLOG segment file, make it a long header.
                 */
                if ((XLogSegmentOffset(NewPage->xlp_pageaddr, wal_segment_size)) == 0)
                {
                        XLogLongPageHeader NewLongPage = (XLogLongPageHeader) NewPage;

                        NewLongPage->xlp_sysid = ControlFile->system_identifier;
                        NewLongPage->xlp_seg_size = wal_segment_size;
                        NewLongPage->xlp_xlog_blcksz = XLOG_BLCKSZ;
                        NewPage->xlp_info |= XLP_LONG_HEADER;
                }

                /*
                 * Make sure the initialization of the page becomes visible to others
                 * before the xlblocks update. GetXLogBuffer() reads xlblocks without
                 * holding a lock.
                 */
                pg_write_barrier();

                *((volatile XLogRecPtr *) &XLogCtl->xlblocks[nextidx]) = NewPageEndPtr;

                XLogCtl->InitializedUpTo = NewPageEndPtr;

                npages++;
        }
        LWLockRelease(WALBufMappingLock);

#ifdef WAL_DEBUG
        if (XLOG_DEBUG && npages > 0)
        {
                elog(DEBUG1, "initialized %d pages, up to %X/%X",
                         npages, LSN_FORMAT_ARGS(NewPageEndPtr));
        }
#endif
}

/*
 * Calculate CheckPointSegments based on max_wal_size_mb and
 * checkpoint_completion_target.
 */
static void
CalculateCheckpointSegments(void)
{
        double          target;

        /*-------
         * Calculate the distance at which to trigger a checkpoint, to avoid
         * exceeding max_wal_size_mb. This is based on two assumptions:
         *
         * a) we keep WAL for only one checkpoint cycle (prior to PG11 we kept
         *    WAL for two checkpoint cycles to allow us to recover from the
         *    secondary checkpoint if the first checkpoint failed, though we
         *    only did this on the primary anyway, not on standby. Keeping just
         *    one checkpoint simplifies processing and reduces disk space in
         *    many smaller databases.)
         * b) during checkpoint, we consume checkpoint_completion_target *
         *        number of segments consumed between checkpoints.
         *-------
         */
        target = (double) ConvertToXSegs(max_wal_size_mb, wal_segment_size) /
                (1.0 + CheckPointCompletionTarget);

        /* round down */
        CheckPointSegments = (int) target;

        if (CheckPointSegments < 1)
                CheckPointSegments = 1;
}

void
assign_max_wal_size(int newval, void *extra)
{
        max_wal_size_mb = newval;
        CalculateCheckpointSegments();
}

void
assign_checkpoint_completion_target(double newval, void *extra)
{
        CheckPointCompletionTarget = newval;
        CalculateCheckpointSegments();
}

/*
 * At a checkpoint, how many WAL segments to recycle as preallocated future
 * XLOG segments? Returns the highest segment that should be preallocated.
 */
static XLogSegNo
XLOGfileslop(XLogRecPtr lastredoptr)
{
        XLogSegNo       minSegNo;
        XLogSegNo       maxSegNo;
        double          distance;
        XLogSegNo       recycleSegNo;

        /*
         * Calculate the segment numbers that min_wal_size_mb and max_wal_size_mb
         * correspond to. Always recycle enough segments to meet the minimum, and
         * remove enough segments to stay below the maximum.
         */
        minSegNo = lastredoptr / wal_segment_size +
                ConvertToXSegs(min_wal_size_mb, wal_segment_size) - 1;
        maxSegNo = lastredoptr / wal_segment_size +
                ConvertToXSegs(max_wal_size_mb, wal_segment_size) - 1;

        /*
         * Between those limits, recycle enough segments to get us through to the
         * estimated end of next checkpoint.
         *
         * To estimate where the next checkpoint will finish, assume that the
         * system runs steadily consuming CheckPointDistanceEstimate bytes between
         * every checkpoint.
         */
        distance = (1.0 + CheckPointCompletionTarget) * CheckPointDistanceEstimate;
        /* add 10% for good measure. */
        distance *= 1.10;

        recycleSegNo = (XLogSegNo) ceil(((double) lastredoptr + distance) /
                                                                        wal_segment_size);

        if (recycleSegNo < minSegNo)
                recycleSegNo = minSegNo;
        if (recycleSegNo > maxSegNo)
                recycleSegNo = maxSegNo;

        return recycleSegNo;
}

/*
 * Check whether we've consumed enough xlog space that a checkpoint is needed.
 *
 * new_segno indicates a log file that has just been filled up (or read
 * during recovery). We measure the distance from RedoRecPtr to new_segno
 * and see if that exceeds CheckPointSegments.
 *
 * Note: it is caller's responsibility that RedoRecPtr is up-to-date.
 */
bool
XLogCheckpointNeeded(XLogSegNo new_segno)
{
        XLogSegNo       old_segno;

        XLByteToSeg(RedoRecPtr, old_segno, wal_segment_size);

        if (new_segno >= old_segno + (uint64) (CheckPointSegments - 1))
                return true;
        return false;
}

/*
 * Write and/or fsync the log at least as far as WriteRqst indicates.
 *
 * If flexible == true, we don't have to write as far as WriteRqst, but
 * may stop at any convenient boundary (such as a cache or logfile boundary).
 * This option allows us to avoid uselessly issuing multiple writes when a
 * single one would do.
 *
 * Must be called with WALWriteLock held. WaitXLogInsertionsToFinish(WriteRqst)
 * must be called before grabbing the lock, to make sure the data is ready to
 * write.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
{
        bool            ispartialpage;
        bool            last_iteration;
        bool            finishing_seg;
        int                     curridx;
        int                     npages;
        int                     startidx;
        uint32          startoffset;

        /* We should always be inside a critical section here */
        Assert(CritSectionCount > 0);

        /*
         * Update local LogwrtResult (caller probably did this already, but...)
         */
        LogwrtResult = XLogCtl->LogwrtResult;

        /*
         * Since successive pages in the xlog cache are consecutively allocated,
         * we can usually gather multiple pages together and issue just one
         * write() call.  npages is the number of pages we have determined can be
         * written together; startidx is the cache block index of the first one,
         * and startoffset is the file offset at which it should go. The latter
         * two variables are only valid when npages > 0, but we must initialize
         * all of them to keep the compiler quiet.
         */
        npages = 0;
        startidx = 0;
        startoffset = 0;

        /*
         * Within the loop, curridx is the cache block index of the page to
         * consider writing.  Begin at the buffer containing the next unwritten
         * page, or last partially written page.
         */
        curridx = XLogRecPtrToBufIdx(LogwrtResult.Write);

        while (LogwrtResult.Write < WriteRqst.Write)
        {
                /*
                 * Make sure we're not ahead of the insert process.  This could happen
                 * if we're passed a bogus WriteRqst.Write that is past the end of the
                 * last page that's been initialized by AdvanceXLInsertBuffer.
                 */
                XLogRecPtr      EndPtr = XLogCtl->xlblocks[curridx];

                if (LogwrtResult.Write >= EndPtr)
                        elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
                                 LSN_FORMAT_ARGS(LogwrtResult.Write),
                                 LSN_FORMAT_ARGS(EndPtr));

                /* Advance LogwrtResult.Write to end of current buffer page */
                LogwrtResult.Write = EndPtr;
                ispartialpage = WriteRqst.Write < LogwrtResult.Write;

                if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo,
                                                         wal_segment_size))
                {
                        /*
                         * Switch to new logfile segment.  We cannot have any pending
                         * pages here (since we dump what we have at segment end).
                         */
                        Assert(npages == 0);
                        if (openLogFile >= 0)
                                XLogFileClose();
                        XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo,
                                                        wal_segment_size);
                        openLogTLI = tli;

                        /* create/use new log file */
                        openLogFile = XLogFileInit(openLogSegNo, tli);
                        ReserveExternalFD();
                }

                /* Make sure we have the current logfile open */
                if (openLogFile < 0)
                {
                        XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo,
                                                        wal_segment_size);
                        openLogTLI = tli;
                        openLogFile = XLogFileOpen(openLogSegNo, tli);
                        ReserveExternalFD();
                }

                /* Add current page to the set of pending pages-to-dump */
                if (npages == 0)
                {
                        /* first of group */
                        startidx = curridx;
                        startoffset = XLogSegmentOffset(LogwrtResult.Write - XLOG_BLCKSZ,
                                                                                        wal_segment_size);
                }
                npages++;

                /*
                 * Dump the set if this will be the last loop iteration, or if we are
                 * at the last page of the cache area (since the next page won't be
                 * contiguous in memory), or if we are at the end of the logfile
                 * segment.
                 */
                last_iteration = WriteRqst.Write <= LogwrtResult.Write;

                finishing_seg = !ispartialpage &&
                        (startoffset + npages * XLOG_BLCKSZ) >= wal_segment_size;

                if (last_iteration ||
                        curridx == XLogCtl->XLogCacheBlck ||
                        finishing_seg)
                {
                        char       *from;
                        Size            nbytes;
                        Size            nleft;
                        int                     written;
                        instr_time      start;

                        /* OK to write the page(s) */
                        from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
                        nbytes = npages * (Size) XLOG_BLCKSZ;
                        nleft = nbytes;
                        do
                        {
                                errno = 0;

                                /* Measure I/O timing to write WAL data */
                                if (track_wal_io_timing)
                                        INSTR_TIME_SET_CURRENT(start);

                                pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
                                written = pg_pwrite(openLogFile, from, nleft, startoffset);
                                pgstat_report_wait_end();

                                /*
                                 * Increment the I/O timing and the number of times WAL data
                                 * were written out to disk.
                                 */
                                if (track_wal_io_timing)
                                {
                                        instr_time      duration;

                                        INSTR_TIME_SET_CURRENT(duration);
                                        INSTR_TIME_SUBTRACT(duration, start);
                                        WalStats.m_wal_write_time += INSTR_TIME_GET_MICROSEC(duration);
                                }

                                WalStats.m_wal_write++;

                                if (written <= 0)
                                {
                                        char            xlogfname[MAXFNAMELEN];
                                        int                     save_errno;

                                        if (errno == EINTR)
                                                continue;

                                        save_errno = errno;
                                        XLogFileName(xlogfname, tli, openLogSegNo,
                                                                 wal_segment_size);
                                        errno = save_errno;
                                        ereport(PANIC,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not write to log file %s "
                                                                        "at offset %u, length %zu: %m",
                                                                        xlogfname, startoffset, nleft)));
                                }
                                nleft -= written;
                                from += written;
                                startoffset += written;
                        } while (nleft > 0);

                        npages = 0;

                        /*
                         * If we just wrote the whole last page of a logfile segment,
                         * fsync the segment immediately.  This avoids having to go back
                         * and re-open prior segments when an fsync request comes along
                         * later. Doing it here ensures that one and only one backend will
                         * perform this fsync.
                         *
                         * This is also the right place to notify the Archiver that the
                         * segment is ready to copy to archival storage, and to update the
                         * timer for archive_timeout, and to signal for a checkpoint if
                         * too many logfile segments have been used since the last
                         * checkpoint.
                         */
                        if (finishing_seg)
                        {
                                issue_xlog_fsync(openLogFile, openLogSegNo, tli);

                                /* signal that we need to wakeup walsenders later */
                                WalSndWakeupRequest();

                                LogwrtResult.Flush = LogwrtResult.Write;        /* end of page */

                                if (XLogArchivingActive())
                                        XLogArchiveNotifySeg(openLogSegNo, tli);

                                XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);
                                XLogCtl->lastSegSwitchLSN = LogwrtResult.Flush;

                                /*
                                 * Request a checkpoint if we've consumed too much xlog since
                                 * the last one.  For speed, we first check using the local
                                 * copy of RedoRecPtr, which might be out of date; if it looks
                                 * like a checkpoint is needed, forcibly update RedoRecPtr and
                                 * recheck.
                                 */
                                if (IsUnderPostmaster && XLogCheckpointNeeded(openLogSegNo))
                                {
                                        (void) GetRedoRecPtr();
                                        if (XLogCheckpointNeeded(openLogSegNo))
                                                RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
                                }
                        }
                }

                if (ispartialpage)
                {
                        /* Only asked to write a partial page */
                        LogwrtResult.Write = WriteRqst.Write;
                        break;
                }
                curridx = NextBufIdx(curridx);

                /* If flexible, break out of loop as soon as we wrote something */
                if (flexible && npages == 0)
                        break;
        }

        Assert(npages == 0);

        /*
         * If asked to flush, do so
         */
        if (LogwrtResult.Flush < WriteRqst.Flush &&
                LogwrtResult.Flush < LogwrtResult.Write)
        {
                /*
                 * Could get here without iterating above loop, in which case we might
                 * have no open file or the wrong one.  However, we do not need to
                 * fsync more than one file.
                 */
                if (sync_method != SYNC_METHOD_OPEN &&
                        sync_method != SYNC_METHOD_OPEN_DSYNC)
                {
                        if (openLogFile >= 0 &&
                                !XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo,
                                                                 wal_segment_size))
                                XLogFileClose();
                        if (openLogFile < 0)
                        {
                                XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo,
                                                                wal_segment_size);
                                openLogTLI = tli;
                                openLogFile = XLogFileOpen(openLogSegNo, tli);
                                ReserveExternalFD();
                        }

                        issue_xlog_fsync(openLogFile, openLogSegNo, tli);
                }

                /* signal that we need to wakeup walsenders later */
                WalSndWakeupRequest();

                LogwrtResult.Flush = LogwrtResult.Write;
        }

        /*
         * Update shared-memory status
         *
         * We make sure that the shared 'request' values do not fall behind the
         * 'result' values.  This is not absolutely essential, but it saves some
         * code in a couple of places.
         */
        {
                SpinLockAcquire(&XLogCtl->info_lck);
                XLogCtl->LogwrtResult = LogwrtResult;
                if (XLogCtl->LogwrtRqst.Write < LogwrtResult.Write)
                        XLogCtl->LogwrtRqst.Write = LogwrtResult.Write;
                if (XLogCtl->LogwrtRqst.Flush < LogwrtResult.Flush)
                        XLogCtl->LogwrtRqst.Flush = LogwrtResult.Flush;
                SpinLockRelease(&XLogCtl->info_lck);
        }
}

/*
 * Record the LSN for an asynchronous transaction commit/abort
 * and nudge the WALWriter if there is work for it to do.
 * (This should not be called for synchronous commits.)
 */
void
XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
{
        XLogRecPtr      WriteRqstPtr = asyncXactLSN;
        bool            sleeping;

        SpinLockAcquire(&XLogCtl->info_lck);
        LogwrtResult = XLogCtl->LogwrtResult;
        sleeping = XLogCtl->WalWriterSleeping;
        if (XLogCtl->asyncXactLSN < asyncXactLSN)
                XLogCtl->asyncXactLSN = asyncXactLSN;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * If the WALWriter is sleeping, we should kick it to make it come out of
         * low-power mode.  Otherwise, determine whether there's a full page of
         * WAL available to write.
         */
        if (!sleeping)
        {
                /* back off to last completed page boundary */
                WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

                /* if we have already flushed that far, we're done */
                if (WriteRqstPtr <= LogwrtResult.Flush)
                        return;
        }

        /*
         * Nudge the WALWriter: it has a full page of WAL to write, or we want it
         * to come out of low-power mode so that this async commit will reach disk
         * within the expected amount of time.
         */
        if (ProcGlobal->walwriterLatch)
                SetLatch(ProcGlobal->walwriterLatch);
}

/*
 * Record the LSN up to which we can remove WAL because it's not required by
 * any replication slot.
 */
void
XLogSetReplicationSlotMinimumLSN(XLogRecPtr lsn)
{
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->replicationSlotMinLSN = lsn;
        SpinLockRelease(&XLogCtl->info_lck);
}


/*
 * Return the oldest LSN we must retain to satisfy the needs of some
 * replication slot.
 */
static XLogRecPtr
XLogGetReplicationSlotMinimumLSN(void)
{
        XLogRecPtr      retval;

        SpinLockAcquire(&XLogCtl->info_lck);
        retval = XLogCtl->replicationSlotMinLSN;
        SpinLockRelease(&XLogCtl->info_lck);

        return retval;
}

/*
 * Advance minRecoveryPoint in control file.
 *
 * If we crash during recovery, we must reach this point again before the
 * database is consistent.
 *
 * If 'force' is true, 'lsn' argument is ignored. Otherwise, minRecoveryPoint
 * is only updated if it's not already greater than or equal to 'lsn'.
 */
static void
UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force)
{
        /* Quick check using our local copy of the variable */
        if (!updateMinRecoveryPoint || (!force && lsn <= LocalMinRecoveryPoint))
                return;

        /*
         * An invalid minRecoveryPoint means that we need to recover all the WAL,
         * i.e., we're doing crash recovery.  We never modify the control file's
         * value in that case, so we can short-circuit future checks here too. The
         * local values of minRecoveryPoint and minRecoveryPointTLI should not be
         * updated until crash recovery finishes.  We only do this for the startup
         * process as it should not update its own reference of minRecoveryPoint
         * until it has finished crash recovery to make sure that all WAL
         * available is replayed in this case.  This also saves from extra locks
         * taken on the control file from the startup process.
         */
        if (XLogRecPtrIsInvalid(LocalMinRecoveryPoint) && InRecovery)
        {
                updateMinRecoveryPoint = false;
                return;
        }

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

        /* update local copy */
        LocalMinRecoveryPoint = ControlFile->minRecoveryPoint;
        LocalMinRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

        if (XLogRecPtrIsInvalid(LocalMinRecoveryPoint))
                updateMinRecoveryPoint = false;
        else if (force || LocalMinRecoveryPoint < lsn)
        {
                XLogRecPtr      newMinRecoveryPoint;
                TimeLineID      newMinRecoveryPointTLI;

                /*
                 * To avoid having to update the control file too often, we update it
                 * all the way to the last record being replayed, even though 'lsn'
                 * would suffice for correctness.  This also allows the 'force' case
                 * to not need a valid 'lsn' value.
                 *
                 * Another important reason for doing it this way is that the passed
                 * 'lsn' value could be bogus, i.e., past the end of available WAL, if
                 * the caller got it from a corrupted heap page.  Accepting such a
                 * value as the min recovery point would prevent us from coming up at
                 * all.  Instead, we just log a warning and continue with recovery.
                 * (See also the comments about corrupt LSNs in XLogFlush.)
                 */
                newMinRecoveryPoint = GetCurrentReplayRecPtr(&newMinRecoveryPointTLI);
                if (!force && newMinRecoveryPoint < lsn)
                        elog(WARNING,
                                 "xlog min recovery request %X/%X is past current point %X/%X",
                                 LSN_FORMAT_ARGS(lsn), LSN_FORMAT_ARGS(newMinRecoveryPoint));

                /* update control file */
                if (ControlFile->minRecoveryPoint < newMinRecoveryPoint)
                {
                        ControlFile->minRecoveryPoint = newMinRecoveryPoint;
                        ControlFile->minRecoveryPointTLI = newMinRecoveryPointTLI;
                        UpdateControlFile();
                        LocalMinRecoveryPoint = newMinRecoveryPoint;
                        LocalMinRecoveryPointTLI = newMinRecoveryPointTLI;

                        ereport(DEBUG2,
                                        (errmsg_internal("updated min recovery point to %X/%X on timeline %u",
                                                                         LSN_FORMAT_ARGS(newMinRecoveryPoint),
                                                                         newMinRecoveryPointTLI)));
                }
        }
        LWLockRelease(ControlFileLock);
}

/*
 * Ensure that all XLOG data through the given position is flushed to disk.
 *
 * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not
 * already held, and we try to avoid acquiring it if possible.
 */
void
XLogFlush(XLogRecPtr record)
{
        XLogRecPtr      WriteRqstPtr;
        XLogwrtRqst WriteRqst;
        TimeLineID      insertTLI = XLogCtl->InsertTimeLineID;

        /*
         * During REDO, we are reading not writing WAL.  Therefore, instead of
         * trying to flush the WAL, we should update minRecoveryPoint instead. We
         * test XLogInsertAllowed(), not InRecovery, because we need checkpointer
         * to act this way too, and because when it tries to write the
         * end-of-recovery checkpoint, it should indeed flush.
         */
        if (!XLogInsertAllowed())
        {
                UpdateMinRecoveryPoint(record, false);
                return;
        }

        /* Quick exit if already known flushed */
        if (record <= LogwrtResult.Flush)
                return;

#ifdef WAL_DEBUG
        if (XLOG_DEBUG)
                elog(LOG, "xlog flush request %X/%X; write %X/%X; flush %X/%X",
                         LSN_FORMAT_ARGS(record),
                         LSN_FORMAT_ARGS(LogwrtResult.Write),
                         LSN_FORMAT_ARGS(LogwrtResult.Flush));
#endif

        START_CRIT_SECTION();

        /*
         * Since fsync is usually a horribly expensive operation, we try to
         * piggyback as much data as we can on each fsync: if we see any more data
         * entered into the xlog buffer, we'll write and fsync that too, so that
         * the final value of LogwrtResult.Flush is as large as possible. This
         * gives us some chance of avoiding another fsync immediately after.
         */

        /* initialize to given target; may increase below */
        WriteRqstPtr = record;

        /*
         * Now wait until we get the write lock, or someone else does the flush
         * for us.
         */
        for (;;)
        {
                XLogRecPtr      insertpos;

                /* read LogwrtResult and update local state */
                SpinLockAcquire(&XLogCtl->info_lck);
                if (WriteRqstPtr < XLogCtl->LogwrtRqst.Write)
                        WriteRqstPtr = XLogCtl->LogwrtRqst.Write;
                LogwrtResult = XLogCtl->LogwrtResult;
                SpinLockRelease(&XLogCtl->info_lck);

                /* done already? */
                if (record <= LogwrtResult.Flush)
                        break;

                /*
                 * Before actually performing the write, wait for all in-flight
                 * insertions to the pages we're about to write to finish.
                 */
                insertpos = WaitXLogInsertionsToFinish(WriteRqstPtr);

                /*
                 * Try to get the write lock. If we can't get it immediately, wait
                 * until it's released, and recheck if we still need to do the flush
                 * or if the backend that held the lock did it for us already. This
                 * helps to maintain a good rate of group committing when the system
                 * is bottlenecked by the speed of fsyncing.
                 */
                if (!LWLockAcquireOrWait(WALWriteLock, LW_EXCLUSIVE))
                {
                        /*
                         * The lock is now free, but we didn't acquire it yet. Before we
                         * do, loop back to check if someone else flushed the record for
                         * us already.
                         */
                        continue;
                }

                /* Got the lock; recheck whether request is satisfied */
                LogwrtResult = XLogCtl->LogwrtResult;
                if (record <= LogwrtResult.Flush)
                {
                        LWLockRelease(WALWriteLock);
                        break;
                }

                /*
                 * Sleep before flush! By adding a delay here, we may give further
                 * backends the opportunity to join the backlog of group commit
                 * followers; this can significantly improve transaction throughput,
                 * at the risk of increasing transaction latency.
                 *
                 * We do not sleep if enableFsync is not turned on, nor if there are
                 * fewer than CommitSiblings other backends with active transactions.
                 */
                if (CommitDelay > 0 && enableFsync &&
                        MinimumActiveBackends(CommitSiblings))
                {
                        pg_usleep(CommitDelay);

                        /*
                         * Re-check how far we can now flush the WAL. It's generally not
                         * safe to call WaitXLogInsertionsToFinish while holding
                         * WALWriteLock, because an in-progress insertion might need to
                         * also grab WALWriteLock to make progress. But we know that all
                         * the insertions up to insertpos have already finished, because
                         * that's what the earlier WaitXLogInsertionsToFinish() returned.
                         * We're only calling it again to allow insertpos to be moved
                         * further forward, not to actually wait for anyone.
                         */
                        insertpos = WaitXLogInsertionsToFinish(insertpos);
                }

                /* try to write/flush later additions to XLOG as well */
                WriteRqst.Write = insertpos;
                WriteRqst.Flush = insertpos;

                XLogWrite(WriteRqst, insertTLI, false);

                LWLockRelease(WALWriteLock);
                /* done */
                break;
        }

        END_CRIT_SECTION();

        /* wake up walsenders now that we've released heavily contended locks */
        WalSndWakeupProcessRequests();

        /*
         * If we still haven't flushed to the request point then we have a
         * problem; most likely, the requested flush point is past end of XLOG.
         * This has been seen to occur when a disk page has a corrupted LSN.
         *
         * Formerly we treated this as a PANIC condition, but that hurts the
         * system's robustness rather than helping it: we do not want to take down
         * the whole system due to corruption on one data page.  In particular, if
         * the bad page is encountered again during recovery then we would be
         * unable to restart the database at all!  (This scenario actually
         * happened in the field several times with 7.1 releases.)      As of 8.4, bad
         * LSNs encountered during recovery are UpdateMinRecoveryPoint's problem;
         * the only time we can reach here during recovery is while flushing the
         * end-of-recovery checkpoint record, and we don't expect that to have a
         * bad LSN.
         *
         * Note that for calls from xact.c, the ERROR will be promoted to PANIC
         * since xact.c calls this routine inside a critical section.  However,
         * calls from bufmgr.c are not within critical sections and so we will not
         * force a restart for a bad LSN on a data page.
         */
        if (LogwrtResult.Flush < record)
                elog(ERROR,
                         "xlog flush request %X/%X is not satisfied --- flushed only to %X/%X",
                         LSN_FORMAT_ARGS(record),
                         LSN_FORMAT_ARGS(LogwrtResult.Flush));
}

/*
 * Write & flush xlog, but without specifying exactly where to.
 *
 * We normally write only completed blocks; but if there is nothing to do on
 * that basis, we check for unwritten async commits in the current incomplete
 * block, and write through the latest one of those.  Thus, if async commits
 * are not being used, we will write complete blocks only.
 *
 * If, based on the above, there's anything to write we do so immediately. But
 * to avoid calling fsync, fdatasync et. al. at a rate that'd impact
 * concurrent IO, we only flush WAL every wal_writer_delay ms, or if there's
 * more than wal_writer_flush_after unflushed blocks.
 *
 * We can guarantee that async commits reach disk after at most three
 * wal_writer_delay cycles. (When flushing complete blocks, we allow XLogWrite
 * to write "flexibly", meaning it can stop at the end of the buffer ring;
 * this makes a difference only with very high load or long wal_writer_delay,
 * but imposes one extra cycle for the worst case for async commits.)
 *
 * This routine is invoked periodically by the background walwriter process.
 *
 * Returns true if there was any work to do, even if we skipped flushing due
 * to wal_writer_delay/wal_writer_flush_after.
 */
bool
XLogBackgroundFlush(void)
{
        XLogwrtRqst WriteRqst;
        bool            flexible = true;
        static TimestampTz lastflush;
        TimestampTz now;
        int                     flushbytes;
        TimeLineID      insertTLI;

        /* XLOG doesn't need flushing during recovery */
        if (RecoveryInProgress())
                return false;

        /*
         * Since we're not in recovery, InsertTimeLineID is set and can't change,
         * so we can read it without a lock.
         */
        insertTLI = XLogCtl->InsertTimeLineID;

        /* read LogwrtResult and update local state */
        SpinLockAcquire(&XLogCtl->info_lck);
        LogwrtResult = XLogCtl->LogwrtResult;
        WriteRqst = XLogCtl->LogwrtRqst;
        SpinLockRelease(&XLogCtl->info_lck);

        /* back off to last completed page boundary */
        WriteRqst.Write -= WriteRqst.Write % XLOG_BLCKSZ;

        /* if we have already flushed that far, consider async commit records */
        if (WriteRqst.Write <= LogwrtResult.Flush)
        {
                SpinLockAcquire(&XLogCtl->info_lck);
                WriteRqst.Write = XLogCtl->asyncXactLSN;
                SpinLockRelease(&XLogCtl->info_lck);
                flexible = false;               /* ensure it all gets written */
        }

        /*
         * If already known flushed, we're done. Just need to check if we are
         * holding an open file handle to a logfile that's no longer in use,
         * preventing the file from being deleted.
         */
        if (WriteRqst.Write <= LogwrtResult.Flush)
        {
                if (openLogFile >= 0)
                {
                        if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo,
                                                                 wal_segment_size))
                        {
                                XLogFileClose();
                        }
                }
                return false;
        }

        /*
         * Determine how far to flush WAL, based on the wal_writer_delay and
         * wal_writer_flush_after GUCs.
         */
        now = GetCurrentTimestamp();
        flushbytes =
                WriteRqst.Write / XLOG_BLCKSZ - LogwrtResult.Flush / XLOG_BLCKSZ;

        if (WalWriterFlushAfter == 0 || lastflush == 0)
        {
                /* first call, or block based limits disabled */
                WriteRqst.Flush = WriteRqst.Write;
                lastflush = now;
        }
        else if (TimestampDifferenceExceeds(lastflush, now, WalWriterDelay))
        {
                /*
                 * Flush the writes at least every WalWriterDelay ms. This is
                 * important to bound the amount of time it takes for an asynchronous
                 * commit to hit disk.
                 */
                WriteRqst.Flush = WriteRqst.Write;
                lastflush = now;
        }
        else if (flushbytes >= WalWriterFlushAfter)
        {
                /* exceeded wal_writer_flush_after blocks, flush */
                WriteRqst.Flush = WriteRqst.Write;
                lastflush = now;
        }
        else
        {
                /* no flushing, this time round */
                WriteRqst.Flush = 0;
        }

#ifdef WAL_DEBUG
        if (XLOG_DEBUG)
                elog(LOG, "xlog bg flush request write %X/%X; flush: %X/%X, current is write %X/%X; flush %X/%X",
                         LSN_FORMAT_ARGS(WriteRqst.Write),
                         LSN_FORMAT_ARGS(WriteRqst.Flush),
                         LSN_FORMAT_ARGS(LogwrtResult.Write),
                         LSN_FORMAT_ARGS(LogwrtResult.Flush));
#endif

        START_CRIT_SECTION();

        /* now wait for any in-progress insertions to finish and get write lock */
        WaitXLogInsertionsToFinish(WriteRqst.Write);
        LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
        LogwrtResult = XLogCtl->LogwrtResult;
        if (WriteRqst.Write > LogwrtResult.Write ||
                WriteRqst.Flush > LogwrtResult.Flush)
        {
                XLogWrite(WriteRqst, insertTLI, flexible);
        }
        LWLockRelease(WALWriteLock);

        END_CRIT_SECTION();

        /* wake up walsenders now that we've released heavily contended locks */
        WalSndWakeupProcessRequests();

        /*
         * Great, done. To take some work off the critical path, try to initialize
         * as many of the no-longer-needed WAL buffers for future use as we can.
         */
        AdvanceXLInsertBuffer(InvalidXLogRecPtr, insertTLI, true);

        /*
         * If we determined that we need to write data, but somebody else
         * wrote/flushed already, it should be considered as being active, to
         * avoid hibernating too early.
         */
        return true;
}

/*
 * Test whether XLOG data has been flushed up to (at least) the given position.
 *
 * Returns true if a flush is still needed.  (It may be that someone else
 * is already in process of flushing that far, however.)
 */
bool
XLogNeedsFlush(XLogRecPtr record)
{
        /*
         * During recovery, we don't flush WAL but update minRecoveryPoint
         * instead. So "needs flush" is taken to mean whether minRecoveryPoint
         * would need to be updated.
         */
        if (RecoveryInProgress())
        {
                /*
                 * An invalid minRecoveryPoint means that we need to recover all the
                 * WAL, i.e., we're doing crash recovery.  We never modify the control
                 * file's value in that case, so we can short-circuit future checks
                 * here too.  This triggers a quick exit path for the startup process,
                 * which cannot update its local copy of minRecoveryPoint as long as
                 * it has not replayed all WAL available when doing crash recovery.
                 */
                if (XLogRecPtrIsInvalid(LocalMinRecoveryPoint) && InRecovery)
                        updateMinRecoveryPoint = false;

                /* Quick exit if already known to be updated or cannot be updated */
                if (record <= LocalMinRecoveryPoint || !updateMinRecoveryPoint)
                        return false;

                /*
                 * Update local copy of minRecoveryPoint. But if the lock is busy,
                 * just return a conservative guess.
                 */
                if (!LWLockConditionalAcquire(ControlFileLock, LW_SHARED))
                        return true;
                LocalMinRecoveryPoint = ControlFile->minRecoveryPoint;
                LocalMinRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
                LWLockRelease(ControlFileLock);

                /*
                 * Check minRecoveryPoint for any other process than the startup
                 * process doing crash recovery, which should not update the control
                 * file value if crash recovery is still running.
                 */
                if (XLogRecPtrIsInvalid(LocalMinRecoveryPoint))
                        updateMinRecoveryPoint = false;

                /* check again */
                if (record <= LocalMinRecoveryPoint || !updateMinRecoveryPoint)
                        return false;
                else
                        return true;
        }

        /* Quick exit if already known flushed */
        if (record <= LogwrtResult.Flush)
                return false;

        /* read LogwrtResult and update local state */
        SpinLockAcquire(&XLogCtl->info_lck);
        LogwrtResult = XLogCtl->LogwrtResult;
        SpinLockRelease(&XLogCtl->info_lck);

        /* check again */
        if (record <= LogwrtResult.Flush)
                return false;

        return true;
}

/*
 * Try to make a given XLOG file segment exist.
 *
 * logsegno: identify segment.
 *
 * *added: on return, true if this call raised the number of extant segments.
 *
 * path: on return, this char[MAXPGPATH] has the path to the logsegno file.
 *
 * Returns -1 or FD of opened file.  A -1 here is not an error; a caller
 * wanting an open segment should attempt to open "path", which usually will
 * succeed.  (This is weird, but it's efficient for the callers.)
 */
static int
XLogFileInitInternal(XLogSegNo logsegno, TimeLineID logtli,
                                         bool *added, char *path)
{
        char            tmppath[MAXPGPATH];
        PGAlignedXLogBlock zbuffer;
        XLogSegNo       installed_segno;
        XLogSegNo       max_segno;
        int                     fd;
        int                     save_errno;

        Assert(logtli != 0);

        XLogFilePath(path, logtli, logsegno, wal_segment_size);

        /*
         * Try to use existent file (checkpoint maker may have created it already)
         */
        *added = false;
        fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
        if (fd < 0)
        {
                if (errno != ENOENT)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not open file \"%s\": %m", path)));
        }
        else
                return fd;

        /*
         * Initialize an empty (all zeroes) segment.  NOTE: it is possible that
         * another process is doing the same thing.  If so, we will end up
         * pre-creating an extra log segment.  That seems OK, and better than
         * holding the lock throughout this lengthy process.
         */
        elog(DEBUG2, "creating and filling new WAL file");

        snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

        unlink(tmppath);

        /* do not use get_sync_bit() here --- want to fsync only at end of fill */
        fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m", tmppath)));

        memset(zbuffer.data, 0, XLOG_BLCKSZ);

        pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_WRITE);
        save_errno = 0;
        if (wal_init_zero)
        {
                struct iovec iov[PG_IOV_MAX];
                int                     blocks;

                /*
                 * Zero-fill the file.  With this setting, we do this the hard way to
                 * ensure that all the file space has really been allocated.  On
                 * platforms that allow "holes" in files, just seeking to the end
                 * doesn't allocate intermediate space.  This way, we know that we
                 * have all the space and (after the fsync below) that all the
                 * indirect blocks are down on disk.  Therefore, fdatasync(2) or
                 * O_DSYNC will be sufficient to sync future writes to the log file.
                 */

                /* Prepare to write out a lot of copies of our zero buffer at once. */
                for (int i = 0; i < lengthof(iov); ++i)
                {
                        iov[i].iov_base = zbuffer.data;
                        iov[i].iov_len = XLOG_BLCKSZ;
                }

                /* Loop, writing as many blocks as we can for each system call. */
                blocks = wal_segment_size / XLOG_BLCKSZ;
                for (int i = 0; i < blocks;)
                {
                        int                     iovcnt = Min(blocks - i, lengthof(iov));
                        off_t           offset = i * XLOG_BLCKSZ;

                        if (pg_pwritev_with_retry(fd, iov, iovcnt, offset) < 0)
                        {
                                save_errno = errno;
                                break;
                        }

                        i += iovcnt;
                }
        }
        else
        {
                /*
                 * Otherwise, seeking to the end and writing a solitary byte is
                 * enough.
                 */
                errno = 0;
                if (pg_pwrite(fd, zbuffer.data, 1, wal_segment_size - 1) != 1)
                {
                        /* if write didn't set errno, assume no disk space */
                        save_errno = errno ? errno : ENOSPC;
                }
        }
        pgstat_report_wait_end();

        if (save_errno)
        {
                /*
                 * If we fail to make the file, delete it to release disk space
                 */
                unlink(tmppath);

                close(fd);

                errno = save_errno;

                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write to file \"%s\": %m", tmppath)));
        }

        pgstat_report_wait_start(WAIT_EVENT_WAL_INIT_SYNC);
        if (pg_fsync(fd) != 0)
        {
                int                     save_errno = errno;

                close(fd);
                errno = save_errno;
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m", tmppath)));
        }
        pgstat_report_wait_end();

        if (close(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", tmppath)));

        /*
         * Now move the segment into place with its final name.  Cope with
         * possibility that someone else has created the file while we were
         * filling ours: if so, use ours to pre-create a future log segment.
         */
        installed_segno = logsegno;

        /*
         * XXX: What should we use as max_segno? We used to use XLOGfileslop when
         * that was a constant, but that was always a bit dubious: normally, at a
         * checkpoint, XLOGfileslop was the offset from the checkpoint record, but
         * here, it was the offset from the insert location. We can't do the
         * normal XLOGfileslop calculation here because we don't have access to
         * the prior checkpoint's redo location. So somewhat arbitrarily, just use
         * CheckPointSegments.
         */
        max_segno = logsegno + CheckPointSegments;
        if (InstallXLogFileSegment(&installed_segno, tmppath, true, max_segno,
                                                           logtli))
        {
                *added = true;
                elog(DEBUG2, "done creating and filling new WAL file");
        }
        else
        {
                /*
                 * No need for any more future segments, or InstallXLogFileSegment()
                 * failed to rename the file into place. If the rename failed, a
                 * caller opening the file may fail.
                 */
                unlink(tmppath);
                elog(DEBUG2, "abandoned new WAL file");
        }

        return -1;
}

/*
 * Create a new XLOG file segment, or open a pre-existing one.
 *
 * logsegno: identify segment to be created/opened.
 *
 * Returns FD of opened file.
 *
 * Note: errors here are ERROR not PANIC because we might or might not be
 * inside a critical section (eg, during checkpoint there is no reason to
 * take down the system on failure).  They will promote to PANIC if we are
 * in a critical section.
 */
int
XLogFileInit(XLogSegNo logsegno, TimeLineID logtli)
{
        bool            ignore_added;
        char            path[MAXPGPATH];
        int                     fd;

        Assert(logtli != 0);

        fd = XLogFileInitInternal(logsegno, logtli, &ignore_added, path);
        if (fd >= 0)
                return fd;

        /* Now open original target segment (might not be file I just made) */
        fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));
        return fd;
}

/*
 * Create a new XLOG file segment by copying a pre-existing one.
 *
 * destsegno: identify segment to be created.
 *
 * srcTLI, srcsegno: identify segment to be copied (could be from
 *              a different timeline)
 *
 * upto: how much of the source file to copy (the rest is filled with
 *              zeros)
 *
 * Currently this is only used during recovery, and so there are no locking
 * considerations.  But we should be just as tense as XLogFileInit to avoid
 * emplacing a bogus file.
 */
static void
XLogFileCopy(TimeLineID destTLI, XLogSegNo destsegno,
                         TimeLineID srcTLI, XLogSegNo srcsegno,
                         int upto)
{
        char            path[MAXPGPATH];
        char            tmppath[MAXPGPATH];
        PGAlignedXLogBlock buffer;
        int                     srcfd;
        int                     fd;
        int                     nbytes;

        /*
         * Open the source file
         */
        XLogFilePath(path, srcTLI, srcsegno, wal_segment_size);
        srcfd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
        if (srcfd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));

        /*
         * Copy into a temp file name.
         */
        snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

        unlink(tmppath);

        /* do not use get_sync_bit() here --- want to fsync only at end of fill */
        fd = OpenTransientFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m", tmppath)));

        /*
         * Do the data copying.
         */
        for (nbytes = 0; nbytes < wal_segment_size; nbytes += sizeof(buffer))
        {
                int                     nread;

                nread = upto - nbytes;

                /*
                 * The part that is not read from the source file is filled with
                 * zeros.
                 */
                if (nread < sizeof(buffer))
                        memset(buffer.data, 0, sizeof(buffer));

                if (nread > 0)
                {
                        int                     r;

                        if (nread > sizeof(buffer))
                                nread = sizeof(buffer);
                        pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_READ);
                        r = read(srcfd, buffer.data, nread);
                        if (r != nread)
                        {
                                if (r < 0)
                                        ereport(ERROR,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not read file \"%s\": %m",
                                                                        path)));
                                else
                                        ereport(ERROR,
                                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                                         errmsg("could not read file \"%s\": read %d of %zu",
                                                                        path, r, (Size) nread)));
                        }
                        pgstat_report_wait_end();
                }
                errno = 0;
                pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_WRITE);
                if ((int) write(fd, buffer.data, sizeof(buffer)) != (int) sizeof(buffer))
                {
                        int                     save_errno = errno;

                        /*
                         * If we fail to make the file, delete it to release disk space
                         */
                        unlink(tmppath);
                        /* if write didn't set errno, assume problem is no disk space */
                        errno = save_errno ? save_errno : ENOSPC;

                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not write to file \"%s\": %m", tmppath)));
                }
                pgstat_report_wait_end();
        }

        pgstat_report_wait_start(WAIT_EVENT_WAL_COPY_SYNC);
        if (pg_fsync(fd) != 0)
                ereport(data_sync_elevel(ERROR),
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m", tmppath)));
        pgstat_report_wait_end();

        if (CloseTransientFile(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", tmppath)));

        if (CloseTransientFile(srcfd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", path)));

        /*
         * Now move the segment into place with its final name.
         */
        if (!InstallXLogFileSegment(&destsegno, tmppath, false, 0, destTLI))
                elog(ERROR, "InstallXLogFileSegment should not have failed");
}

/*
 * Install a new XLOG segment file as a current or future log segment.
 *
 * This is used both to install a newly-created segment (which has a temp
 * filename while it's being created) and to recycle an old segment.
 *
 * *segno: identify segment to install as (or first possible target).
 * When find_free is true, this is modified on return to indicate the
 * actual installation location or last segment searched.
 *
 * tmppath: initial name of file to install.  It will be renamed into place.
 *
 * find_free: if true, install the new segment at the first empty segno
 * number at or after the passed numbers.  If false, install the new segment
 * exactly where specified, deleting any existing segment file there.
 *
 * max_segno: maximum segment number to install the new file as.  Fail if no
 * free slot is found between *segno and max_segno. (Ignored when find_free
 * is false.)
 *
 * tli: The timeline on which the new segment should be installed.
 *
 * Returns true if the file was installed successfully.  false indicates that
 * max_segno limit was exceeded, the startup process has disabled this
 * function for now, or an error occurred while renaming the file into place.
 */
static bool
InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
                                           bool find_free, XLogSegNo max_segno, TimeLineID tli)
{
        char            path[MAXPGPATH];
        struct stat stat_buf;

        Assert(tli != 0);

        XLogFilePath(path, tli, *segno, wal_segment_size);

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        if (!XLogCtl->InstallXLogFileSegmentActive)
        {
                LWLockRelease(ControlFileLock);
                return false;
        }

        if (!find_free)
        {
                /* Force installation: get rid of any pre-existing segment file */
                durable_unlink(path, DEBUG1);
        }
        else
        {
                /* Find a free slot to put it in */
                while (stat(path, &stat_buf) == 0)
                {
                        if ((*segno) >= max_segno)
                        {
                                /* Failed to find a free slot within specified range */
                                LWLockRelease(ControlFileLock);
                                return false;
                        }
                        (*segno)++;
                        XLogFilePath(path, tli, *segno, wal_segment_size);
                }
        }

        /*
         * Perform the rename using link if available, paranoidly trying to avoid
         * overwriting an existing file (there shouldn't be one).
         */
        if (durable_rename_excl(tmppath, path, LOG) != 0)
        {
                LWLockRelease(ControlFileLock);
                /* durable_rename_excl already emitted log message */
                return false;
        }

        LWLockRelease(ControlFileLock);

        return true;
}

/*
 * Open a pre-existing logfile segment for writing.
 */
int
XLogFileOpen(XLogSegNo segno, TimeLineID tli)
{
        char            path[MAXPGPATH];
        int                     fd;

        XLogFilePath(path, tli, segno, wal_segment_size);

        fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method));
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));

        return fd;
}

/*
 * Close the current logfile segment for writing.
 */
static void
XLogFileClose(void)
{
        Assert(openLogFile >= 0);

        /*
         * WAL segment files will not be re-read in normal operation, so we advise
         * the OS to release any cached pages.  But do not do so if WAL archiving
         * or streaming is active, because archiver and walsender process could
         * use the cache to read the WAL segment.
         */
#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
        if (!XLogIsNeeded())
                (void) posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
#endif

        if (close(openLogFile) != 0)
        {
                char            xlogfname[MAXFNAMELEN];
                int                     save_errno = errno;

                XLogFileName(xlogfname, openLogTLI, openLogSegNo, wal_segment_size);
                errno = save_errno;
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", xlogfname)));
        }

        openLogFile = -1;
        ReleaseExternalFD();
}

/*
 * Preallocate log files beyond the specified log endpoint.
 *
 * XXX this is currently extremely conservative, since it forces only one
 * future log segment to exist, and even that only if we are 75% done with
 * the current one.  This is only appropriate for very low-WAL-volume systems.
 * High-volume systems will be OK once they've built up a sufficient set of
 * recycled log segments, but the startup transient is likely to include
 * a lot of segment creations by foreground processes, which is not so good.
 *
 * XLogFileInitInternal() can ereport(ERROR).  All known causes indicate big
 * trouble; for example, a full filesystem is one cause.  The checkpoint WAL
 * and/or ControlFile updates already completed.  If a RequestCheckpoint()
 * initiated the present checkpoint and an ERROR ends this function, the
 * command that called RequestCheckpoint() fails.  That's not ideal, but it's
 * not worth contorting more functions to use caller-specified elevel values.
 * (With or without RequestCheckpoint(), an ERROR forestalls some inessential
 * reporting and resource reclamation.)
 */
static void
PreallocXlogFiles(XLogRecPtr endptr, TimeLineID tli)
{
        XLogSegNo       _logSegNo;
        int                     lf;
        bool            added;
        char            path[MAXPGPATH];
        uint64          offset;

        if (!XLogCtl->InstallXLogFileSegmentActive)
                return;                                 /* unlocked check says no */

        XLByteToPrevSeg(endptr, _logSegNo, wal_segment_size);
        offset = XLogSegmentOffset(endptr - 1, wal_segment_size);
        if (offset >= (uint32) (0.75 * wal_segment_size))
        {
                _logSegNo++;
                lf = XLogFileInitInternal(_logSegNo, tli, &added, path);
                if (lf >= 0)
                        close(lf);
                if (added)
                        CheckpointStats.ckpt_segs_added++;
        }
}

/*
 * Throws an error if the given log segment has already been removed or
 * recycled. The caller should only pass a segment that it knows to have
 * existed while the server has been running, as this function always
 * succeeds if no WAL segments have been removed since startup.
 * 'tli' is only used in the error message.
 *
 * Note: this function guarantees to keep errno unchanged on return.
 * This supports callers that use this to possibly deliver a better
 * error message about a missing file, while still being able to throw
 * a normal file-access error afterwards, if this does return.
 */
void
CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
{
        int                     save_errno = errno;
        XLogSegNo       lastRemovedSegNo;

        SpinLockAcquire(&XLogCtl->info_lck);
        lastRemovedSegNo = XLogCtl->lastRemovedSegNo;
        SpinLockRelease(&XLogCtl->info_lck);

        if (segno <= lastRemovedSegNo)
        {
                char            filename[MAXFNAMELEN];

                XLogFileName(filename, tli, segno, wal_segment_size);
                errno = save_errno;
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("requested WAL segment %s has already been removed",
                                                filename)));
        }
        errno = save_errno;
}

/*
 * Return the last WAL segment removed, or 0 if no segment has been removed
 * since startup.
 *
 * NB: the result can be out of date arbitrarily fast, the caller has to deal
 * with that.
 */
XLogSegNo
XLogGetLastRemovedSegno(void)
{
        XLogSegNo       lastRemovedSegNo;

        SpinLockAcquire(&XLogCtl->info_lck);
        lastRemovedSegNo = XLogCtl->lastRemovedSegNo;
        SpinLockRelease(&XLogCtl->info_lck);

        return lastRemovedSegNo;
}


/*
 * Update the last removed segno pointer in shared memory, to reflect that the
 * given XLOG file has been removed.
 */
static void
UpdateLastRemovedPtr(char *filename)
{
        uint32          tli;
        XLogSegNo       segno;

        XLogFromFileName(filename, &tli, &segno, wal_segment_size);

        SpinLockAcquire(&XLogCtl->info_lck);
        if (segno > XLogCtl->lastRemovedSegNo)
                XLogCtl->lastRemovedSegNo = segno;
        SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * Remove all temporary log files in pg_wal
 *
 * This is called at the beginning of recovery after a previous crash,
 * at a point where no other processes write fresh WAL data.
 */
static void
RemoveTempXlogFiles(void)
{
        DIR                *xldir;
        struct dirent *xlde;

        elog(DEBUG2, "removing all temporary WAL segments");

        xldir = AllocateDir(XLOGDIR);
        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                char            path[MAXPGPATH];

                if (strncmp(xlde->d_name, "xlogtemp.", 9) != 0)
                        continue;

                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);
                unlink(path);
                elog(DEBUG2, "removed temporary WAL segment \"%s\"", path);
        }
        FreeDir(xldir);
}

/*
 * Recycle or remove all log files older or equal to passed segno.
 *
 * endptr is current (or recent) end of xlog, and lastredoptr is the
 * redo pointer of the last checkpoint. These are used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 *
 * insertTLI is the current timeline for XLOG insertion. Any recycled
 * segments should be reused for this timeline.
 */
static void
RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr lastredoptr, XLogRecPtr endptr,
                                   TimeLineID insertTLI)
{
        DIR                *xldir;
        struct dirent *xlde;
        char            lastoff[MAXFNAMELEN];
        XLogSegNo       endlogSegNo;
        XLogSegNo       recycleSegNo;

        /* Initialize info about where to try to recycle to */
        XLByteToSeg(endptr, endlogSegNo, wal_segment_size);
        recycleSegNo = XLOGfileslop(lastredoptr);

        /*
         * Construct a filename of the last segment to be kept. The timeline ID
         * doesn't matter, we ignore that in the comparison. (During recovery,
         * InsertTimeLineID isn't set, so we can't use that.)
         */
        XLogFileName(lastoff, 0, segno, wal_segment_size);

        elog(DEBUG2, "attempting to remove WAL segments older than log file %s",
                 lastoff);

        xldir = AllocateDir(XLOGDIR);

        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                /* Ignore files that are not XLOG segments */
                if (!IsXLogFileName(xlde->d_name) &&
                        !IsPartialXLogFileName(xlde->d_name))
                        continue;

                /*
                 * We ignore the timeline part of the XLOG segment identifiers in
                 * deciding whether a segment is still needed.  This ensures that we
                 * won't prematurely remove a segment from a parent timeline. We could
                 * probably be a little more proactive about removing segments of
                 * non-parent timelines, but that would be a whole lot more
                 * complicated.
                 *
                 * We use the alphanumeric sorting property of the filenames to decide
                 * which ones are earlier than the lastoff segment.
                 */
                if (strcmp(xlde->d_name + 8, lastoff + 8) <= 0)
                {
                        if (XLogArchiveCheckDone(xlde->d_name))
                        {
                                /* Update the last removed location in shared memory first */
                                UpdateLastRemovedPtr(xlde->d_name);

                                RemoveXlogFile(xlde->d_name, recycleSegNo, &endlogSegNo,
                                                           insertTLI);
                        }
                }
        }

        FreeDir(xldir);
}

/*
 * Remove WAL files that are not part of the given timeline's history.
 *
 * This is called during recovery, whenever we switch to follow a new
 * timeline, and at the end of recovery when we create a new timeline. We
 * wouldn't otherwise care about extra WAL files lying in pg_wal, but they
 * might be leftover pre-allocated or recycled WAL segments on the old timeline
 * that we haven't used yet, and contain garbage. If we just leave them in
 * pg_wal, they will eventually be archived, and we can't let that happen.
 * Files that belong to our timeline history are valid, because we have
 * successfully replayed them, but from others we can't be sure.
 *
 * 'switchpoint' is the current point in WAL where we switch to new timeline,
 * and 'newTLI' is the new timeline we switch to.
 */
void
RemoveNonParentXlogFiles(XLogRecPtr switchpoint, TimeLineID newTLI)
{
        DIR                *xldir;
        struct dirent *xlde;
        char            switchseg[MAXFNAMELEN];
        XLogSegNo       endLogSegNo;
        XLogSegNo       switchLogSegNo;
        XLogSegNo       recycleSegNo;

        /*
         * Initialize info about where to begin the work.  This will recycle,
         * somewhat arbitrarily, 10 future segments.
         */
        XLByteToPrevSeg(switchpoint, switchLogSegNo, wal_segment_size);
        XLByteToSeg(switchpoint, endLogSegNo, wal_segment_size);
        recycleSegNo = endLogSegNo + 10;

        /*
         * Construct a filename of the last segment to be kept.
         */
        XLogFileName(switchseg, newTLI, switchLogSegNo, wal_segment_size);

        elog(DEBUG2, "attempting to remove WAL segments newer than log file %s",
                 switchseg);

        xldir = AllocateDir(XLOGDIR);

        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                /* Ignore files that are not XLOG segments */
                if (!IsXLogFileName(xlde->d_name))
                        continue;

                /*
                 * Remove files that are on a timeline older than the new one we're
                 * switching to, but with a segment number >= the first segment on the
                 * new timeline.
                 */
                if (strncmp(xlde->d_name, switchseg, 8) < 0 &&
                        strcmp(xlde->d_name + 8, switchseg + 8) > 0)
                {
                        /*
                         * If the file has already been marked as .ready, however, don't
                         * remove it yet. It should be OK to remove it - files that are
                         * not part of our timeline history are not required for recovery
                         * - but seems safer to let them be archived and removed later.
                         */
                        if (!XLogArchiveIsReady(xlde->d_name))
                                RemoveXlogFile(xlde->d_name, recycleSegNo, &endLogSegNo,
                                                           newTLI);
                }
        }

        FreeDir(xldir);
}

/*
 * Recycle or remove a log file that's no longer needed.
 *
 * segname is the name of the segment to recycle or remove.  recycleSegNo
 * is the segment number to recycle up to.  endlogSegNo is the segment
 * number of the current (or recent) end of WAL.
 *
 * endlogSegNo gets incremented if the segment is recycled so as it is not
 * checked again with future callers of this function.
 *
 * insertTLI is the current timeline for XLOG insertion. Any recycled segments
 * should be used for this timeline.
 */
static void
RemoveXlogFile(const char *segname, XLogSegNo recycleSegNo,
                           XLogSegNo *endlogSegNo, TimeLineID insertTLI)
{
        char            path[MAXPGPATH];
#ifdef WIN32
        char            newpath[MAXPGPATH];
#endif
        struct stat statbuf;

        snprintf(path, MAXPGPATH, XLOGDIR "/%s", segname);

        /*
         * Before deleting the file, see if it can be recycled as a future log
         * segment. Only recycle normal files, because we don't want to recycle
         * symbolic links pointing to a separate archive directory.
         */
        if (wal_recycle &&
                *endlogSegNo <= recycleSegNo &&
                XLogCtl->InstallXLogFileSegmentActive &&        /* callee rechecks this */
                lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
                InstallXLogFileSegment(endlogSegNo, path,
                                                           true, recycleSegNo, insertTLI))
        {
                ereport(DEBUG2,
                                (errmsg_internal("recycled write-ahead log file \"%s\"",
                                                                 segname)));
                CheckpointStats.ckpt_segs_recycled++;
                /* Needn't recheck that slot on future iterations */
                (*endlogSegNo)++;
        }
        else
        {
                /* No need for any more future segments, or recycling failed ... */
                int                     rc;

                ereport(DEBUG2,
                                (errmsg_internal("removing write-ahead log file \"%s\"",
                                                                 segname)));

#ifdef WIN32

                /*
                 * On Windows, if another process (e.g another backend) holds the file
                 * open in FILE_SHARE_DELETE mode, unlink will succeed, but the file
                 * will still show up in directory listing until the last handle is
                 * closed. To avoid confusing the lingering deleted file for a live
                 * WAL file that needs to be archived, rename it before deleting it.
                 *
                 * If another process holds the file open without FILE_SHARE_DELETE
                 * flag, rename will fail. We'll try again at the next checkpoint.
                 */
                snprintf(newpath, MAXPGPATH, "%s.deleted", path);
                if (rename(path, newpath) != 0)
                {
                        ereport(LOG,
                                        (errcode_for_file_access(),
                                         errmsg("could not rename file \"%s\": %m",
                                                        path)));
                        return;
                }
                rc = durable_unlink(newpath, LOG);
#else
                rc = durable_unlink(path, LOG);
#endif
                if (rc != 0)
                {
                        /* Message already logged by durable_unlink() */
                        return;
                }
                CheckpointStats.ckpt_segs_removed++;
        }

        XLogArchiveCleanup(segname);
}

/*
 * Verify whether pg_wal and pg_wal/archive_status exist.
 * If the latter does not exist, recreate it.
 *
 * It is not the goal of this function to verify the contents of these
 * directories, but to help in cases where someone has performed a cluster
 * copy for PITR purposes but omitted pg_wal from the copy.
 *
 * We could also recreate pg_wal if it doesn't exist, but a deliberate
 * policy decision was made not to.  It is fairly common for pg_wal to be
 * a symlink, and if that was the DBA's intent then automatically making a
 * plain directory would result in degraded performance with no notice.
 */
static void
ValidateXLOGDirectoryStructure(void)
{
        char            path[MAXPGPATH];
        struct stat stat_buf;

        /* Check for pg_wal; if it doesn't exist, error out */
        if (stat(XLOGDIR, &stat_buf) != 0 ||
                !S_ISDIR(stat_buf.st_mode))
                ereport(FATAL,
                                (errmsg("required WAL directory \"%s\" does not exist",
                                                XLOGDIR)));

        /* Check for archive_status */
        snprintf(path, MAXPGPATH, XLOGDIR "/archive_status");
        if (stat(path, &stat_buf) == 0)
        {
                /* Check for weird cases where it exists but isn't a directory */
                if (!S_ISDIR(stat_buf.st_mode))
                        ereport(FATAL,
                                        (errmsg("required WAL directory \"%s\" does not exist",
                                                        path)));
        }
        else
        {
                ereport(LOG,
                                (errmsg("creating missing WAL directory \"%s\"", path)));
                if (MakePGDirectory(path) < 0)
                        ereport(FATAL,
                                        (errmsg("could not create missing directory \"%s\": %m",
                                                        path)));
        }
}

/*
 * Remove previous backup history files.  This also retries creation of
 * .ready files for any backup history files for which XLogArchiveNotify
 * failed earlier.
 */
static void
CleanupBackupHistory(void)
{
        DIR                *xldir;
        struct dirent *xlde;
        char            path[MAXPGPATH + sizeof(XLOGDIR)];

        xldir = AllocateDir(XLOGDIR);

        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                if (IsBackupHistoryFileName(xlde->d_name))
                {
                        if (XLogArchiveCheckDone(xlde->d_name))
                        {
                                elog(DEBUG2, "removing WAL backup history file \"%s\"",
                                         xlde->d_name);
                                snprintf(path, sizeof(path), XLOGDIR "/%s", xlde->d_name);
                                unlink(path);
                                XLogArchiveCleanup(xlde->d_name);
                        }
                }
        }

        FreeDir(xldir);
}

/*
 * I/O routines for pg_control
 *
 * *ControlFile is a buffer in shared memory that holds an image of the
 * contents of pg_control.  WriteControlFile() initializes pg_control
 * given a preloaded buffer, ReadControlFile() loads the buffer from
 * the pg_control file (during postmaster or standalone-backend startup),
 * and UpdateControlFile() rewrites pg_control after we modify xlog state.
 * InitControlFile() fills the buffer with initial values.
 *
 * For simplicity, WriteControlFile() initializes the fields of pg_control
 * that are related to checking backend/database compatibility, and
 * ReadControlFile() verifies they are correct.  We could split out the
 * I/O and compatibility-check functions, but there seems no need currently.
 */

static void
InitControlFile(uint64 sysidentifier)
{
        char            mock_auth_nonce[MOCK_AUTH_NONCE_LEN];

        /*
         * Generate a random nonce. This is used for authentication requests that
         * will fail because the user does not exist. The nonce is used to create
         * a genuine-looking password challenge for the non-existent user, in lieu
         * of an actual stored password.
         */
        if (!pg_strong_random(mock_auth_nonce, MOCK_AUTH_NONCE_LEN))
                ereport(PANIC,
                                (errcode(ERRCODE_INTERNAL_ERROR),
                                 errmsg("could not generate secret authorization token")));

        memset(ControlFile, 0, sizeof(ControlFileData));
        /* Initialize pg_control status fields */
        ControlFile->system_identifier = sysidentifier;
        memcpy(ControlFile->mock_authentication_nonce, mock_auth_nonce, MOCK_AUTH_NONCE_LEN);
        ControlFile->state = DB_SHUTDOWNED;
        ControlFile->unloggedLSN = FirstNormalUnloggedLSN;

        /* Set important parameter values for use when replaying WAL */
        ControlFile->MaxConnections = MaxConnections;
        ControlFile->max_worker_processes = max_worker_processes;
        ControlFile->max_wal_senders = max_wal_senders;
        ControlFile->max_prepared_xacts = max_prepared_xacts;
        ControlFile->max_locks_per_xact = max_locks_per_xact;
        ControlFile->wal_level = wal_level;
        ControlFile->wal_log_hints = wal_log_hints;
        ControlFile->track_commit_timestamp = track_commit_timestamp;
        ControlFile->data_checksum_version = bootstrap_data_checksum_version;
}

static void
WriteControlFile(void)
{
        int                     fd;
        char            buffer[PG_CONTROL_FILE_SIZE];   /* need not be aligned */

        /*
         * Ensure that the size of the pg_control data structure is sane.  See the
         * comments for these symbols in pg_control.h.
         */
        StaticAssertStmt(sizeof(ControlFileData) <= PG_CONTROL_MAX_SAFE_SIZE,
                                         "pg_control is too large for atomic disk writes");
        StaticAssertStmt(sizeof(ControlFileData) <= PG_CONTROL_FILE_SIZE,
                                         "sizeof(ControlFileData) exceeds PG_CONTROL_FILE_SIZE");

        /*
         * Initialize version and compatibility-check fields
         */
        ControlFile->pg_control_version = PG_CONTROL_VERSION;
        ControlFile->catalog_version_no = CATALOG_VERSION_NO;

        ControlFile->maxAlign = MAXIMUM_ALIGNOF;
        ControlFile->floatFormat = FLOATFORMAT_VALUE;

        ControlFile->blcksz = BLCKSZ;
        ControlFile->relseg_size = RELSEG_SIZE;
        ControlFile->xlog_blcksz = XLOG_BLCKSZ;
        ControlFile->xlog_seg_size = wal_segment_size;

        ControlFile->nameDataLen = NAMEDATALEN;
        ControlFile->indexMaxKeys = INDEX_MAX_KEYS;

        ControlFile->toast_max_chunk_size = TOAST_MAX_CHUNK_SIZE;
        ControlFile->loblksize = LOBLKSIZE;

        ControlFile->float8ByVal = FLOAT8PASSBYVAL;

        /* Contents are protected with a CRC */
        INIT_CRC32C(ControlFile->crc);
        COMP_CRC32C(ControlFile->crc,
                                (char *) ControlFile,
                                offsetof(ControlFileData, crc));
        FIN_CRC32C(ControlFile->crc);

        /*
         * We write out PG_CONTROL_FILE_SIZE bytes into pg_control, zero-padding
         * the excess over sizeof(ControlFileData).  This reduces the odds of
         * premature-EOF errors when reading pg_control.  We'll still fail when we
         * check the contents of the file, but hopefully with a more specific
         * error than "couldn't read pg_control".
         */
        memset(buffer, 0, PG_CONTROL_FILE_SIZE);
        memcpy(buffer, ControlFile, sizeof(ControlFileData));

        fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                           O_RDWR | O_CREAT | O_EXCL | PG_BINARY);
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));

        errno = 0;
        pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_WRITE);
        if (write(fd, buffer, PG_CONTROL_FILE_SIZE) != PG_CONTROL_FILE_SIZE)
        {
                /* if write didn't set errno, assume problem is no disk space */
                if (errno == 0)
                        errno = ENOSPC;
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not write to file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));
        }
        pgstat_report_wait_end();

        pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_SYNC);
        if (pg_fsync(fd) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));
        pgstat_report_wait_end();

        if (close(fd) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));
}

static void
ReadControlFile(void)
{
        pg_crc32c       crc;
        int                     fd;
        static char wal_segsz_str[20];
        int                     r;

        /*
         * Read data...
         */
        fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                           O_RDWR | PG_BINARY);
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));

        pgstat_report_wait_start(WAIT_EVENT_CONTROL_FILE_READ);
        r = read(fd, ControlFile, sizeof(ControlFileData));
        if (r != sizeof(ControlFileData))
        {
                if (r < 0)
                        ereport(PANIC,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m",
                                                        XLOG_CONTROL_FILE)));
                else
                        ereport(PANIC,
                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                         errmsg("could not read file \"%s\": read %d of %zu",
                                                        XLOG_CONTROL_FILE, r, sizeof(ControlFileData))));
        }
        pgstat_report_wait_end();

        close(fd);

        /*
         * Check for expected pg_control format version.  If this is wrong, the
         * CRC check will likely fail because we'll be checking the wrong number
         * of bytes.  Complaining about wrong version will probably be more
         * enlightening than complaining about wrong CRC.
         */

        if (ControlFile->pg_control_version != PG_CONTROL_VERSION && ControlFile->pg_control_version % 65536 == 0 && ControlFile->pg_control_version / 65536 != 0)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d (0x%08x),"
                                                   " but the server was compiled with PG_CONTROL_VERSION %d (0x%08x).",
                                                   ControlFile->pg_control_version, ControlFile->pg_control_version,
                                                   PG_CONTROL_VERSION, PG_CONTROL_VERSION),
                                 errhint("This could be a problem of mismatched byte ordering.  It looks like you need to initdb.")));

        if (ControlFile->pg_control_version != PG_CONTROL_VERSION)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d,"
                                                   " but the server was compiled with PG_CONTROL_VERSION %d.",
                                                   ControlFile->pg_control_version, PG_CONTROL_VERSION),
                                 errhint("It looks like you need to initdb.")));

        /* Now check the CRC. */
        INIT_CRC32C(crc);
        COMP_CRC32C(crc,
                                (char *) ControlFile,
                                offsetof(ControlFileData, crc));
        FIN_CRC32C(crc);

        if (!EQ_CRC32C(crc, ControlFile->crc))
                ereport(FATAL,
                                (errmsg("incorrect checksum in control file")));

        /*
         * Do compatibility checking immediately.  If the database isn't
         * compatible with the backend executable, we want to abort before we can
         * possibly do any damage.
         */
        if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with CATALOG_VERSION_NO %d,"
                                                   " but the server was compiled with CATALOG_VERSION_NO %d.",
                                                   ControlFile->catalog_version_no, CATALOG_VERSION_NO),
                                 errhint("It looks like you need to initdb.")));
        if (ControlFile->maxAlign != MAXIMUM_ALIGNOF)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with MAXALIGN %d,"
                                                   " but the server was compiled with MAXALIGN %d.",
                                                   ControlFile->maxAlign, MAXIMUM_ALIGNOF),
                                 errhint("It looks like you need to initdb.")));
        if (ControlFile->floatFormat != FLOATFORMAT_VALUE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster appears to use a different floating-point number format than the server executable."),
                                 errhint("It looks like you need to initdb.")));
        if (ControlFile->blcksz != BLCKSZ)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with BLCKSZ %d,"
                                                   " but the server was compiled with BLCKSZ %d.",
                                                   ControlFile->blcksz, BLCKSZ),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->relseg_size != RELSEG_SIZE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with RELSEG_SIZE %d,"
                                                   " but the server was compiled with RELSEG_SIZE %d.",
                                                   ControlFile->relseg_size, RELSEG_SIZE),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->xlog_blcksz != XLOG_BLCKSZ)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with XLOG_BLCKSZ %d,"
                                                   " but the server was compiled with XLOG_BLCKSZ %d.",
                                                   ControlFile->xlog_blcksz, XLOG_BLCKSZ),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->nameDataLen != NAMEDATALEN)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with NAMEDATALEN %d,"
                                                   " but the server was compiled with NAMEDATALEN %d.",
                                                   ControlFile->nameDataLen, NAMEDATALEN),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->indexMaxKeys != INDEX_MAX_KEYS)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with INDEX_MAX_KEYS %d,"
                                                   " but the server was compiled with INDEX_MAX_KEYS %d.",
                                                   ControlFile->indexMaxKeys, INDEX_MAX_KEYS),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->toast_max_chunk_size != TOAST_MAX_CHUNK_SIZE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with TOAST_MAX_CHUNK_SIZE %d,"
                                                   " but the server was compiled with TOAST_MAX_CHUNK_SIZE %d.",
                                                   ControlFile->toast_max_chunk_size, (int) TOAST_MAX_CHUNK_SIZE),
                                 errhint("It looks like you need to recompile or initdb.")));
        if (ControlFile->loblksize != LOBLKSIZE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with LOBLKSIZE %d,"
                                                   " but the server was compiled with LOBLKSIZE %d.",
                                                   ControlFile->loblksize, (int) LOBLKSIZE),
                                 errhint("It looks like you need to recompile or initdb.")));

#ifdef USE_FLOAT8_BYVAL
        if (ControlFile->float8ByVal != true)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized without USE_FLOAT8_BYVAL"
                                                   " but the server was compiled with USE_FLOAT8_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#else
        if (ControlFile->float8ByVal != false)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with USE_FLOAT8_BYVAL"
                                                   " but the server was compiled without USE_FLOAT8_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#endif

        wal_segment_size = ControlFile->xlog_seg_size;

        if (!IsValidWalSegSize(wal_segment_size))
                ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                errmsg_plural("WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d byte",
                                                                          "WAL segment size must be a power of two between 1 MB and 1 GB, but the control file specifies %d bytes",
                                                                          wal_segment_size,
                                                                          wal_segment_size)));

        snprintf(wal_segsz_str, sizeof(wal_segsz_str), "%d", wal_segment_size);
        SetConfigOption("wal_segment_size", wal_segsz_str, PGC_INTERNAL,
                                        PGC_S_OVERRIDE);

        /* check and update variables dependent on wal_segment_size */
        if (ConvertToXSegs(min_wal_size_mb, wal_segment_size) < 2)
                ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                errmsg("\"min_wal_size\" must be at least twice \"wal_segment_size\"")));

        if (ConvertToXSegs(max_wal_size_mb, wal_segment_size) < 2)
                ereport(ERROR, (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                                errmsg("\"max_wal_size\" must be at least twice \"wal_segment_size\"")));

        UsableBytesInSegment =
                (wal_segment_size / XLOG_BLCKSZ * UsableBytesInPage) -
                (SizeOfXLogLongPHD - SizeOfXLogShortPHD);

        CalculateCheckpointSegments();

        /* Make the initdb settings visible as GUC variables, too */
        SetConfigOption("data_checksums", DataChecksumsEnabled() ? "yes" : "no",
                                        PGC_INTERNAL, PGC_S_OVERRIDE);
}

/*
 * Utility wrapper to update the control file.  Note that the control
 * file gets flushed.
 */
static void
UpdateControlFile(void)
{
        update_controlfile(DataDir, ControlFile, true);
}

/*
 * Returns the unique system identifier from control file.
 */
uint64
GetSystemIdentifier(void)
{
        Assert(ControlFile != NULL);
        return ControlFile->system_identifier;
}

/*
 * Returns the random nonce from control file.
 */
char *
GetMockAuthenticationNonce(void)
{
        Assert(ControlFile != NULL);
        return ControlFile->mock_authentication_nonce;
}

/*
 * Are checksums enabled for data pages?
 */
bool
DataChecksumsEnabled(void)
{
        Assert(ControlFile != NULL);
        return (ControlFile->data_checksum_version > 0);
}

/*
 * Returns a fake LSN for unlogged relations.
 *
 * Each call generates an LSN that is greater than any previous value
 * returned. The current counter value is saved and restored across clean
 * shutdowns, but like unlogged relations, does not survive a crash. This can
 * be used in lieu of real LSN values returned by XLogInsert, if you need an
 * LSN-like increasing sequence of numbers without writing any WAL.
 */
XLogRecPtr
GetFakeLSNForUnloggedRel(void)
{
        XLogRecPtr      nextUnloggedLSN;

        /* increment the unloggedLSN counter, need SpinLock */
        SpinLockAcquire(&XLogCtl->ulsn_lck);
        nextUnloggedLSN = XLogCtl->unloggedLSN++;
        SpinLockRelease(&XLogCtl->ulsn_lck);

        return nextUnloggedLSN;
}

/*
 * Auto-tune the number of XLOG buffers.
 *
 * The preferred setting for wal_buffers is about 3% of shared_buffers, with
 * a maximum of one XLOG segment (there is little reason to think that more
 * is helpful, at least so long as we force an fsync when switching log files)
 * and a minimum of 8 blocks (which was the default value prior to PostgreSQL
 * 9.1, when auto-tuning was added).
 *
 * This should not be called until NBuffers has received its final value.
 */
static int
XLOGChooseNumBuffers(void)
{
        int                     xbuffers;

        xbuffers = NBuffers / 32;
        if (xbuffers > (wal_segment_size / XLOG_BLCKSZ))
                xbuffers = (wal_segment_size / XLOG_BLCKSZ);
        if (xbuffers < 8)
                xbuffers = 8;
        return xbuffers;
}

/*
 * GUC check_hook for wal_buffers
 */
bool
check_wal_buffers(int *newval, void **extra, GucSource source)
{
        /*
         * -1 indicates a request for auto-tune.
         */
        if (*newval == -1)
        {
                /*
                 * If we haven't yet changed the boot_val default of -1, just let it
                 * be.  We'll fix it when XLOGShmemSize is called.
                 */
                if (XLOGbuffers == -1)
                        return true;

                /* Otherwise, substitute the auto-tune value */
                *newval = XLOGChooseNumBuffers();
        }

        /*
         * We clamp manually-set values to at least 4 blocks.  Prior to PostgreSQL
         * 9.1, a minimum of 4 was enforced by guc.c, but since that is no longer
         * the case, we just silently treat such values as a request for the
         * minimum.  (We could throw an error instead, but that doesn't seem very
         * helpful.)
         */
        if (*newval < 4)
                *newval = 4;

        return true;
}

/*
 * Read the control file, set respective GUCs.
 *
 * This is to be called during startup, including a crash recovery cycle,
 * unless in bootstrap mode, where no control file yet exists.  As there's no
 * usable shared memory yet (its sizing can depend on the contents of the
 * control file!), first store the contents in local memory. XLOGShmemInit()
 * will then copy it to shared memory later.
 *
 * reset just controls whether previous contents are to be expected (in the
 * reset case, there's a dangling pointer into old shared memory), or not.
 */
void
LocalProcessControlFile(bool reset)
{
        Assert(reset || ControlFile == NULL);
        ControlFile = palloc(sizeof(ControlFileData));
        ReadControlFile();
}

/*
 * Initialization of shared memory for XLOG
 */
Size
XLOGShmemSize(void)
{
        Size            size;

        /*
         * If the value of wal_buffers is -1, use the preferred auto-tune value.
         * This isn't an amazingly clean place to do this, but we must wait till
         * NBuffers has received its final value, and must do it before using the
         * value of XLOGbuffers to do anything important.
         */
        if (XLOGbuffers == -1)
        {
                char            buf[32];

                snprintf(buf, sizeof(buf), "%d", XLOGChooseNumBuffers());
                SetConfigOption("wal_buffers", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
        }
        Assert(XLOGbuffers > 0);

        /* XLogCtl */
        size = sizeof(XLogCtlData);

        /* WAL insertion locks, plus alignment */
        size = add_size(size, mul_size(sizeof(WALInsertLockPadded), NUM_XLOGINSERT_LOCKS + 1));
        /* xlblocks array */
        size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
        /* extra alignment padding for XLOG I/O buffers */
        size = add_size(size, XLOG_BLCKSZ);
        /* and the buffers themselves */
        size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));

        /*
         * Note: we don't count ControlFileData, it comes out of the "slop factor"
         * added by CreateSharedMemoryAndSemaphores.  This lets us use this
         * routine again below to compute the actual allocation size.
         */

        return size;
}

void
XLOGShmemInit(void)
{
        bool            foundCFile,
                                foundXLog;
        char       *allocptr;
        int                     i;
        ControlFileData *localControlFile;

#ifdef WAL_DEBUG

        /*
         * Create a memory context for WAL debugging that's exempt from the normal
         * "no pallocs in critical section" rule. Yes, that can lead to a PANIC if
         * an allocation fails, but wal_debug is not for production use anyway.
         */
        if (walDebugCxt == NULL)
        {
                walDebugCxt = AllocSetContextCreate(TopMemoryContext,
                                                                                        "WAL Debug",
                                                                                        ALLOCSET_DEFAULT_SIZES);
                MemoryContextAllowInCriticalSection(walDebugCxt, true);
        }
#endif


        XLogCtl = (XLogCtlData *)
                ShmemInitStruct("XLOG Ctl", XLOGShmemSize(), &foundXLog);

        localControlFile = ControlFile;
        ControlFile = (ControlFileData *)
                ShmemInitStruct("Control File", sizeof(ControlFileData), &foundCFile);

        if (foundCFile || foundXLog)
        {
                /* both should be present or neither */
                Assert(foundCFile && foundXLog);

                /* Initialize local copy of WALInsertLocks */
                WALInsertLocks = XLogCtl->Insert.WALInsertLocks;

                if (localControlFile)
                        pfree(localControlFile);
                return;
        }
        memset(XLogCtl, 0, sizeof(XLogCtlData));

        /*
         * Already have read control file locally, unless in bootstrap mode. Move
         * contents into shared memory.
         */
        if (localControlFile)
        {
                memcpy(ControlFile, localControlFile, sizeof(ControlFileData));
                pfree(localControlFile);
        }

        /*
         * Since XLogCtlData contains XLogRecPtr fields, its sizeof should be a
         * multiple of the alignment for same, so no extra alignment padding is
         * needed here.
         */
        allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
        XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
        memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
        allocptr += sizeof(XLogRecPtr) * XLOGbuffers;


        /* WAL insertion locks. Ensure they're aligned to the full padded size */
        allocptr += sizeof(WALInsertLockPadded) -
                ((uintptr_t) allocptr) % sizeof(WALInsertLockPadded);
        WALInsertLocks = XLogCtl->Insert.WALInsertLocks =
                (WALInsertLockPadded *) allocptr;
        allocptr += sizeof(WALInsertLockPadded) * NUM_XLOGINSERT_LOCKS;

        for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
        {
                LWLockInitialize(&WALInsertLocks[i].l.lock, LWTRANCHE_WAL_INSERT);
                WALInsertLocks[i].l.insertingAt = InvalidXLogRecPtr;
                WALInsertLocks[i].l.lastImportantAt = InvalidXLogRecPtr;
        }

        /*
         * Align the start of the page buffers to a full xlog block size boundary.
         * This simplifies some calculations in XLOG insertion. It is also
         * required for O_DIRECT.
         */
        allocptr = (char *) TYPEALIGN(XLOG_BLCKSZ, allocptr);
        XLogCtl->pages = allocptr;
        memset(XLogCtl->pages, 0, (Size) XLOG_BLCKSZ * XLOGbuffers);

        /*
         * Do basic initialization of XLogCtl shared data. (StartupXLOG will fill
         * in additional info.)
         */
        XLogCtl->XLogCacheBlck = XLOGbuffers - 1;
        XLogCtl->SharedRecoveryState = RECOVERY_STATE_CRASH;
        XLogCtl->InstallXLogFileSegmentActive = false;
        XLogCtl->WalWriterSleeping = false;

        SpinLockInit(&XLogCtl->Insert.insertpos_lck);
        SpinLockInit(&XLogCtl->info_lck);
        SpinLockInit(&XLogCtl->ulsn_lck);
}

/*
 * This func must be called ONCE on system install.  It creates pg_control
 * and the initial XLOG segment.
 */
void
BootStrapXLOG(void)
{
        CheckPoint      checkPoint;
        char       *buffer;
        XLogPageHeader page;
        XLogLongPageHeader longpage;
        XLogRecord *record;
        char       *recptr;
        uint64          sysidentifier;
        struct timeval tv;
        pg_crc32c       crc;

        /* allow ordinary WAL segment creation, like StartupXLOG() would */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        XLogCtl->InstallXLogFileSegmentActive = true;
        LWLockRelease(ControlFileLock);

        /*
         * Select a hopefully-unique system identifier code for this installation.
         * We use the result of gettimeofday(), including the fractional seconds
         * field, as being about as unique as we can easily get.  (Think not to
         * use random(), since it hasn't been seeded and there's no portable way
         * to seed it other than the system clock value...)  The upper half of the
         * uint64 value is just the tv_sec part, while the lower half contains the
         * tv_usec part (which must fit in 20 bits), plus 12 bits from our current
         * PID for a little extra uniqueness.  A person knowing this encoding can
         * determine the initialization time of the installation, which could
         * perhaps be useful sometimes.
         */
        gettimeofday(&tv, NULL);
        sysidentifier = ((uint64) tv.tv_sec) << 32;
        sysidentifier |= ((uint64) tv.tv_usec) << 12;
        sysidentifier |= getpid() & 0xFFF;

        /* page buffer must be aligned suitably for O_DIRECT */
        buffer = (char *) palloc(XLOG_BLCKSZ + XLOG_BLCKSZ);
        page = (XLogPageHeader) TYPEALIGN(XLOG_BLCKSZ, buffer);
        memset(page, 0, XLOG_BLCKSZ);

        /*
         * Set up information for the initial checkpoint record
         *
         * The initial checkpoint record is written to the beginning of the WAL
         * segment with logid=0 logseg=1. The very first WAL segment, 0/0, is not
         * used, so that we can use 0/0 to mean "before any valid WAL segment".
         */
        checkPoint.redo = wal_segment_size + SizeOfXLogLongPHD;
        checkPoint.ThisTimeLineID = BootstrapTimeLineID;
        checkPoint.PrevTimeLineID = BootstrapTimeLineID;
        checkPoint.fullPageWrites = fullPageWrites;
        checkPoint.nextXid =
                FullTransactionIdFromEpochAndXid(0, FirstNormalTransactionId);
        checkPoint.nextOid = FirstGenbkiObjectId;
        checkPoint.nextMulti = FirstMultiXactId;
        checkPoint.nextMultiOffset = 0;
        checkPoint.oldestXid = FirstNormalTransactionId;
        checkPoint.oldestXidDB = TemplateDbOid;
        checkPoint.oldestMulti = FirstMultiXactId;
        checkPoint.oldestMultiDB = TemplateDbOid;
        checkPoint.oldestCommitTsXid = InvalidTransactionId;
        checkPoint.newestCommitTsXid = InvalidTransactionId;
        checkPoint.time = (pg_time_t) time(NULL);
        checkPoint.oldestActiveXid = InvalidTransactionId;

        ShmemVariableCache->nextXid = checkPoint.nextXid;
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
        AdvanceOldestClogXid(checkPoint.oldestXid);
        SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
        SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB, true);
        SetCommitTsLimit(InvalidTransactionId, InvalidTransactionId);

        /* Set up the XLOG page header */
        page->xlp_magic = XLOG_PAGE_MAGIC;
        page->xlp_info = XLP_LONG_HEADER;
        page->xlp_tli = BootstrapTimeLineID;
        page->xlp_pageaddr = wal_segment_size;
        longpage = (XLogLongPageHeader) page;
        longpage->xlp_sysid = sysidentifier;
        longpage->xlp_seg_size = wal_segment_size;
        longpage->xlp_xlog_blcksz = XLOG_BLCKSZ;

        /* Insert the initial checkpoint record */
        recptr = ((char *) page + SizeOfXLogLongPHD);
        record = (XLogRecord *) recptr;
        record->xl_prev = 0;
        record->xl_xid = InvalidTransactionId;
        record->xl_tot_len = SizeOfXLogRecord + SizeOfXLogRecordDataHeaderShort + sizeof(checkPoint);
        record->xl_info = XLOG_CHECKPOINT_SHUTDOWN;
        record->xl_rmid = RM_XLOG_ID;
        recptr += SizeOfXLogRecord;
        /* fill the XLogRecordDataHeaderShort struct */
        *(recptr++) = (char) XLR_BLOCK_ID_DATA_SHORT;
        *(recptr++) = sizeof(checkPoint);
        memcpy(recptr, &checkPoint, sizeof(checkPoint));
        recptr += sizeof(checkPoint);
        Assert(recptr - (char *) record == record->xl_tot_len);

        INIT_CRC32C(crc);
        COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord);
        COMP_CRC32C(crc, (char *) record, offsetof(XLogRecord, xl_crc));
        FIN_CRC32C(crc);
        record->xl_crc = crc;

        /* Create first XLOG segment file */
        openLogTLI = BootstrapTimeLineID;
        openLogFile = XLogFileInit(1, BootstrapTimeLineID);

        /*
         * We needn't bother with Reserve/ReleaseExternalFD here, since we'll
         * close the file again in a moment.
         */

        /* Write the first page with the initial record */
        errno = 0;
        pgstat_report_wait_start(WAIT_EVENT_WAL_BOOTSTRAP_WRITE);
        if (write(openLogFile, page, XLOG_BLCKSZ) != XLOG_BLCKSZ)
        {
                /* if write didn't set errno, assume problem is no disk space */
                if (errno == 0)
                        errno = ENOSPC;
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not write bootstrap write-ahead log file: %m")));
        }
        pgstat_report_wait_end();

        pgstat_report_wait_start(WAIT_EVENT_WAL_BOOTSTRAP_SYNC);
        if (pg_fsync(openLogFile) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync bootstrap write-ahead log file: %m")));
        pgstat_report_wait_end();

        if (close(openLogFile) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close bootstrap write-ahead log file: %m")));

        openLogFile = -1;

        /* Now create pg_control */
        InitControlFile(sysidentifier);
        ControlFile->time = checkPoint.time;
        ControlFile->checkPoint = checkPoint.redo;
        ControlFile->checkPointCopy = checkPoint;

        /* some additional ControlFile fields are set in WriteControlFile() */
        WriteControlFile();

        /* Bootstrap the commit log, too */
        BootStrapCLOG();
        BootStrapCommitTs();
        BootStrapSUBTRANS();
        BootStrapMultiXact();

        pfree(buffer);

        /*
         * Force control file to be read - in contrast to normal processing we'd
         * otherwise never run the checks and GUC related initializations therein.
         */
        ReadControlFile();
}

static char *
str_time(pg_time_t tnow)
{
        static char buf[128];

        pg_strftime(buf, sizeof(buf),
                                "%Y-%m-%d %H:%M:%S %Z",
                                pg_localtime(&tnow, log_timezone));

        return buf;
}

/*
 * Initialize the first WAL segment on new timeline.
 */
static void
XLogInitNewTimeline(TimeLineID endTLI, XLogRecPtr endOfLog, TimeLineID newTLI)
{
        char            xlogfname[MAXFNAMELEN];
        XLogSegNo       endLogSegNo;
        XLogSegNo       startLogSegNo;

        /* we always switch to a new timeline after archive recovery */
        Assert(endTLI != newTLI);

        /*
         * Update min recovery point one last time.
         */
        UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);

        /*
         * Calculate the last segment on the old timeline, and the first segment
         * on the new timeline. If the switch happens in the middle of a segment,
         * they are the same, but if the switch happens exactly at a segment
         * boundary, startLogSegNo will be endLogSegNo + 1.
         */
        XLByteToPrevSeg(endOfLog, endLogSegNo, wal_segment_size);
        XLByteToSeg(endOfLog, startLogSegNo, wal_segment_size);

        /*
         * Initialize the starting WAL segment for the new timeline. If the switch
         * happens in the middle of a segment, copy data from the last WAL segment
         * of the old timeline up to the switch point, to the starting WAL segment
         * on the new timeline.
         */
        if (endLogSegNo == startLogSegNo)
        {
                /*
                 * Make a copy of the file on the new timeline.
                 *
                 * Writing WAL isn't allowed yet, so there are no locking
                 * considerations. But we should be just as tense as XLogFileInit to
                 * avoid emplacing a bogus file.
                 */
                XLogFileCopy(newTLI, endLogSegNo, endTLI, endLogSegNo,
                                         XLogSegmentOffset(endOfLog, wal_segment_size));
        }
        else
        {
                /*
                 * The switch happened at a segment boundary, so just create the next
                 * segment on the new timeline.
                 */
                int                     fd;

                fd = XLogFileInit(startLogSegNo, newTLI);

                if (close(fd) != 0)
                {
                        char            xlogfname[MAXFNAMELEN];
                        int                     save_errno = errno;

                        XLogFileName(xlogfname, newTLI, startLogSegNo, wal_segment_size);
                        errno = save_errno;
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not close file \"%s\": %m", xlogfname)));
                }
        }

        /*
         * Let's just make real sure there are not .ready or .done flags posted
         * for the new segment.
         */
        XLogFileName(xlogfname, newTLI, startLogSegNo, wal_segment_size);
        XLogArchiveCleanup(xlogfname);
}

/*
 * Perform cleanup actions at the conclusion of archive recovery.
 */
static void
CleanupAfterArchiveRecovery(TimeLineID EndOfLogTLI, XLogRecPtr EndOfLog,
                                                        TimeLineID newTLI)
{
        /*
         * Execute the recovery_end_command, if any.
         */
        if (recoveryEndCommand && strcmp(recoveryEndCommand, "") != 0)
                ExecuteRecoveryCommand(recoveryEndCommand,
                                                           "recovery_end_command",
                                                           true,
                                                           WAIT_EVENT_RECOVERY_END_COMMAND);

        /*
         * We switched to a new timeline. Clean up segments on the old timeline.
         *
         * If there are any higher-numbered segments on the old timeline, remove
         * them. They might contain valid WAL, but they might also be
         * pre-allocated files containing garbage. In any case, they are not part
         * of the new timeline's history so we don't need them.
         */
        RemoveNonParentXlogFiles(EndOfLog, newTLI);

        /*
         * If the switch happened in the middle of a segment, what to do with the
         * last, partial segment on the old timeline? If we don't archive it, and
         * the server that created the WAL never archives it either (e.g. because
         * it was hit by a meteor), it will never make it to the archive. That's
         * OK from our point of view, because the new segment that we created with
         * the new TLI contains all the WAL from the old timeline up to the switch
         * point. But if you later try to do PITR to the "missing" WAL on the old
         * timeline, recovery won't find it in the archive. It's physically
         * present in the new file with new TLI, but recovery won't look there
         * when it's recovering to the older timeline. On the other hand, if we
         * archive the partial segment, and the original server on that timeline
         * is still running and archives the completed version of the same segment
         * later, it will fail. (We used to do that in 9.4 and below, and it
         * caused such problems).
         *
         * As a compromise, we rename the last segment with the .partial suffix,
         * and archive it. Archive recovery will never try to read .partial
         * segments, so they will normally go unused. But in the odd PITR case,
         * the administrator can copy them manually to the pg_wal directory
         * (removing the suffix). They can be useful in debugging, too.
         *
         * If a .done or .ready file already exists for the old timeline, however,
         * we had already determined that the segment is complete, so we can let
         * it be archived normally. (In particular, if it was restored from the
         * archive to begin with, it's expected to have a .done file).
         */
        if (XLogSegmentOffset(EndOfLog, wal_segment_size) != 0 &&
                XLogArchivingActive())
        {
                char            origfname[MAXFNAMELEN];
                XLogSegNo       endLogSegNo;

                XLByteToPrevSeg(EndOfLog, endLogSegNo, wal_segment_size);
                XLogFileName(origfname, EndOfLogTLI, endLogSegNo, wal_segment_size);

                if (!XLogArchiveIsReadyOrDone(origfname))
                {
                        char            origpath[MAXPGPATH];
                        char            partialfname[MAXFNAMELEN];
                        char            partialpath[MAXPGPATH];

                        XLogFilePath(origpath, EndOfLogTLI, endLogSegNo, wal_segment_size);
                        snprintf(partialfname, MAXFNAMELEN, "%s.partial", origfname);
                        snprintf(partialpath, MAXPGPATH, "%s.partial", origpath);

                        /*
                         * Make sure there's no .done or .ready file for the .partial
                         * file.
                         */
                        XLogArchiveCleanup(partialfname);

                        durable_rename(origpath, partialpath, ERROR);
                        XLogArchiveNotify(partialfname);
                }
        }
}

/*
 * Check to see if required parameters are set high enough on this server
 * for various aspects of recovery operation.
 *
 * Note that all the parameters which this function tests need to be
 * listed in Administrator's Overview section in high-availability.sgml.
 * If you change them, don't forget to update the list.
 */
static void
CheckRequiredParameterValues(void)
{
        /*
         * For archive recovery, the WAL must be generated with at least 'replica'
         * wal_level.
         */
        if (ArchiveRecoveryRequested && ControlFile->wal_level == WAL_LEVEL_MINIMAL)
        {
                ereport(FATAL,
                                (errmsg("WAL was generated with wal_level=minimal, cannot continue recovering"),
                                 errdetail("This happens if you temporarily set wal_level=minimal on the server."),
                                 errhint("Use a backup taken after setting wal_level to higher than minimal.")));
        }

        /*
         * For Hot Standby, the WAL must be generated with 'replica' mode, and we
         * must have at least as many backend slots as the primary.
         */
        if (ArchiveRecoveryRequested && EnableHotStandby)
        {
                /* We ignore autovacuum_max_workers when we make this test. */
                RecoveryRequiresIntParameter("max_connections",
                                                                         MaxConnections,
                                                                         ControlFile->MaxConnections);
                RecoveryRequiresIntParameter("max_worker_processes",
                                                                         max_worker_processes,
                                                                         ControlFile->max_worker_processes);
                RecoveryRequiresIntParameter("max_wal_senders",
                                                                         max_wal_senders,
                                                                         ControlFile->max_wal_senders);
                RecoveryRequiresIntParameter("max_prepared_transactions",
                                                                         max_prepared_xacts,
                                                                         ControlFile->max_prepared_xacts);
                RecoveryRequiresIntParameter("max_locks_per_transaction",
                                                                         max_locks_per_xact,
                                                                         ControlFile->max_locks_per_xact);
        }
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
        XLogCtlInsert *Insert;
        CheckPoint      checkPoint;
        bool            wasShutdown;
        bool            haveTblspcMap;
        bool            haveBackupLabel;
        XLogRecPtr      EndOfLog;
        TimeLineID      EndOfLogTLI;
        TimeLineID      newTLI;
        bool            performedWalRecovery;
        EndOfWalRecoveryInfo *endOfRecoveryInfo;
        XLogRecPtr      abortedRecPtr;
        XLogRecPtr      missingContrecPtr;
        TransactionId oldestActiveXID;
        bool            promoted = false;

        /*
         * We should have an aux process resource owner to use, and we should not
         * be in a transaction that's installed some other resowner.
         */
        Assert(AuxProcessResourceOwner != NULL);
        Assert(CurrentResourceOwner == NULL ||
                   CurrentResourceOwner == AuxProcessResourceOwner);
        CurrentResourceOwner = AuxProcessResourceOwner;

        /*
         * Check that contents look valid.
         */
        if (!XRecOffIsValid(ControlFile->checkPoint))
                ereport(FATAL,
                                (errmsg("control file contains invalid checkpoint location")));

        switch (ControlFile->state)
        {
                case DB_SHUTDOWNED:

                        /*
                         * This is the expected case, so don't be chatty in standalone
                         * mode
                         */
                        ereport(IsPostmasterEnvironment ? LOG : NOTICE,
                                        (errmsg("database system was shut down at %s",
                                                        str_time(ControlFile->time))));
                        break;

                case DB_SHUTDOWNED_IN_RECOVERY:
                        ereport(LOG,
                                        (errmsg("database system was shut down in recovery at %s",
                                                        str_time(ControlFile->time))));
                        break;

                case DB_SHUTDOWNING:
                        ereport(LOG,
                                        (errmsg("database system shutdown was interrupted; last known up at %s",
                                                        str_time(ControlFile->time))));
                        break;

                case DB_IN_CRASH_RECOVERY:
                        ereport(LOG,
                                        (errmsg("database system was interrupted while in recovery at %s",
                                                        str_time(ControlFile->time)),
                                         errhint("This probably means that some data is corrupted and"
                                                         " you will have to use the last backup for recovery.")));
                        break;

                case DB_IN_ARCHIVE_RECOVERY:
                        ereport(LOG,
                                        (errmsg("database system was interrupted while in recovery at log time %s",
                                                        str_time(ControlFile->checkPointCopy.time)),
                                         errhint("If this has occurred more than once some data might be corrupted"
                                                         " and you might need to choose an earlier recovery target.")));
                        break;

                case DB_IN_PRODUCTION:
                        ereport(LOG,
                                        (errmsg("database system was interrupted; last known up at %s",
                                                        str_time(ControlFile->time))));
                        break;

                default:
                        ereport(FATAL,
                                        (errmsg("control file contains invalid database cluster state")));
        }

        /* This is just to allow attaching to startup process with a debugger */
#ifdef XLOG_REPLAY_DELAY
        if (ControlFile->state != DB_SHUTDOWNED)
                pg_usleep(60000000L);
#endif

        /*
         * Verify that pg_wal and pg_wal/archive_status exist.  In cases where
         * someone has performed a copy for PITR, these directories may have been
         * excluded and need to be re-created.
         */
        ValidateXLOGDirectoryStructure();

        /* Set up timeout handler needed to report startup progress. */
        if (!IsBootstrapProcessingMode())
                RegisterTimeout(STARTUP_PROGRESS_TIMEOUT,
                                                startup_progress_timeout_handler);

        /*----------
         * If we previously crashed, perform a couple of actions:
         *
         * - The pg_wal directory may still include some temporary WAL segments
         *   used when creating a new segment, so perform some clean up to not
         *   bloat this path.  This is done first as there is no point to sync
         *   this temporary data.
         *
         * - There might be data which we had written, intending to fsync it, but
         *   which we had not actually fsync'd yet.  Therefore, a power failure in
         *   the near future might cause earlier unflushed writes to be lost, even
         *   though more recent data written to disk from here on would be
         *   persisted.  To avoid that, fsync the entire data directory.
         */
        if (ControlFile->state != DB_SHUTDOWNED &&
                ControlFile->state != DB_SHUTDOWNED_IN_RECOVERY)
        {
                RemoveTempXlogFiles();
                SyncDataDirectory();
        }

        /*
         * Prepare for WAL recovery if needed.
         *
         * InitWalRecovery analyzes the control file and the backup label file, if
         * any.  It updates the in-memory ControlFile buffer according to the
         * starting checkpoint, and sets InRecovery and ArchiveRecoveryRequested.
         * It also applies the tablespace map file, if any.
         */
        InitWalRecovery(ControlFile, &wasShutdown,
                                        &haveBackupLabel, &haveTblspcMap);
        checkPoint = ControlFile->checkPointCopy;

        /* initialize shared memory variables from the checkpoint record */
        ShmemVariableCache->nextXid = checkPoint.nextXid;
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
        AdvanceOldestClogXid(checkPoint.oldestXid);
        SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
        SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB, true);
        SetCommitTsLimit(checkPoint.oldestCommitTsXid,
                                         checkPoint.newestCommitTsXid);
        XLogCtl->ckptFullXid = checkPoint.nextXid;

        /*
         * Clear out any old relcache cache files.  This is *necessary* if we do
         * any WAL replay, since that would probably result in the cache files
         * being out of sync with database reality.  In theory we could leave them
         * in place if the database had been cleanly shut down, but it seems
         * safest to just remove them always and let them be rebuilt during the
         * first backend startup.  These files needs to be removed from all
         * directories including pg_tblspc, however the symlinks are created only
         * after reading tablespace_map file in case of archive recovery from
         * backup, so needs to clear old relcache files here after creating
         * symlinks.
         */
        RelationCacheInitFileRemove();

        /*
         * Initialize replication slots, before there's a chance to remove
         * required resources.
         */
        StartupReplicationSlots();

        /*
         * Startup logical state, needs to be setup now so we have proper data
         * during crash recovery.
         */
        StartupReorderBuffer();

        /*
         * Startup CLOG. This must be done after ShmemVariableCache->nextXid has
         * been initialized and before we accept connections or begin WAL replay.
         */
        StartupCLOG();

        /*
         * Startup MultiXact. We need to do this early to be able to replay
         * truncations.
         */
        StartupMultiXact();

        /*
         * Ditto for commit timestamps.  Activate the facility if the setting is
         * enabled in the control file, as there should be no tracking of commit
         * timestamps done when the setting was disabled.  This facility can be
         * started or stopped when replaying a XLOG_PARAMETER_CHANGE record.
         */
        if (ControlFile->track_commit_timestamp)
                StartupCommitTs();

        /*
         * Recover knowledge about replay progress of known replication partners.
         */
        StartupReplicationOrigin();

        /*
         * Initialize unlogged LSN. On a clean shutdown, it's restored from the
         * control file. On recovery, all unlogged relations are blown away, so
         * the unlogged LSN counter can be reset too.
         */
        if (ControlFile->state == DB_SHUTDOWNED)
                XLogCtl->unloggedLSN = ControlFile->unloggedLSN;
        else
                XLogCtl->unloggedLSN = FirstNormalUnloggedLSN;

        /*
         * Copy any missing timeline history files between 'now' and the recovery
         * target timeline from archive to pg_wal. While we don't need those files
         * ourselves - the history file of the recovery target timeline covers all
         * the previous timelines in the history too - a cascading standby server
         * might be interested in them. Or, if you archive the WAL from this
         * server to a different archive than the primary, it'd be good for all
         * the history files to get archived there after failover, so that you can
         * use one of the old timelines as a PITR target. Timeline history files
         * are small, so it's better to copy them unnecessarily than not copy them
         * and regret later.
         */
        restoreTimeLineHistoryFiles(checkPoint.ThisTimeLineID, recoveryTargetTLI);

        /*
         * Before running in recovery, scan pg_twophase and fill in its status to
         * be able to work on entries generated by redo.  Doing a scan before
         * taking any recovery action has the merit to discard any 2PC files that
         * are newer than the first record to replay, saving from any conflicts at
         * replay.  This avoids as well any subsequent scans when doing recovery
         * of the on-disk two-phase data.
         */
        restoreTwoPhaseData();

        lastFullPageWrites = checkPoint.fullPageWrites;

        RedoRecPtr = XLogCtl->RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;
        doPageWrites = lastFullPageWrites;

        /* REDO */
        if (InRecovery)
        {
                /* Initialize state for RecoveryInProgress() */
                SpinLockAcquire(&XLogCtl->info_lck);
                if (InArchiveRecovery)
                        XLogCtl->SharedRecoveryState = RECOVERY_STATE_ARCHIVE;
                else
                        XLogCtl->SharedRecoveryState = RECOVERY_STATE_CRASH;
                SpinLockRelease(&XLogCtl->info_lck);

                /*
                 * Update pg_control to show that we are recovering and to show the
                 * selected checkpoint as the place we are starting from. We also mark
                 * pg_control with any minimum recovery stop point obtained from a
                 * backup history file.
                 *
                 * No need to hold ControlFileLock yet, we aren't up far enough.
                 */
                UpdateControlFile();

                /*
                 * If there was a backup label file, it's done its job and the info
                 * has now been propagated into pg_control.  We must get rid of the
                 * label file so that if we crash during recovery, we'll pick up at
                 * the latest recovery restartpoint instead of going all the way back
                 * to the backup start point.  It seems prudent though to just rename
                 * the file out of the way rather than delete it completely.
                 */
                if (haveBackupLabel)
                {
                        unlink(BACKUP_LABEL_OLD);
                        durable_rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD, FATAL);
                }

                /*
                 * If there was a tablespace_map file, it's done its job and the
                 * symlinks have been created.  We must get rid of the map file so
                 * that if we crash during recovery, we don't create symlinks again.
                 * It seems prudent though to just rename the file out of the way
                 * rather than delete it completely.
                 */
                if (haveTblspcMap)
                {
                        unlink(TABLESPACE_MAP_OLD);
                        durable_rename(TABLESPACE_MAP, TABLESPACE_MAP_OLD, FATAL);
                }

                /*
                 * Initialize our local copy of minRecoveryPoint.  When doing crash
                 * recovery we want to replay up to the end of WAL.  Particularly, in
                 * the case of a promoted standby minRecoveryPoint value in the
                 * control file is only updated after the first checkpoint.  However,
                 * if the instance crashes before the first post-recovery checkpoint
                 * is completed then recovery will use a stale location causing the
                 * startup process to think that there are still invalid page
                 * references when checking for data consistency.
                 */
                if (InArchiveRecovery)
                {
                        LocalMinRecoveryPoint = ControlFile->minRecoveryPoint;
                        LocalMinRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
                }
                else
                {
                        LocalMinRecoveryPoint = InvalidXLogRecPtr;
                        LocalMinRecoveryPointTLI = 0;
                }

                /*
                 * Reset pgstat data, because it may be invalid after recovery.
                 */
                pgstat_reset_all();

                /* Check that the GUCs used to generate the WAL allow recovery */
                CheckRequiredParameterValues();

                /*
                 * We're in recovery, so unlogged relations may be trashed and must be
                 * reset.  This should be done BEFORE allowing Hot Standby
                 * connections, so that read-only backends don't try to read whatever
                 * garbage is left over from before.
                 */
                ResetUnloggedRelations(UNLOGGED_RELATION_CLEANUP);

                /*
                 * Likewise, delete any saved transaction snapshot files that got left
                 * behind by crashed backends.
                 */
                DeleteAllExportedSnapshotFiles();

                /*
                 * Initialize for Hot Standby, if enabled. We won't let backends in
                 * yet, not until we've reached the min recovery point specified in
                 * control file and we've established a recovery snapshot from a
                 * running-xacts WAL record.
                 */
                if (ArchiveRecoveryRequested && EnableHotStandby)
                {
                        TransactionId *xids;
                        int                     nxids;

                        ereport(DEBUG1,
                                        (errmsg_internal("initializing for hot standby")));

                        InitRecoveryTransactionEnvironment();

                        if (wasShutdown)
                                oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);
                        else
                                oldestActiveXID = checkPoint.oldestActiveXid;
                        Assert(TransactionIdIsValid(oldestActiveXID));

                        /* Tell procarray about the range of xids it has to deal with */
                        ProcArrayInitRecovery(XidFromFullTransactionId(ShmemVariableCache->nextXid));

                        /*
                         * Startup subtrans only.  CLOG, MultiXact and commit timestamp
                         * have already been started up and other SLRUs are not maintained
                         * during recovery and need not be started yet.
                         */
                        StartupSUBTRANS(oldestActiveXID);

                        /*
                         * If we're beginning at a shutdown checkpoint, we know that
                         * nothing was running on the primary at this point. So fake-up an
                         * empty running-xacts record and use that here and now. Recover
                         * additional standby state for prepared transactions.
                         */
                        if (wasShutdown)
                        {
                                RunningTransactionsData running;
                                TransactionId latestCompletedXid;

                                /*
                                 * Construct a RunningTransactions snapshot representing a
                                 * shut down server, with only prepared transactions still
                                 * alive. We're never overflowed at this point because all
                                 * subxids are listed with their parent prepared transactions.
                                 */
                                running.xcnt = nxids;
                                running.subxcnt = 0;
                                running.subxid_overflow = false;
                                running.nextXid = XidFromFullTransactionId(checkPoint.nextXid);
                                running.oldestRunningXid = oldestActiveXID;
                                latestCompletedXid = XidFromFullTransactionId(checkPoint.nextXid);
                                TransactionIdRetreat(latestCompletedXid);
                                Assert(TransactionIdIsNormal(latestCompletedXid));
                                running.latestCompletedXid = latestCompletedXid;
                                running.xids = xids;

                                ProcArrayApplyRecoveryInfo(&running);

                                StandbyRecoverPreparedTransactions();
                        }
                }

                /*
                 * We're all set for replaying the WAL now. Do it.
                 */
                PerformWalRecovery();
                performedWalRecovery = true;
        }
        else
                performedWalRecovery = false;

        /*
         * Finish WAL recovery.
         */
        endOfRecoveryInfo = FinishWalRecovery();
        EndOfLog = endOfRecoveryInfo->endOfLog;
        EndOfLogTLI = endOfRecoveryInfo->endOfLogTLI;
        abortedRecPtr = endOfRecoveryInfo->abortedRecPtr;
        missingContrecPtr = endOfRecoveryInfo->missingContrecPtr;

        /*
         * Complain if we did not roll forward far enough to render the backup
         * dump consistent.  Note: it is indeed okay to look at the local variable
         * LocalMinRecoveryPoint here, even though ControlFile->minRecoveryPoint
         * might be further ahead --- ControlFile->minRecoveryPoint cannot have
         * been advanced beyond the WAL we processed.
         */
        if (InRecovery &&
                (EndOfLog < LocalMinRecoveryPoint ||
                 !XLogRecPtrIsInvalid(ControlFile->backupStartPoint)))
        {
                /*
                 * Ran off end of WAL before reaching end-of-backup WAL record, or
                 * minRecoveryPoint. That's usually a bad sign, indicating that you
                 * tried to recover from an online backup but never called
                 * pg_stop_backup(), or you didn't archive all the WAL up to that
                 * point. However, this also happens in crash recovery, if the system
                 * crashes while an online backup is in progress. We must not treat
                 * that as an error, or the database will refuse to start up.
                 */
                if (ArchiveRecoveryRequested || ControlFile->backupEndRequired)
                {
                        if (ControlFile->backupEndRequired)
                                ereport(FATAL,
                                                (errmsg("WAL ends before end of online backup"),
                                                 errhint("All WAL generated while online backup was taken must be available at recovery.")));
                        else if (!XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
                                ereport(FATAL,
                                                (errmsg("WAL ends before end of online backup"),
                                                 errhint("Online backup started with pg_start_backup() must be ended with pg_stop_backup(), and all WAL up to that point must be available at recovery.")));
                        else
                                ereport(FATAL,
                                                (errmsg("WAL ends before consistent recovery point")));
                }
        }

        /*
         * Reset unlogged relations to the contents of their INIT fork. This is
         * done AFTER recovery is complete so as to include any unlogged relations
         * created during recovery, but BEFORE recovery is marked as having
         * completed successfully. Otherwise we'd not retry if any of the post
         * end-of-recovery steps fail.
         */
        if (InRecovery)
                ResetUnloggedRelations(UNLOGGED_RELATION_INIT);

        /*
         * Pre-scan prepared transactions to find out the range of XIDs present.
         * This information is not quite needed yet, but it is positioned here so
         * as potential problems are detected before any on-disk change is done.
         */
        oldestActiveXID = PrescanPreparedTransactions(NULL, NULL);

        /*
         * Allow ordinary WAL segment creation before possibly switching to a new
         * timeline, which creates a new segment, and after the last ReadRecord().
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        XLogCtl->InstallXLogFileSegmentActive = true;
        LWLockRelease(ControlFileLock);

        /*
         * Consider whether we need to assign a new timeline ID.
         *
         * If we did archive recovery, we always assign a new ID.  This handles a
         * couple of issues.  If we stopped short of the end of WAL during
         * recovery, then we are clearly generating a new timeline and must assign
         * it a unique new ID.  Even if we ran to the end, modifying the current
         * last segment is problematic because it may result in trying to
         * overwrite an already-archived copy of that segment, and we encourage
         * DBAs to make their archive_commands reject that.  We can dodge the
         * problem by making the new active segment have a new timeline ID.
         *
         * In a normal crash recovery, we can just extend the timeline we were in.
         */
        newTLI = endOfRecoveryInfo->lastRecTLI;
        if (ArchiveRecoveryRequested)
        {
                newTLI = findNewestTimeLine(recoveryTargetTLI) + 1;
                ereport(LOG,
                                (errmsg("selected new timeline ID: %u", newTLI)));

                /*
                 * Make a writable copy of the last WAL segment.  (Note that we also
                 * have a copy of the last block of the old WAL in
                 * endOfRecovery->lastPage; we will use that below.)
                 */
                XLogInitNewTimeline(EndOfLogTLI, EndOfLog, newTLI);

                /*
                 * Remove the signal files out of the way, so that we don't
                 * accidentally re-enter archive recovery mode in a subsequent crash.
                 */
                if (endOfRecoveryInfo->standby_signal_file_found)
                        durable_unlink(STANDBY_SIGNAL_FILE, FATAL);

                if (endOfRecoveryInfo->recovery_signal_file_found)
                        durable_unlink(RECOVERY_SIGNAL_FILE, FATAL);

                /*
                 * Write the timeline history file, and have it archived. After this
                 * point (or rather, as soon as the file is archived), the timeline
                 * will appear as "taken" in the WAL archive and to any standby
                 * servers.  If we crash before actually switching to the new
                 * timeline, standby servers will nevertheless think that we switched
                 * to the new timeline, and will try to connect to the new timeline.
                 * To minimize the window for that, try to do as little as possible
                 * between here and writing the end-of-recovery record.
                 */
                writeTimeLineHistory(newTLI, recoveryTargetTLI,
                                                         EndOfLog, endOfRecoveryInfo->recoveryStopReason);

                ereport(LOG,
                                (errmsg("archive recovery complete")));
        }

        /* Save the selected TimeLineID in shared memory, too */
        XLogCtl->InsertTimeLineID = newTLI;
        XLogCtl->PrevTimeLineID = endOfRecoveryInfo->lastRecTLI;

        /*
         * Actually, if WAL ended in an incomplete record, skip the parts that
         * made it through and start writing after the portion that persisted.
         * (It's critical to first write an OVERWRITE_CONTRECORD message, which
         * we'll do as soon as we're open for writing new WAL.)
         */
        if (!XLogRecPtrIsInvalid(missingContrecPtr))
        {
                Assert(!XLogRecPtrIsInvalid(abortedRecPtr));
                EndOfLog = missingContrecPtr;
        }

        /*
         * Prepare to write WAL starting at EndOfLog location, and init xlog
         * buffer cache using the block containing the last record from the
         * previous incarnation.
         */
        Insert = &XLogCtl->Insert;
        Insert->PrevBytePos = XLogRecPtrToBytePos(endOfRecoveryInfo->lastRec);
        Insert->CurrBytePos = XLogRecPtrToBytePos(EndOfLog);

        /*
         * Tricky point here: lastPage contains the *last* block that the LastRec
         * record spans, not the one it starts in.  The last block is indeed the
         * one we want to use.
         */
        if (EndOfLog % XLOG_BLCKSZ != 0)
        {
                char       *page;
                int                     len;
                int                     firstIdx;

                firstIdx = XLogRecPtrToBufIdx(EndOfLog);
                len = EndOfLog - endOfRecoveryInfo->lastPageBeginPtr;
                Assert(len < XLOG_BLCKSZ);

                /* Copy the valid part of the last block, and zero the rest */
                page = &XLogCtl->pages[firstIdx * XLOG_BLCKSZ];
                memcpy(page, endOfRecoveryInfo->lastPage, len);
                memset(page + len, 0, XLOG_BLCKSZ - len);

                XLogCtl->xlblocks[firstIdx] = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
                XLogCtl->InitializedUpTo = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
        }
        else
        {
                /*
                 * There is no partial block to copy. Just set InitializedUpTo, and
                 * let the first attempt to insert a log record to initialize the next
                 * buffer.
                 */
                XLogCtl->InitializedUpTo = EndOfLog;
        }

        LogwrtResult.Write = LogwrtResult.Flush = EndOfLog;

        XLogCtl->LogwrtResult = LogwrtResult;

        XLogCtl->LogwrtRqst.Write = EndOfLog;
        XLogCtl->LogwrtRqst.Flush = EndOfLog;

        /*
         * Preallocate additional log files, if wanted.
         */
        PreallocXlogFiles(EndOfLog, newTLI);

        /*
         * Okay, we're officially UP.
         */
        InRecovery = false;

        /* start the archive_timeout timer and LSN running */
        XLogCtl->lastSegSwitchTime = (pg_time_t) time(NULL);
        XLogCtl->lastSegSwitchLSN = EndOfLog;

        /* also initialize latestCompletedXid, to nextXid - 1 */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
        FullTransactionIdRetreat(&ShmemVariableCache->latestCompletedXid);
        LWLockRelease(ProcArrayLock);

        /*
         * Start up subtrans, if not already done for hot standby.  (commit
         * timestamps are started below, if necessary.)
         */
        if (standbyState == STANDBY_DISABLED)
                StartupSUBTRANS(oldestActiveXID);

        /*
         * Perform end of recovery actions for any SLRUs that need it.
         */
        TrimCLOG();
        TrimMultiXact();

        /* Reload shared-memory state for prepared transactions */
        RecoverPreparedTransactions();

        /* Shut down xlogreader */
        ShutdownWalRecovery();

        /* Enable WAL writes for this backend only. */
        LocalSetXLogInsertAllowed();

        /* If necessary, write overwrite-contrecord before doing anything else */
        if (!XLogRecPtrIsInvalid(abortedRecPtr))
        {
                Assert(!XLogRecPtrIsInvalid(missingContrecPtr));
                CreateOverwriteContrecordRecord(abortedRecPtr, missingContrecPtr, newTLI);
        }

        /*
         * Update full_page_writes in shared memory and write an XLOG_FPW_CHANGE
         * record before resource manager writes cleanup WAL records or checkpoint
         * record is written.
         */
        Insert->fullPageWrites = lastFullPageWrites;
        UpdateFullPageWrites();

        /*
         * Emit checkpoint or end-of-recovery record in XLOG, if required.
         */
        if (performedWalRecovery)
                promoted = PerformRecoveryXLogAction();

        /*
         * If any of the critical GUCs have changed, log them before we allow
         * backends to write WAL.
         */
        XLogReportParameters();

        /* If this is archive recovery, perform post-recovery cleanup actions. */
        if (ArchiveRecoveryRequested)
                CleanupAfterArchiveRecovery(EndOfLogTLI, EndOfLog, newTLI);

        /*
         * Local WAL inserts enabled, so it's time to finish initialization of
         * commit timestamp.
         */
        CompleteCommitTsInitialization();

        /*
         * All done with end-of-recovery actions.
         *
         * Now allow backends to write WAL and update the control file status in
         * consequence.  SharedRecoveryState, that controls if backends can write
         * WAL, is updated while holding ControlFileLock to prevent other backends
         * to look at an inconsistent state of the control file in shared memory.
         * There is still a small window during which backends can write WAL and
         * the control file is still referring to a system not in DB_IN_PRODUCTION
         * state while looking at the on-disk control file.
         *
         * Also, we use info_lck to update SharedRecoveryState to ensure that
         * there are no race conditions concerning visibility of other recent
         * updates to shared memory.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->state = DB_IN_PRODUCTION;

        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->SharedRecoveryState = RECOVERY_STATE_DONE;
        SpinLockRelease(&XLogCtl->info_lck);

        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        /*
         * Shutdown the recovery environment.  This must occur after
         * RecoverPreparedTransactions() (see notes in lock_twophase_recover())
         * and after switching SharedRecoveryState to RECOVERY_STATE_DONE so as
         * any session building a snapshot will not rely on KnownAssignedXids as
         * RecoveryInProgress() would return false at this stage.  This is
         * particularly critical for prepared 2PC transactions, that would still
         * need to be included in snapshots once recovery has ended.
         */
        if (standbyState != STANDBY_DISABLED)
                ShutdownRecoveryTransactionEnvironment();

        /*
         * If there were cascading standby servers connected to us, nudge any wal
         * sender processes to notice that we've been promoted.
         */
        WalSndWakeup();

        /*
         * If this was a promotion, request an (online) checkpoint now. This isn't
         * required for consistency, but the last restartpoint might be far back,
         * and in case of a crash, recovering from it might take a longer than is
         * appropriate now that we're not in standby mode anymore.
         */
        if (promoted)
                RequestCheckpoint(CHECKPOINT_FORCE);
}

/*
 * Callback from PerformWalRecovery(), called when we switch from crash
 * recovery to archive recovery mode.  Updates the control file accordingly.
 */
void
SwitchIntoArchiveRecovery(XLogRecPtr EndRecPtr, TimeLineID replayTLI)
{
        /* initialize minRecoveryPoint to this record */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
        if (ControlFile->minRecoveryPoint < EndRecPtr)
        {
                ControlFile->minRecoveryPoint = EndRecPtr;
                ControlFile->minRecoveryPointTLI = replayTLI;
        }
        /* update local copy */
        LocalMinRecoveryPoint = ControlFile->minRecoveryPoint;
        LocalMinRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

        /*
         * The startup process can update its local copy of minRecoveryPoint from
         * this point.
         */
        updateMinRecoveryPoint = true;

        UpdateControlFile();

        /*
         * We update SharedRecoveryState while holding the lock on ControlFileLock
         * so both states are consistent in shared memory.
         */
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->SharedRecoveryState = RECOVERY_STATE_ARCHIVE;
        SpinLockRelease(&XLogCtl->info_lck);

        LWLockRelease(ControlFileLock);
}

/*
 * Callback from PerformWalRecovery(), called when we reach the end of backup.
 * Updates the control file accordingly.
 */
void
ReachedEndOfBackup(XLogRecPtr EndRecPtr, TimeLineID tli)
{
        /*
         * We have reached the end of base backup, as indicated by pg_control. The
         * data on disk is now consistent (unless minRecovery point is further
         * ahead, which can happen if we crashed during previous recovery).  Reset
         * backupStartPoint and backupEndPoint, and update minRecoveryPoint to
         * make sure we don't allow starting up at an earlier point even if
         * recovery is stopped and restarted soon after this.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

        if (ControlFile->minRecoveryPoint < EndRecPtr)
        {
                ControlFile->minRecoveryPoint = EndRecPtr;
                ControlFile->minRecoveryPointTLI = tli;
        }

        ControlFile->backupStartPoint = InvalidXLogRecPtr;
        ControlFile->backupEndPoint = InvalidXLogRecPtr;
        ControlFile->backupEndRequired = false;
        UpdateControlFile();

        LWLockRelease(ControlFileLock);
}

/*
 * Perform whatever XLOG actions are necessary at end of REDO.
 *
 * The goal here is to make sure that we'll be able to recover properly if
 * we crash again. If we choose to write a checkpoint, we'll write a shutdown
 * checkpoint rather than an on-line one. This is not particularly critical,
 * but since we may be assigning a new TLI, using a shutdown checkpoint allows
 * us to have the rule that TLI only changes in shutdown checkpoints, which
 * allows some extra error checking in xlog_redo.
 */
static bool
PerformRecoveryXLogAction(void)
{
        bool            promoted = false;

        /*
         * Perform a checkpoint to update all our recovery activity to disk.
         *
         * Note that we write a shutdown checkpoint rather than an on-line one.
         * This is not particularly critical, but since we may be assigning a new
         * TLI, using a shutdown checkpoint allows us to have the rule that TLI
         * only changes in shutdown checkpoints, which allows some extra error
         * checking in xlog_redo.
         *
         * In promotion, only create a lightweight end-of-recovery record instead
         * of a full checkpoint. A checkpoint is requested later, after we're
         * fully out of recovery mode and already accepting queries.
         */
        if (ArchiveRecoveryRequested && IsUnderPostmaster &&
                PromoteIsTriggered())
        {
                promoted = true;

                /*
                 * Insert a special WAL record to mark the end of recovery, since we
                 * aren't doing a checkpoint. That means that the checkpointer process
                 * may likely be in the middle of a time-smoothed restartpoint and
                 * could continue to be for minutes after this.  That sounds strange,
                 * but the effect is roughly the same and it would be stranger to try
                 * to come out of the restartpoint and then checkpoint. We request a
                 * checkpoint later anyway, just for safety.
                 */
                CreateEndOfRecoveryRecord();
        }
        else
        {
                RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |
                                                  CHECKPOINT_IMMEDIATE |
                                                  CHECKPOINT_WAIT);
        }

        return promoted;
}

/*
 * Is the system still in recovery?
 *
 * Unlike testing InRecovery, this works in any process that's connected to
 * shared memory.
 */
bool
RecoveryInProgress(void)
{
        /*
         * We check shared state each time only until we leave recovery mode. We
         * can't re-enter recovery, so there's no need to keep checking after the
         * shared variable has once been seen false.
         */
        if (!LocalRecoveryInProgress)
                return false;
        else
        {
                /*
                 * use volatile pointer to make sure we make a fresh read of the
                 * shared variable.
                 */
                volatile XLogCtlData *xlogctl = XLogCtl;

                LocalRecoveryInProgress = (xlogctl->SharedRecoveryState != RECOVERY_STATE_DONE);

                /*
                 * Note: We don't need a memory barrier when we're still in recovery.
                 * We might exit recovery immediately after return, so the caller
                 * can't rely on 'true' meaning that we're still in recovery anyway.
                 */

                return LocalRecoveryInProgress;
        }
}

/*
 * Returns current recovery state from shared memory.
 *
 * This returned state is kept consistent with the contents of the control
 * file.  See details about the possible values of RecoveryState in xlog.h.
 */
RecoveryState
GetRecoveryState(void)
{
        RecoveryState retval;

        SpinLockAcquire(&XLogCtl->info_lck);
        retval = XLogCtl->SharedRecoveryState;
        SpinLockRelease(&XLogCtl->info_lck);

        return retval;
}

/*
 * Is this process allowed to insert new WAL records?
 *
 * Ordinarily this is essentially equivalent to !RecoveryInProgress().
 * But we also have provisions for forcing the result "true" or "false"
 * within specific processes regardless of the global state.
 */
bool
XLogInsertAllowed(void)
{
        /*
         * If value is "unconditionally true" or "unconditionally false", just
         * return it.  This provides the normal fast path once recovery is known
         * done.
         */
        if (LocalXLogInsertAllowed >= 0)
                return (bool) LocalXLogInsertAllowed;

        /*
         * Else, must check to see if we're still in recovery.
         */
        if (RecoveryInProgress())
                return false;

        /*
         * On exit from recovery, reset to "unconditionally true", since there is
         * no need to keep checking.
         */
        LocalXLogInsertAllowed = 1;
        return true;
}

/*
 * Make XLogInsertAllowed() return true in the current process only.
 *
 * Note: it is allowed to switch LocalXLogInsertAllowed back to -1 later,
 * and even call LocalSetXLogInsertAllowed() again after that.
 *
 * Returns the previous value of LocalXLogInsertAllowed.
 */
static int
LocalSetXLogInsertAllowed(void)
{
        int                     oldXLogAllowed = LocalXLogInsertAllowed;

        LocalXLogInsertAllowed = 1;

        return oldXLogAllowed;
}

/*
 * Return the current Redo pointer from shared memory.
 *
 * As a side-effect, the local RedoRecPtr copy is updated.
 */
XLogRecPtr
GetRedoRecPtr(void)
{
        XLogRecPtr      ptr;

        /*
         * The possibly not up-to-date copy in XlogCtl is enough. Even if we
         * grabbed a WAL insertion lock to read the authoritative value in
         * Insert->RedoRecPtr, someone might update it just after we've released
         * the lock.
         */
        SpinLockAcquire(&XLogCtl->info_lck);
        ptr = XLogCtl->RedoRecPtr;
        SpinLockRelease(&XLogCtl->info_lck);

        if (RedoRecPtr < ptr)
                RedoRecPtr = ptr;

        return RedoRecPtr;
}

/*
 * Return information needed to decide whether a modified block needs a
 * full-page image to be included in the WAL record.
 *
 * The returned values are cached copies from backend-private memory, and
 * possibly out-of-date or, indeed, uninitialized, in which case they will
 * be InvalidXLogRecPtr and false, respectively.  XLogInsertRecord will
 * re-check them against up-to-date values, while holding the WAL insert lock.
 */
void
GetFullPageWriteInfo(XLogRecPtr *RedoRecPtr_p, bool *doPageWrites_p)
{
        *RedoRecPtr_p = RedoRecPtr;
        *doPageWrites_p = doPageWrites;
}

/*
 * GetInsertRecPtr -- Returns the current insert position.
 *
 * NOTE: The value *actually* returned is the position of the last full
 * xlog page. It lags behind the real insert position by at most 1 page.
 * For that, we don't need to scan through WAL insertion locks, and an
 * approximation is enough for the current usage of this function.
 */
XLogRecPtr
GetInsertRecPtr(void)
{
        XLogRecPtr      recptr;

        SpinLockAcquire(&XLogCtl->info_lck);
        recptr = XLogCtl->LogwrtRqst.Write;
        SpinLockRelease(&XLogCtl->info_lck);

        return recptr;
}

/*
 * GetFlushRecPtr -- Returns the current flush position, ie, the last WAL
 * position known to be fsync'd to disk. This should only be used on a
 * system that is known not to be in recovery.
 */
XLogRecPtr
GetFlushRecPtr(TimeLineID *insertTLI)
{
        Assert(XLogCtl->SharedRecoveryState == RECOVERY_STATE_DONE);

        SpinLockAcquire(&XLogCtl->info_lck);
        LogwrtResult = XLogCtl->LogwrtResult;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * If we're writing and flushing WAL, the time line can't be changing, so
         * no lock is required.
         */
        if (insertTLI)
                *insertTLI = XLogCtl->InsertTimeLineID;

        return LogwrtResult.Flush;
}

/*
 * GetWALInsertionTimeLine -- Returns the current timeline of a system that
 * is not in recovery.
 */
TimeLineID
GetWALInsertionTimeLine(void)
{
        Assert(XLogCtl->SharedRecoveryState == RECOVERY_STATE_DONE);

        /* Since the value can't be changing, no lock is required. */
        return XLogCtl->InsertTimeLineID;
}

/*
 * GetLastImportantRecPtr -- Returns the LSN of the last important record
 * inserted. All records not explicitly marked as unimportant are considered
 * important.
 *
 * The LSN is determined by computing the maximum of
 * WALInsertLocks[i].lastImportantAt.
 */
XLogRecPtr
GetLastImportantRecPtr(void)
{
        XLogRecPtr      res = InvalidXLogRecPtr;
        int                     i;

        for (i = 0; i < NUM_XLOGINSERT_LOCKS; i++)
        {
                XLogRecPtr      last_important;

                /*
                 * Need to take a lock to prevent torn reads of the LSN, which are
                 * possible on some of the supported platforms. WAL insert locks only
                 * support exclusive mode, so we have to use that.
                 */
                LWLockAcquire(&WALInsertLocks[i].l.lock, LW_EXCLUSIVE);
                last_important = WALInsertLocks[i].l.lastImportantAt;
                LWLockRelease(&WALInsertLocks[i].l.lock);

                if (res < last_important)
                        res = last_important;
        }

        return res;
}

/*
 * Get the time and LSN of the last xlog segment switch
 */
pg_time_t
GetLastSegSwitchData(XLogRecPtr *lastSwitchLSN)
{
        pg_time_t       result;

        /* Need WALWriteLock, but shared lock is sufficient */
        LWLockAcquire(WALWriteLock, LW_SHARED);
        result = XLogCtl->lastSegSwitchTime;
        *lastSwitchLSN = XLogCtl->lastSegSwitchLSN;
        LWLockRelease(WALWriteLock);

        return result;
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownXLOG(int code, Datum arg)
{
        /*
         * We should have an aux process resource owner to use, and we should not
         * be in a transaction that's installed some other resowner.
         */
        Assert(AuxProcessResourceOwner != NULL);
        Assert(CurrentResourceOwner == NULL ||
                   CurrentResourceOwner == AuxProcessResourceOwner);
        CurrentResourceOwner = AuxProcessResourceOwner;

        /* Don't be chatty in standalone mode */
        ereport(IsPostmasterEnvironment ? LOG : NOTICE,
                        (errmsg("shutting down")));

        /*
         * Signal walsenders to move to stopping state.
         */
        WalSndInitStopping();

        /*
         * Wait for WAL senders to be in stopping state.  This prevents commands
         * from writing new WAL.
         */
        WalSndWaitStopping();

        if (RecoveryInProgress())
                CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
        else
        {
                /*
                 * If archiving is enabled, rotate the last XLOG file so that all the
                 * remaining records are archived (postmaster wakes up the archiver
                 * process one more time at the end of shutdown). The checkpoint
                 * record will go to the next XLOG file and won't be archived (yet).
                 */
                if (XLogArchivingActive())
                        RequestXLogSwitch(false);

                CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
        }
}

/*
 * Log start of a checkpoint.
 */
static void
LogCheckpointStart(int flags, bool restartpoint)
{
        if (restartpoint)
                ereport(LOG,
                /* translator: the placeholders show checkpoint options */
                                (errmsg("restartpoint starting:%s%s%s%s%s%s%s%s",
                                                (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
                                                (flags & CHECKPOINT_END_OF_RECOVERY) ? " end-of-recovery" : "",
                                                (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
                                                (flags & CHECKPOINT_FORCE) ? " force" : "",
                                                (flags & CHECKPOINT_WAIT) ? " wait" : "",
                                                (flags & CHECKPOINT_CAUSE_XLOG) ? " wal" : "",
                                                (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "",
                                                (flags & CHECKPOINT_FLUSH_ALL) ? " flush-all" : "")));
        else
                ereport(LOG,
                /* translator: the placeholders show checkpoint options */
                                (errmsg("checkpoint starting:%s%s%s%s%s%s%s%s",
                                                (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
                                                (flags & CHECKPOINT_END_OF_RECOVERY) ? " end-of-recovery" : "",
                                                (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
                                                (flags & CHECKPOINT_FORCE) ? " force" : "",
                                                (flags & CHECKPOINT_WAIT) ? " wait" : "",
                                                (flags & CHECKPOINT_CAUSE_XLOG) ? " wal" : "",
                                                (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "",
                                                (flags & CHECKPOINT_FLUSH_ALL) ? " flush-all" : "")));
}

/*
 * Log end of a checkpoint.
 */
static void
LogCheckpointEnd(bool restartpoint)
{
        long            write_msecs,
                                sync_msecs,
                                total_msecs,
                                longest_msecs,
                                average_msecs;
        uint64          average_sync_time;

        CheckpointStats.ckpt_end_t = GetCurrentTimestamp();

        write_msecs = TimestampDifferenceMilliseconds(CheckpointStats.ckpt_write_t,
                                                                                                  CheckpointStats.ckpt_sync_t);

        sync_msecs = TimestampDifferenceMilliseconds(CheckpointStats.ckpt_sync_t,
                                                                                                 CheckpointStats.ckpt_sync_end_t);

        /* Accumulate checkpoint timing summary data, in milliseconds. */
        PendingCheckpointerStats.m_checkpoint_write_time += write_msecs;
        PendingCheckpointerStats.m_checkpoint_sync_time += sync_msecs;

        /*
         * All of the published timing statistics are accounted for.  Only
         * continue if a log message is to be written.
         */
        if (!log_checkpoints)
                return;

        total_msecs = TimestampDifferenceMilliseconds(CheckpointStats.ckpt_start_t,
                                                                                                  CheckpointStats.ckpt_end_t);

        /*
         * Timing values returned from CheckpointStats are in microseconds.
         * Convert to milliseconds for consistent printing.
         */
        longest_msecs = (long) ((CheckpointStats.ckpt_longest_sync + 999) / 1000);

        average_sync_time = 0;
        if (CheckpointStats.ckpt_sync_rels > 0)
                average_sync_time = CheckpointStats.ckpt_agg_sync_time /
                        CheckpointStats.ckpt_sync_rels;
        average_msecs = (long) ((average_sync_time + 999) / 1000);

        if (restartpoint)
                ereport(LOG,
                                (errmsg("restartpoint complete: wrote %d buffers (%.1f%%); "
                                                "%d WAL file(s) added, %d removed, %d recycled; "
                                                "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
                                                "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s; "
                                                "distance=%d kB, estimate=%d kB",
                                                CheckpointStats.ckpt_bufs_written,
                                                (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
                                                CheckpointStats.ckpt_segs_added,
                                                CheckpointStats.ckpt_segs_removed,
                                                CheckpointStats.ckpt_segs_recycled,
                                                write_msecs / 1000, (int) (write_msecs % 1000),
                                                sync_msecs / 1000, (int) (sync_msecs % 1000),
                                                total_msecs / 1000, (int) (total_msecs % 1000),
                                                CheckpointStats.ckpt_sync_rels,
                                                longest_msecs / 1000, (int) (longest_msecs % 1000),
                                                average_msecs / 1000, (int) (average_msecs % 1000),
                                                (int) (PrevCheckPointDistance / 1024.0),
                                                (int) (CheckPointDistanceEstimate / 1024.0))));
        else
                ereport(LOG,
                                (errmsg("checkpoint complete: wrote %d buffers (%.1f%%); "
                                                "%d WAL file(s) added, %d removed, %d recycled; "
                                                "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
                                                "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s; "
                                                "distance=%d kB, estimate=%d kB",
                                                CheckpointStats.ckpt_bufs_written,
                                                (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
                                                CheckpointStats.ckpt_segs_added,
                                                CheckpointStats.ckpt_segs_removed,
                                                CheckpointStats.ckpt_segs_recycled,
                                                write_msecs / 1000, (int) (write_msecs % 1000),
                                                sync_msecs / 1000, (int) (sync_msecs % 1000),
                                                total_msecs / 1000, (int) (total_msecs % 1000),
                                                CheckpointStats.ckpt_sync_rels,
                                                longest_msecs / 1000, (int) (longest_msecs % 1000),
                                                average_msecs / 1000, (int) (average_msecs % 1000),
                                                (int) (PrevCheckPointDistance / 1024.0),
                                                (int) (CheckPointDistanceEstimate / 1024.0))));
}

/*
 * Update the estimate of distance between checkpoints.
 *
 * The estimate is used to calculate the number of WAL segments to keep
 * preallocated, see XLOGfileslop().
 */
static void
UpdateCheckPointDistanceEstimate(uint64 nbytes)
{
        /*
         * To estimate the number of segments consumed between checkpoints, keep a
         * moving average of the amount of WAL generated in previous checkpoint
         * cycles. However, if the load is bursty, with quiet periods and busy
         * periods, we want to cater for the peak load. So instead of a plain
         * moving average, let the average decline slowly if the previous cycle
         * used less WAL than estimated, but bump it up immediately if it used
         * more.
         *
         * When checkpoints are triggered by max_wal_size, this should converge to
         * CheckpointSegments * wal_segment_size,
         *
         * Note: This doesn't pay any attention to what caused the checkpoint.
         * Checkpoints triggered manually with CHECKPOINT command, or by e.g.
         * starting a base backup, are counted the same as those created
         * automatically. The slow-decline will largely mask them out, if they are
         * not frequent. If they are frequent, it seems reasonable to count them
         * in as any others; if you issue a manual checkpoint every 5 minutes and
         * never let a timed checkpoint happen, it makes sense to base the
         * preallocation on that 5 minute interval rather than whatever
         * checkpoint_timeout is set to.
         */
        PrevCheckPointDistance = nbytes;
        if (CheckPointDistanceEstimate < nbytes)
                CheckPointDistanceEstimate = nbytes;
        else
                CheckPointDistanceEstimate =
                        (0.90 * CheckPointDistanceEstimate + 0.10 * (double) nbytes);
}

/*
 * Update the ps display for a process running a checkpoint.  Note that
 * this routine should not do any allocations so as it can be called
 * from a critical section.
 */
static void
update_checkpoint_display(int flags, bool restartpoint, bool reset)
{
        /*
         * The status is reported only for end-of-recovery and shutdown
         * checkpoints or shutdown restartpoints.  Updating the ps display is
         * useful in those situations as it may not be possible to rely on
         * pg_stat_activity to see the status of the checkpointer or the startup
         * process.
         */
        if ((flags & (CHECKPOINT_END_OF_RECOVERY | CHECKPOINT_IS_SHUTDOWN)) == 0)
                return;

        if (reset)
                set_ps_display("");
        else
        {
                char            activitymsg[128];

                snprintf(activitymsg, sizeof(activitymsg), "performing %s%s%s",
                                 (flags & CHECKPOINT_END_OF_RECOVERY) ? "end-of-recovery " : "",
                                 (flags & CHECKPOINT_IS_SHUTDOWN) ? "shutdown " : "",
                                 restartpoint ? "restartpoint" : "checkpoint");
                set_ps_display(activitymsg);
        }
}


/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 *
 * flags is a bitwise OR of the following:
 *      CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
 *      CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
 *      CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
 *              ignoring checkpoint_completion_target parameter.
 *      CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occurred
 *              since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
 *              CHECKPOINT_END_OF_RECOVERY).
 *      CHECKPOINT_FLUSH_ALL: also flush buffers of unlogged tables.
 *
 * Note: flags contains other bits, of interest here only for logging purposes.
 * In particular note that this routine is synchronous and does not pay
 * attention to CHECKPOINT_WAIT.
 *
 * If !shutdown then we are writing an online checkpoint. This is a very special
 * kind of operation and WAL record because the checkpoint action occurs over
 * a period of time yet logically occurs at just a single LSN. The logical
 * position of the WAL record (redo ptr) is the same or earlier than the
 * physical position. When we replay WAL we locate the checkpoint via its
 * physical position then read the redo ptr and actually start replay at the
 * earlier logical position. Note that we don't write *anything* to WAL at
 * the logical position, so that location could be any other kind of WAL record.
 * All of this mechanism allows us to continue working while we checkpoint.
 * As a result, timing of actions is critical here and be careful to note that
 * this function will likely take minutes to execute on a busy system.
 */
void
CreateCheckPoint(int flags)
{
        bool            shutdown;
        CheckPoint      checkPoint;
        XLogRecPtr      recptr;
        XLogSegNo       _logSegNo;
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        uint32          freespace;
        XLogRecPtr      PriorRedoPtr;
        XLogRecPtr      curInsert;
        XLogRecPtr      last_important_lsn;
        VirtualTransactionId *vxids;
        int                     nvxids;
        int                     oldXLogAllowed = 0;

        /*
         * An end-of-recovery checkpoint is really a shutdown checkpoint, just
         * issued at a different time.
         */
        if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
                shutdown = true;
        else
                shutdown = false;

        /* sanity check */
        if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
                elog(ERROR, "can't create a checkpoint during recovery");

        /*
         * Prepare to accumulate statistics.
         *
         * Note: because it is possible for log_checkpoints to change while a
         * checkpoint proceeds, we always accumulate stats, even if
         * log_checkpoints is currently off.
         */
        MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
        CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

        /*
         * Let smgr prepare for checkpoint; this has to happen outside the
         * critical section and before we determine the REDO pointer.  Note that
         * smgr must not do anything that'd have to be undone if we decide no
         * checkpoint is needed.
         */
        SyncPreCheckpoint();

        /*
         * Use a critical section to force system panic if we have trouble.
         */
        START_CRIT_SECTION();

        if (shutdown)
        {
                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                ControlFile->state = DB_SHUTDOWNING;
                UpdateControlFile();
                LWLockRelease(ControlFileLock);
        }

        /* Begin filling in the checkpoint WAL record */
        MemSet(&checkPoint, 0, sizeof(checkPoint));
        checkPoint.time = (pg_time_t) time(NULL);

        /*
         * For Hot Standby, derive the oldestActiveXid before we fix the redo
         * pointer. This allows us to begin accumulating changes to assemble our
         * starting snapshot of locks and transactions.
         */
        if (!shutdown && XLogStandbyInfoActive())
                checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
        else
                checkPoint.oldestActiveXid = InvalidTransactionId;

        /*
         * Get location of last important record before acquiring insert locks (as
         * GetLastImportantRecPtr() also locks WAL locks).
         */
        last_important_lsn = GetLastImportantRecPtr();

        /*
         * We must block concurrent insertions while examining insert state to
         * determine the checkpoint REDO pointer.
         */
        WALInsertLockAcquireExclusive();
        curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);

        /*
         * If this isn't a shutdown or forced checkpoint, and if there has been no
         * WAL activity requiring a checkpoint, skip it.  The idea here is to
         * avoid inserting duplicate checkpoints when the system is idle.
         */
        if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
                                  CHECKPOINT_FORCE)) == 0)
        {
                if (last_important_lsn == ControlFile->checkPoint)
                {
                        WALInsertLockRelease();
                        END_CRIT_SECTION();
                        ereport(DEBUG1,
                                        (errmsg_internal("checkpoint skipped because system is idle")));
                        return;
                }
        }

        /*
         * An end-of-recovery checkpoint is created before anyone is allowed to
         * write WAL. To allow us to write the checkpoint record, temporarily
         * enable XLogInsertAllowed.
         */
        if (flags & CHECKPOINT_END_OF_RECOVERY)
                oldXLogAllowed = LocalSetXLogInsertAllowed();

        checkPoint.ThisTimeLineID = XLogCtl->InsertTimeLineID;
        if (flags & CHECKPOINT_END_OF_RECOVERY)
                checkPoint.PrevTimeLineID = XLogCtl->PrevTimeLineID;
        else
                checkPoint.PrevTimeLineID = checkPoint.ThisTimeLineID;

        checkPoint.fullPageWrites = Insert->fullPageWrites;

        /*
         * Compute new REDO record ptr = location of next XLOG record.
         *
         * NB: this is NOT necessarily where the checkpoint record itself will be,
         * since other backends may insert more XLOG records while we're off doing
         * the buffer flush work.  Those XLOG records are logically after the
         * checkpoint, even though physically before it.  Got that?
         */
        freespace = INSERT_FREESPACE(curInsert);
        if (freespace == 0)
        {
                if (XLogSegmentOffset(curInsert, wal_segment_size) == 0)
                        curInsert += SizeOfXLogLongPHD;
                else
                        curInsert += SizeOfXLogShortPHD;
        }
        checkPoint.redo = curInsert;

        /*
         * Here we update the shared RedoRecPtr for future XLogInsert calls; this
         * must be done while holding all the insertion locks.
         *
         * Note: if we fail to complete the checkpoint, RedoRecPtr will be left
         * pointing past where it really needs to point.  This is okay; the only
         * consequence is that XLogInsert might back up whole buffers that it
         * didn't really need to.  We can't postpone advancing RedoRecPtr because
         * XLogInserts that happen while we are dumping buffers must assume that
         * their buffer changes are not included in the checkpoint.
         */
        RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;

        /*
         * Now we can release the WAL insertion locks, allowing other xacts to
         * proceed while we are flushing disk buffers.
         */
        WALInsertLockRelease();

        /* Update the info_lck-protected copy of RedoRecPtr as well */
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->RedoRecPtr = checkPoint.redo;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * If enabled, log checkpoint start.  We postpone this until now so as not
         * to log anything if we decided to skip the checkpoint.
         */
        if (log_checkpoints)
                LogCheckpointStart(flags, false);

        /* Update the process title */
        update_checkpoint_display(flags, false, false);

        TRACE_POSTGRESQL_CHECKPOINT_START(flags);

        /*
         * Get the other info we need for the checkpoint record.
         *
         * We don't need to save oldestClogXid in the checkpoint, it only matters
         * for the short period in which clog is being truncated, and if we crash
         * during that we'll redo the clog truncation and fix up oldestClogXid
         * there.
         */
        LWLockAcquire(XidGenLock, LW_SHARED);
        checkPoint.nextXid = ShmemVariableCache->nextXid;
        checkPoint.oldestXid = ShmemVariableCache->oldestXid;
        checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
        LWLockRelease(XidGenLock);

        LWLockAcquire(CommitTsLock, LW_SHARED);
        checkPoint.oldestCommitTsXid = ShmemVariableCache->oldestCommitTsXid;
        checkPoint.newestCommitTsXid = ShmemVariableCache->newestCommitTsXid;
        LWLockRelease(CommitTsLock);

        LWLockAcquire(OidGenLock, LW_SHARED);
        checkPoint.nextOid = ShmemVariableCache->nextOid;
        if (!shutdown)
                checkPoint.nextOid += ShmemVariableCache->oidCount;
        LWLockRelease(OidGenLock);

        MultiXactGetCheckptMulti(shutdown,
                                                         &checkPoint.nextMulti,
                                                         &checkPoint.nextMultiOffset,
                                                         &checkPoint.oldestMulti,
                                                         &checkPoint.oldestMultiDB);

        /*
         * Having constructed the checkpoint record, ensure all shmem disk buffers
         * and commit-log buffers are flushed to disk.
         *
         * This I/O could fail for various reasons.  If so, we will fail to
         * complete the checkpoint, but there is no reason to force a system
         * panic. Accordingly, exit critical section while doing it.
         */
        END_CRIT_SECTION();

        /*
         * In some cases there are groups of actions that must all occur on one
         * side or the other of a checkpoint record. Before flushing the
         * checkpoint record we must explicitly wait for any backend currently
         * performing those groups of actions.
         *
         * One example is end of transaction, so we must wait for any transactions
         * that are currently in commit critical sections.  If an xact inserted
         * its commit record into XLOG just before the REDO point, then a crash
         * restart from the REDO point would not replay that record, which means
         * that our flushing had better include the xact's update of pg_xact.  So
         * we wait till he's out of his commit critical section before proceeding.
         * See notes in RecordTransactionCommit().
         *
         * Because we've already released the insertion locks, this test is a bit
         * fuzzy: it is possible that we will wait for xacts we didn't really need
         * to wait for.  But the delay should be short and it seems better to make
         * checkpoint take a bit longer than to hold off insertions longer than
         * necessary. (In fact, the whole reason we have this issue is that xact.c
         * does commit record XLOG insertion and clog update as two separate steps
         * protected by different locks, but again that seems best on grounds of
         * minimizing lock contention.)
         *
         * A transaction that has not yet set delayChkpt when we look cannot be at
         * risk, since he's not inserted his commit record yet; and one that's
         * already cleared it is not at risk either, since he's done fixing clog
         * and we will correctly flush the update below.  So we cannot miss any
         * xacts we need to wait for.
         */
        vxids = GetVirtualXIDsDelayingChkpt(&nvxids);
        if (nvxids > 0)
        {
                do
                {
                        pg_usleep(10000L);      /* wait for 10 msec */
                } while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids));
        }
        pfree(vxids);

        CheckPointGuts(checkPoint.redo, flags);

        /*
         * Take a snapshot of running transactions and write this to WAL. This
         * allows us to reconstruct the state of running transactions during
         * archive recovery, if required. Skip, if this info disabled.
         *
         * If we are shutting down, or Startup process is completing crash
         * recovery we don't need to write running xact data.
         */
        if (!shutdown && XLogStandbyInfoActive())
                LogStandbySnapshot();

        START_CRIT_SECTION();

        /*
         * Now insert the checkpoint record into XLOG.
         */
        XLogBeginInsert();
        XLogRegisterData((char *) (&checkPoint), sizeof(checkPoint));
        recptr = XLogInsert(RM_XLOG_ID,
                                                shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
                                                XLOG_CHECKPOINT_ONLINE);

        XLogFlush(recptr);

        /*
         * We mustn't write any new WAL after a shutdown checkpoint, or it will be
         * overwritten at next startup.  No-one should even try, this just allows
         * sanity-checking.  In the case of an end-of-recovery checkpoint, we want
         * to just temporarily disable writing until the system has exited
         * recovery.
         */
        if (shutdown)
        {
                if (flags & CHECKPOINT_END_OF_RECOVERY)
                        LocalXLogInsertAllowed = oldXLogAllowed;
                else
                        LocalXLogInsertAllowed = 0; /* never again write WAL */
        }

        /*
         * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
         * = end of actual checkpoint record.
         */
        if (shutdown && checkPoint.redo != ProcLastRecPtr)
                ereport(PANIC,
                                (errmsg("concurrent write-ahead log activity while database system is shutting down")));

        /*
         * Remember the prior checkpoint's redo ptr for
         * UpdateCheckPointDistanceEstimate()
         */
        PriorRedoPtr = ControlFile->checkPointCopy.redo;

        /*
         * Update the control file.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        if (shutdown)
                ControlFile->state = DB_SHUTDOWNED;
        ControlFile->checkPoint = ProcLastRecPtr;
        ControlFile->checkPointCopy = checkPoint;
        /* crash recovery should always recover to the end of WAL */
        ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
        ControlFile->minRecoveryPointTLI = 0;

        /*
         * Persist unloggedLSN value. It's reset on crash recovery, so this goes
         * unused on non-shutdown checkpoints, but seems useful to store it always
         * for debugging purposes.
         */
        SpinLockAcquire(&XLogCtl->ulsn_lck);
        ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
        SpinLockRelease(&XLogCtl->ulsn_lck);

        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        /* Update shared-memory copy of checkpoint XID/epoch */
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->ckptFullXid = checkPoint.nextXid;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * We are now done with critical updates; no need for system panic if we
         * have trouble while fooling with old log segments.
         */
        END_CRIT_SECTION();

        /*
         * Let smgr do post-checkpoint cleanup (eg, deleting old files).
         */
        SyncPostCheckpoint();

        /*
         * Update the average distance between checkpoints if the prior checkpoint
         * exists.
         */
        if (PriorRedoPtr != InvalidXLogRecPtr)
                UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);

        /*
         * Delete old log files, those no longer needed for last checkpoint to
         * prevent the disk holding the xlog from growing full.
         */
        XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);
        KeepLogSeg(recptr, &_logSegNo);
        if (InvalidateObsoleteReplicationSlots(_logSegNo))
        {
                /*
                 * Some slots have been invalidated; recalculate the old-segment
                 * horizon, starting again from RedoRecPtr.
                 */
                XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);
                KeepLogSeg(recptr, &_logSegNo);
        }
        _logSegNo--;
        RemoveOldXlogFiles(_logSegNo, RedoRecPtr, recptr,
                                           checkPoint.ThisTimeLineID);

        /*
         * Make more log segments if needed.  (Do this after recycling old log
         * segments, since that may supply some of the needed files.)
         */
        if (!shutdown)
                PreallocXlogFiles(recptr, checkPoint.ThisTimeLineID);

        /*
         * Truncate pg_subtrans if possible.  We can throw away all data before
         * the oldest XMIN of any running transaction.  No future transaction will
         * attempt to reference any pg_subtrans entry older than that (see Asserts
         * in subtrans.c).  During recovery, though, we mustn't do this because
         * StartupSUBTRANS hasn't been called yet.
         */
        if (!RecoveryInProgress())
                TruncateSUBTRANS(GetOldestTransactionIdConsideredRunning());

        /* Real work is done; log and update stats. */
        LogCheckpointEnd(false);

        /* Reset the process title */
        update_checkpoint_display(flags, false, true);

        TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
                                                                         NBuffers,
                                                                         CheckpointStats.ckpt_segs_added,
                                                                         CheckpointStats.ckpt_segs_removed,
                                                                         CheckpointStats.ckpt_segs_recycled);
}

/*
 * Mark the end of recovery in WAL though without running a full checkpoint.
 * We can expect that a restartpoint is likely to be in progress as we
 * do this, though we are unwilling to wait for it to complete.
 *
 * CreateRestartPoint() allows for the case where recovery may end before
 * the restartpoint completes so there is no concern of concurrent behaviour.
 */
static void
CreateEndOfRecoveryRecord(void)
{
        xl_end_of_recovery xlrec;
        XLogRecPtr      recptr;

        /* sanity check */
        if (!RecoveryInProgress())
                elog(ERROR, "can only be used to end recovery");

        xlrec.end_time = GetCurrentTimestamp();

        WALInsertLockAcquireExclusive();
        xlrec.ThisTimeLineID = XLogCtl->InsertTimeLineID;
        xlrec.PrevTimeLineID = XLogCtl->PrevTimeLineID;
        WALInsertLockRelease();

        START_CRIT_SECTION();

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, sizeof(xl_end_of_recovery));
        recptr = XLogInsert(RM_XLOG_ID, XLOG_END_OF_RECOVERY);

        XLogFlush(recptr);

        /*
         * Update the control file so that crash recovery can follow the timeline
         * changes to this point.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->minRecoveryPoint = recptr;
        ControlFile->minRecoveryPointTLI = xlrec.ThisTimeLineID;
        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        END_CRIT_SECTION();
}

/*
 * Write an OVERWRITE_CONTRECORD message.
 *
 * When on WAL replay we expect a continuation record at the start of a page
 * that is not there, recovery ends and WAL writing resumes at that point.
 * But it's wrong to resume writing new WAL back at the start of the record
 * that was broken, because downstream consumers of that WAL (physical
 * replicas) are not prepared to "rewind".  So the first action after
 * finishing replay of all valid WAL must be to write a record of this type
 * at the point where the contrecord was missing; to support xlogreader
 * detecting the special case, XLP_FIRST_IS_OVERWRITE_CONTRECORD is also added
 * to the page header where the record occurs.  xlogreader has an ad-hoc
 * mechanism to report metadata about the broken record, which is what we
 * use here.
 *
 * At replay time, XLP_FIRST_IS_OVERWRITE_CONTRECORD instructs xlogreader to
 * skip the record it was reading, and pass back the LSN of the skipped
 * record, so that its caller can verify (on "replay" of that record) that the
 * XLOG_OVERWRITE_CONTRECORD matches what was effectively overwritten.
 *
 * 'aborted_lsn' is the beginning position of the record that was incomplete.
 * It is included in the WAL record.  'pagePtr' and 'newTLI' point to the
 * beginning of the XLOG page where the record is to be inserted.  They must
 * match the current WAL insert position, they're passed here just so that we
 * can verify that.
 */
static XLogRecPtr
CreateOverwriteContrecordRecord(XLogRecPtr aborted_lsn, XLogRecPtr pagePtr,
                                                                TimeLineID newTLI)
{
        xl_overwrite_contrecord xlrec;
        XLogRecPtr      recptr;
        XLogPageHeader pagehdr;
        XLogRecPtr      startPos;

        /* sanity checks */
        if (!RecoveryInProgress())
                elog(ERROR, "can only be used at end of recovery");
        if (pagePtr % XLOG_BLCKSZ != 0)
                elog(ERROR, "invalid position for missing continuation record %X/%X",
                         LSN_FORMAT_ARGS(pagePtr));

        /* The current WAL insert position should be right after the page header */
        startPos = pagePtr;
        if (XLogSegmentOffset(startPos, wal_segment_size) == 0)
                startPos += SizeOfXLogLongPHD;
        else
                startPos += SizeOfXLogShortPHD;
        recptr = GetXLogInsertRecPtr();
        if (recptr != startPos)
                elog(ERROR, "invalid WAL insert position %X/%X for OVERWRITE_CONTRECORD",
                         LSN_FORMAT_ARGS(recptr));

        START_CRIT_SECTION();

        /*
         * Initialize the XLOG page header (by GetXLogBuffer), and set the
         * XLP_FIRST_IS_OVERWRITE_CONTRECORD flag.
         *
         * No other backend is allowed to write WAL yet, so acquiring the WAL
         * insertion lock is just pro forma.
         */
        WALInsertLockAcquire();
        pagehdr = (XLogPageHeader) GetXLogBuffer(pagePtr, newTLI);
        pagehdr->xlp_info |= XLP_FIRST_IS_OVERWRITE_CONTRECORD;
        WALInsertLockRelease();

        /*
         * Insert the XLOG_OVERWRITE_CONTRECORD record as the first record on the
         * page.  We know it becomes the first record, because no other backend is
         * allowed to write WAL yet.
         */
        XLogBeginInsert();
        xlrec.overwritten_lsn = aborted_lsn;
        xlrec.overwrite_time = GetCurrentTimestamp();
        XLogRegisterData((char *) &xlrec, sizeof(xl_overwrite_contrecord));
        recptr = XLogInsert(RM_XLOG_ID, XLOG_OVERWRITE_CONTRECORD);

        /* check that the record was inserted to the right place */
        if (ProcLastRecPtr != startPos)
                elog(ERROR, "OVERWRITE_CONTRECORD was inserted to unexpected position %X/%X",
                         LSN_FORMAT_ARGS(ProcLastRecPtr));

        XLogFlush(recptr);

        END_CRIT_SECTION();

        return recptr;
}

/*
 * Flush all data in shared memory to disk, and fsync
 *
 * This is the common code shared between regular checkpoints and
 * recovery restartpoints.
 */
static void
CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
{
        CheckPointRelationMap();
        CheckPointReplicationSlots();
        CheckPointSnapBuild();
        CheckPointLogicalRewriteHeap();
        CheckPointReplicationOrigin();

        /* Write out all dirty data in SLRUs and the main buffer pool */
        TRACE_POSTGRESQL_BUFFER_CHECKPOINT_START(flags);
        CheckpointStats.ckpt_write_t = GetCurrentTimestamp();
        CheckPointCLOG();
        CheckPointCommitTs();
        CheckPointSUBTRANS();
        CheckPointMultiXact();
        CheckPointPredicate();
        CheckPointBuffers(flags);

        /* Perform all queued up fsyncs */
        TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
        CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
        ProcessSyncRequests();
        CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
        TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();

        /* We deliberately delay 2PC checkpointing as long as possible */
        CheckPointTwoPhase(checkPointRedo);
}

/*
 * Save a checkpoint for recovery restart if appropriate
 *
 * This function is called each time a checkpoint record is read from XLOG.
 * It must determine whether the checkpoint represents a safe restartpoint or
 * not.  If so, the checkpoint record is stashed in shared memory so that
 * CreateRestartPoint can consult it.  (Note that the latter function is
 * executed by the checkpointer, while this one will be executed by the
 * startup process.)
 */
static void
RecoveryRestartPoint(const CheckPoint *checkPoint, XLogReaderState *record)
{
        /*
         * Also refrain from creating a restartpoint if we have seen any
         * references to non-existent pages. Restarting recovery from the
         * restartpoint would not see the references, so we would lose the
         * cross-check that the pages belonged to a relation that was dropped
         * later.
         */
        if (XLogHaveInvalidPages())
        {
                elog(trace_recovery(DEBUG2),
                         "could not record restart point at %X/%X because there "
                         "are unresolved references to invalid pages",
                         LSN_FORMAT_ARGS(checkPoint->redo));
                return;
        }

        /*
         * Copy the checkpoint record to shared memory, so that checkpointer can
         * work out the next time it wants to perform a restartpoint.
         */
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->lastCheckPointRecPtr = record->ReadRecPtr;
        XLogCtl->lastCheckPointEndPtr = record->EndRecPtr;
        XLogCtl->lastCheckPoint = *checkPoint;
        SpinLockRelease(&XLogCtl->info_lck);
}

/*
 * Establish a restartpoint if possible.
 *
 * This is similar to CreateCheckPoint, but is used during WAL recovery
 * to establish a point from which recovery can roll forward without
 * replaying the entire recovery log.
 *
 * Returns true if a new restartpoint was established. We can only establish
 * a restartpoint if we have replayed a safe checkpoint record since last
 * restartpoint.
 */
bool
CreateRestartPoint(int flags)
{
        XLogRecPtr      lastCheckPointRecPtr;
        XLogRecPtr      lastCheckPointEndPtr;
        CheckPoint      lastCheckPoint;
        XLogRecPtr      PriorRedoPtr;
        XLogRecPtr      receivePtr;
        XLogRecPtr      replayPtr;
        TimeLineID      replayTLI;
        XLogRecPtr      endptr;
        XLogSegNo       _logSegNo;
        TimestampTz xtime;

        /* Get a local copy of the last safe checkpoint record. */
        SpinLockAcquire(&XLogCtl->info_lck);
        lastCheckPointRecPtr = XLogCtl->lastCheckPointRecPtr;
        lastCheckPointEndPtr = XLogCtl->lastCheckPointEndPtr;
        lastCheckPoint = XLogCtl->lastCheckPoint;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * Check that we're still in recovery mode. It's ok if we exit recovery
         * mode after this check, the restart point is valid anyway.
         */
        if (!RecoveryInProgress())
        {
                ereport(DEBUG2,
                                (errmsg_internal("skipping restartpoint, recovery has already ended")));
                return false;
        }

        /*
         * If the last checkpoint record we've replayed is already our last
         * restartpoint, we can't perform a new restart point. We still update
         * minRecoveryPoint in that case, so that if this is a shutdown restart
         * point, we won't start up earlier than before. That's not strictly
         * necessary, but when hot standby is enabled, it would be rather weird if
         * the database opened up for read-only connections at a point-in-time
         * before the last shutdown. Such time travel is still possible in case of
         * immediate shutdown, though.
         *
         * We don't explicitly advance minRecoveryPoint when we do create a
         * restartpoint. It's assumed that flushing the buffers will do that as a
         * side-effect.
         */
        if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) ||
                lastCheckPoint.redo <= ControlFile->checkPointCopy.redo)
        {
                ereport(DEBUG2,
                                (errmsg_internal("skipping restartpoint, already performed at %X/%X",
                                                                 LSN_FORMAT_ARGS(lastCheckPoint.redo))));

                UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);
                if (flags & CHECKPOINT_IS_SHUTDOWN)
                {
                        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                        ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
                        UpdateControlFile();
                        LWLockRelease(ControlFileLock);
                }
                return false;
        }

        /*
         * Update the shared RedoRecPtr so that the startup process can calculate
         * the number of segments replayed since last restartpoint, and request a
         * restartpoint if it exceeds CheckPointSegments.
         *
         * Like in CreateCheckPoint(), hold off insertions to update it, although
         * during recovery this is just pro forma, because no WAL insertions are
         * happening.
         */
        WALInsertLockAcquireExclusive();
        RedoRecPtr = XLogCtl->Insert.RedoRecPtr = lastCheckPoint.redo;
        WALInsertLockRelease();

        /* Also update the info_lck-protected copy */
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->RedoRecPtr = lastCheckPoint.redo;
        SpinLockRelease(&XLogCtl->info_lck);

        /*
         * Prepare to accumulate statistics.
         *
         * Note: because it is possible for log_checkpoints to change while a
         * checkpoint proceeds, we always accumulate stats, even if
         * log_checkpoints is currently off.
         */
        MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
        CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

        if (log_checkpoints)
                LogCheckpointStart(flags, true);

        /* Update the process title */
        update_checkpoint_display(flags, true, false);

        CheckPointGuts(lastCheckPoint.redo, flags);

        /*
         * Remember the prior checkpoint's redo ptr for
         * UpdateCheckPointDistanceEstimate()
         */
        PriorRedoPtr = ControlFile->checkPointCopy.redo;

        /*
         * Update pg_control, using current time.  Check that it still shows
         * DB_IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing;
         * this is a quick hack to make sure nothing really bad happens if somehow
         * we get here after the end-of-recovery checkpoint.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
                ControlFile->checkPointCopy.redo < lastCheckPoint.redo)
        {
                ControlFile->checkPoint = lastCheckPointRecPtr;
                ControlFile->checkPointCopy = lastCheckPoint;

                /*
                 * Ensure minRecoveryPoint is past the checkpoint record.  Normally,
                 * this will have happened already while writing out dirty buffers,
                 * but not necessarily - e.g. because no buffers were dirtied.  We do
                 * this because a non-exclusive base backup uses minRecoveryPoint to
                 * determine which WAL files must be included in the backup, and the
                 * file (or files) containing the checkpoint record must be included,
                 * at a minimum. Note that for an ordinary restart of recovery there's
                 * no value in having the minimum recovery point any earlier than this
                 * anyway, because redo will begin just after the checkpoint record.
                 */
                if (ControlFile->minRecoveryPoint < lastCheckPointEndPtr)
                {
                        ControlFile->minRecoveryPoint = lastCheckPointEndPtr;
                        ControlFile->minRecoveryPointTLI = lastCheckPoint.ThisTimeLineID;

                        /* update local copy */
                        LocalMinRecoveryPoint = ControlFile->minRecoveryPoint;
                        LocalMinRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
                }
                if (flags & CHECKPOINT_IS_SHUTDOWN)
                        ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
                UpdateControlFile();
        }
        LWLockRelease(ControlFileLock);

        /*
         * Update the average distance between checkpoints/restartpoints if the
         * prior checkpoint exists.
         */
        if (PriorRedoPtr != InvalidXLogRecPtr)
                UpdateCheckPointDistanceEstimate(RedoRecPtr - PriorRedoPtr);

        /*
         * Delete old log files, those no longer needed for last restartpoint to
         * prevent the disk holding the xlog from growing full.
         */
        XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);

        /*
         * Retreat _logSegNo using the current end of xlog replayed or received,
         * whichever is later.
         */
        receivePtr = GetWalRcvFlushRecPtr(NULL, NULL);
        replayPtr = GetXLogReplayRecPtr(&replayTLI);
        endptr = (receivePtr < replayPtr) ? replayPtr : receivePtr;
        KeepLogSeg(endptr, &_logSegNo);
        if (InvalidateObsoleteReplicationSlots(_logSegNo))
        {
                /*
                 * Some slots have been invalidated; recalculate the old-segment
                 * horizon, starting again from RedoRecPtr.
                 */
                XLByteToSeg(RedoRecPtr, _logSegNo, wal_segment_size);
                KeepLogSeg(endptr, &_logSegNo);
        }
        _logSegNo--;

        /*
         * Try to recycle segments on a useful timeline. If we've been promoted
         * since the beginning of this restartpoint, use the new timeline chosen
         * at end of recovery.  If we're still in recovery, use the timeline we're
         * currently replaying.
         *
         * There is no guarantee that the WAL segments will be useful on the
         * current timeline; if recovery proceeds to a new timeline right after
         * this, the pre-allocated WAL segments on this timeline will not be used,
         * and will go wasted until recycled on the next restartpoint. We'll live
         * with that.
         */
        if (!RecoveryInProgress())
                replayTLI = XLogCtl->InsertTimeLineID;

        RemoveOldXlogFiles(_logSegNo, RedoRecPtr, endptr, replayTLI);

        /*
         * Make more log segments if needed.  (Do this after recycling old log
         * segments, since that may supply some of the needed files.)
         */
        PreallocXlogFiles(endptr, replayTLI);

        /*
         * Truncate pg_subtrans if possible.  We can throw away all data before
         * the oldest XMIN of any running transaction.  No future transaction will
         * attempt to reference any pg_subtrans entry older than that (see Asserts
         * in subtrans.c).  When hot standby is disabled, though, we mustn't do
         * this because StartupSUBTRANS hasn't been called yet.
         */
        if (EnableHotStandby)
                TruncateSUBTRANS(GetOldestTransactionIdConsideredRunning());

        /* Real work is done; log and update stats. */
        LogCheckpointEnd(true);

        /* Reset the process title */
        update_checkpoint_display(flags, true, true);

        xtime = GetLatestXTime();
        ereport((log_checkpoints ? LOG : DEBUG2),
                        (errmsg("recovery restart point at %X/%X",
                                        LSN_FORMAT_ARGS(lastCheckPoint.redo)),
                         xtime ? errdetail("Last completed transaction was at log time %s.",
                                                           timestamptz_to_str(xtime)) : 0));

        /*
         * Finally, execute archive_cleanup_command, if any.
         */
        if (archiveCleanupCommand && strcmp(archiveCleanupCommand, "") != 0)
                ExecuteRecoveryCommand(archiveCleanupCommand,
                                                           "archive_cleanup_command",
                                                           false,
                                                           WAIT_EVENT_ARCHIVE_CLEANUP_COMMAND);

        return true;
}

/*
 * Report availability of WAL for the given target LSN
 *              (typically a slot's restart_lsn)
 *
 * Returns one of the following enum values:
 *
 * * WALAVAIL_RESERVED means targetLSN is available and it is in the range of
 *   max_wal_size.
 *
 * * WALAVAIL_EXTENDED means it is still available by preserving extra
 *   segments beyond max_wal_size. If max_slot_wal_keep_size is smaller
 *   than max_wal_size, this state is not returned.
 *
 * * WALAVAIL_UNRESERVED means it is being lost and the next checkpoint will
 *   remove reserved segments. The walsender using this slot may return to the
 *   above.
 *
 * * WALAVAIL_REMOVED means it has been removed. A replication stream on
 *   a slot with this LSN cannot continue after a restart.
 *
 * * WALAVAIL_INVALID_LSN means the slot hasn't been set to reserve WAL.
 */
WALAvailability
GetWALAvailability(XLogRecPtr targetLSN)
{
        XLogRecPtr      currpos;                /* current write LSN */
        XLogSegNo       currSeg;                /* segid of currpos */
        XLogSegNo       targetSeg;              /* segid of targetLSN */
        XLogSegNo       oldestSeg;              /* actual oldest segid */
        XLogSegNo       oldestSegMaxWalSize;    /* oldest segid kept by max_wal_size */
        XLogSegNo       oldestSlotSeg;  /* oldest segid kept by slot */
        uint64          keepSegs;

        /*
         * slot does not reserve WAL. Either deactivated, or has never been active
         */
        if (XLogRecPtrIsInvalid(targetLSN))
                return WALAVAIL_INVALID_LSN;

        /*
         * Calculate the oldest segment currently reserved by all slots,
         * considering wal_keep_size and max_slot_wal_keep_size.  Initialize
         * oldestSlotSeg to the current segment.
         */
        currpos = GetXLogWriteRecPtr();
        XLByteToSeg(currpos, oldestSlotSeg, wal_segment_size);
        KeepLogSeg(currpos, &oldestSlotSeg);

        /*
         * Find the oldest extant segment file. We get 1 until checkpoint removes
         * the first WAL segment file since startup, which causes the status being
         * wrong under certain abnormal conditions but that doesn't actually harm.
         */
        oldestSeg = XLogGetLastRemovedSegno() + 1;

        /* calculate oldest segment by max_wal_size */
        XLByteToSeg(currpos, currSeg, wal_segment_size);
        keepSegs = ConvertToXSegs(max_wal_size_mb, wal_segment_size) + 1;

        if (currSeg > keepSegs)
                oldestSegMaxWalSize = currSeg - keepSegs;
        else
                oldestSegMaxWalSize = 1;

        /* the segment we care about */
        XLByteToSeg(targetLSN, targetSeg, wal_segment_size);

        /*
         * No point in returning reserved or extended status values if the
         * targetSeg is known to be lost.
         */
        if (targetSeg >= oldestSlotSeg)
        {
                /* show "reserved" when targetSeg is within max_wal_size */
                if (targetSeg >= oldestSegMaxWalSize)
                        return WALAVAIL_RESERVED;

                /* being retained by slots exceeding max_wal_size */
                return WALAVAIL_EXTENDED;
        }

        /* WAL segments are no longer retained but haven't been removed yet */
        if (targetSeg >= oldestSeg)
                return WALAVAIL_UNRESERVED;

        /* Definitely lost */
        return WALAVAIL_REMOVED;
}


/*
 * Retreat *logSegNo to the last segment that we need to retain because of
 * either wal_keep_size or replication slots.
 *
 * This is calculated by subtracting wal_keep_size from the given xlog
 * location, recptr and by making sure that that result is below the
 * requirement of replication slots.  For the latter criterion we do consider
 * the effects of max_slot_wal_keep_size: reserve at most that much space back
 * from recptr.
 *
 * Note about replication slots: if this function calculates a value
 * that's further ahead than what slots need reserved, then affected
 * slots need to be invalidated and this function invoked again.
 * XXX it might be a good idea to rewrite this function so that
 * invalidation is optionally done here, instead.
 */
static void
KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
{
        XLogSegNo       currSegNo;
        XLogSegNo       segno;
        XLogRecPtr      keep;

        XLByteToSeg(recptr, currSegNo, wal_segment_size);
        segno = currSegNo;

        /*
         * Calculate how many segments are kept by slots first, adjusting for
         * max_slot_wal_keep_size.
         */
        keep = XLogGetReplicationSlotMinimumLSN();
        if (keep != InvalidXLogRecPtr)
        {
                XLByteToSeg(keep, segno, wal_segment_size);

                /* Cap by max_slot_wal_keep_size ... */
                if (max_slot_wal_keep_size_mb >= 0)
                {
                        uint64          slot_keep_segs;

                        slot_keep_segs =
                                ConvertToXSegs(max_slot_wal_keep_size_mb, wal_segment_size);

                        if (currSegNo - segno > slot_keep_segs)
                                segno = currSegNo - slot_keep_segs;
                }
        }

        /* but, keep at least wal_keep_size if that's set */
        if (wal_keep_size_mb > 0)
        {
                uint64          keep_segs;

                keep_segs = ConvertToXSegs(wal_keep_size_mb, wal_segment_size);
                if (currSegNo - segno < keep_segs)
                {
                        /* avoid underflow, don't go below 1 */
                        if (currSegNo <= keep_segs)
                                segno = 1;
                        else
                                segno = currSegNo - keep_segs;
                }
        }

        /* don't delete WAL segments newer than the calculated segment */
        if (segno < *logSegNo)
                *logSegNo = segno;
}

/*
 * Write a NEXTOID log record
 */
void
XLogPutNextOid(Oid nextOid)
{
        XLogBeginInsert();
        XLogRegisterData((char *) (&nextOid), sizeof(Oid));
        (void) XLogInsert(RM_XLOG_ID, XLOG_NEXTOID);

        /*
         * We need not flush the NEXTOID record immediately, because any of the
         * just-allocated OIDs could only reach disk as part of a tuple insert or
         * update that would have its own XLOG record that must follow the NEXTOID
         * record.  Therefore, the standard buffer LSN interlock applied to those
         * records will ensure no such OID reaches disk before the NEXTOID record
         * does.
         *
         * Note, however, that the above statement only covers state "within" the
         * database.  When we use a generated OID as a file or directory name, we
         * are in a sense violating the basic WAL rule, because that filesystem
         * change may reach disk before the NEXTOID WAL record does.  The impact
         * of this is that if a database crash occurs immediately afterward, we
         * might after restart re-generate the same OID and find that it conflicts
         * with the leftover file or directory.  But since for safety's sake we
         * always loop until finding a nonconflicting filename, this poses no real
         * problem in practice. See pgsql-hackers discussion 27-Sep-2006.
         */
}

/*
 * Write an XLOG SWITCH record.
 *
 * Here we just blindly issue an XLogInsert request for the record.
 * All the magic happens inside XLogInsert.
 *
 * The return value is either the end+1 address of the switch record,
 * or the end+1 address of the prior segment if we did not need to
 * write a switch record because we are already at segment start.
 */
XLogRecPtr
RequestXLogSwitch(bool mark_unimportant)
{
        XLogRecPtr      RecPtr;

        /* XLOG SWITCH has no data */
        XLogBeginInsert();

        if (mark_unimportant)
                XLogSetRecordFlags(XLOG_MARK_UNIMPORTANT);
        RecPtr = XLogInsert(RM_XLOG_ID, XLOG_SWITCH);

        return RecPtr;
}

/*
 * Write a RESTORE POINT record
 */
XLogRecPtr
XLogRestorePoint(const char *rpName)
{
        XLogRecPtr      RecPtr;
        xl_restore_point xlrec;

        xlrec.rp_time = GetCurrentTimestamp();
        strlcpy(xlrec.rp_name, rpName, MAXFNAMELEN);

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, sizeof(xl_restore_point));

        RecPtr = XLogInsert(RM_XLOG_ID, XLOG_RESTORE_POINT);

        ereport(LOG,
                        (errmsg("restore point \"%s\" created at %X/%X",
                                        rpName, LSN_FORMAT_ARGS(RecPtr))));

        return RecPtr;
}

/*
 * Check if any of the GUC parameters that are critical for hot standby
 * have changed, and update the value in pg_control file if necessary.
 */
static void
XLogReportParameters(void)
{
        if (wal_level != ControlFile->wal_level ||
                wal_log_hints != ControlFile->wal_log_hints ||
                MaxConnections != ControlFile->MaxConnections ||
                max_worker_processes != ControlFile->max_worker_processes ||
                max_wal_senders != ControlFile->max_wal_senders ||
                max_prepared_xacts != ControlFile->max_prepared_xacts ||
                max_locks_per_xact != ControlFile->max_locks_per_xact ||
                track_commit_timestamp != ControlFile->track_commit_timestamp)
        {
                /*
                 * The change in number of backend slots doesn't need to be WAL-logged
                 * if archiving is not enabled, as you can't start archive recovery
                 * with wal_level=minimal anyway. We don't really care about the
                 * values in pg_control either if wal_level=minimal, but seems better
                 * to keep them up-to-date to avoid confusion.
                 */
                if (wal_level != ControlFile->wal_level || XLogIsNeeded())
                {
                        xl_parameter_change xlrec;
                        XLogRecPtr      recptr;

                        xlrec.MaxConnections = MaxConnections;
                        xlrec.max_worker_processes = max_worker_processes;
                        xlrec.max_wal_senders = max_wal_senders;
                        xlrec.max_prepared_xacts = max_prepared_xacts;
                        xlrec.max_locks_per_xact = max_locks_per_xact;
                        xlrec.wal_level = wal_level;
                        xlrec.wal_log_hints = wal_log_hints;
                        xlrec.track_commit_timestamp = track_commit_timestamp;

                        XLogBeginInsert();
                        XLogRegisterData((char *) &xlrec, sizeof(xlrec));

                        recptr = XLogInsert(RM_XLOG_ID, XLOG_PARAMETER_CHANGE);
                        XLogFlush(recptr);
                }

                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

                ControlFile->MaxConnections = MaxConnections;
                ControlFile->max_worker_processes = max_worker_processes;
                ControlFile->max_wal_senders = max_wal_senders;
                ControlFile->max_prepared_xacts = max_prepared_xacts;
                ControlFile->max_locks_per_xact = max_locks_per_xact;
                ControlFile->wal_level = wal_level;
                ControlFile->wal_log_hints = wal_log_hints;
                ControlFile->track_commit_timestamp = track_commit_timestamp;
                UpdateControlFile();

                LWLockRelease(ControlFileLock);
        }
}

/*
 * Update full_page_writes in shared memory, and write an
 * XLOG_FPW_CHANGE record if necessary.
 *
 * Note: this function assumes there is no other process running
 * concurrently that could update it.
 */
void
UpdateFullPageWrites(void)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        bool            recoveryInProgress;

        /*
         * Do nothing if full_page_writes has not been changed.
         *
         * It's safe to check the shared full_page_writes without the lock,
         * because we assume that there is no concurrently running process which
         * can update it.
         */
        if (fullPageWrites == Insert->fullPageWrites)
                return;

        /*
         * Perform this outside critical section so that the WAL insert
         * initialization done by RecoveryInProgress() doesn't trigger an
         * assertion failure.
         */
        recoveryInProgress = RecoveryInProgress();

        START_CRIT_SECTION();

        /*
         * It's always safe to take full page images, even when not strictly
         * required, but not the other round. So if we're setting full_page_writes
         * to true, first set it true and then write the WAL record. If we're
         * setting it to false, first write the WAL record and then set the global
         * flag.
         */
        if (fullPageWrites)
        {
                WALInsertLockAcquireExclusive();
                Insert->fullPageWrites = true;
                WALInsertLockRelease();
        }

        /*
         * Write an XLOG_FPW_CHANGE record. This allows us to keep track of
         * full_page_writes during archive recovery, if required.
         */
        if (XLogStandbyInfoActive() && !recoveryInProgress)
        {
                XLogBeginInsert();
                XLogRegisterData((char *) (&fullPageWrites), sizeof(bool));

                XLogInsert(RM_XLOG_ID, XLOG_FPW_CHANGE);
        }

        if (!fullPageWrites)
        {
                WALInsertLockAcquireExclusive();
                Insert->fullPageWrites = false;
                WALInsertLockRelease();
        }
        END_CRIT_SECTION();
}

/*
 * XLOG resource manager's routines
 *
 * Definitions of info values are in include/catalog/pg_control.h, though
 * not all record types are related to control file updates.
 *
 * NOTE: Some XLOG record types that are directly related to WAL recovery
 * are handled in xlogrecovery_redo().
 */
void
xlog_redo(XLogReaderState *record)
{
        uint8           info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
        XLogRecPtr      lsn = record->EndRecPtr;

        /*
         * In XLOG rmgr, backup blocks are only used by XLOG_FPI and
         * XLOG_FPI_FOR_HINT records.
         */
        Assert(info == XLOG_FPI || info == XLOG_FPI_FOR_HINT ||
                   !XLogRecHasAnyBlockRefs(record));

        if (info == XLOG_NEXTOID)
        {
                Oid                     nextOid;

                /*
                 * We used to try to take the maximum of ShmemVariableCache->nextOid
                 * and the recorded nextOid, but that fails if the OID counter wraps
                 * around.  Since no OID allocation should be happening during replay
                 * anyway, better to just believe the record exactly.  We still take
                 * OidGenLock while setting the variable, just in case.
                 */
                memcpy(&nextOid, XLogRecGetData(record), sizeof(Oid));
                LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextOid = nextOid;
                ShmemVariableCache->oidCount = 0;
                LWLockRelease(OidGenLock);
        }
        else if (info == XLOG_CHECKPOINT_SHUTDOWN)
        {
                CheckPoint      checkPoint;
                TimeLineID      replayTLI;

                memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                /* In a SHUTDOWN checkpoint, believe the counters exactly */
                LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextXid = checkPoint.nextXid;
                LWLockRelease(XidGenLock);
                LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextOid = checkPoint.nextOid;
                ShmemVariableCache->oidCount = 0;
                LWLockRelease(OidGenLock);
                MultiXactSetNextMXact(checkPoint.nextMulti,
                                                          checkPoint.nextMultiOffset);

                MultiXactAdvanceOldest(checkPoint.oldestMulti,
                                                           checkPoint.oldestMultiDB);

                /*
                 * No need to set oldestClogXid here as well; it'll be set when we
                 * redo an xl_clog_truncate if it changed since initialization.
                 */
                SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);

                /*
                 * If we see a shutdown checkpoint while waiting for an end-of-backup
                 * record, the backup was canceled and the end-of-backup record will
                 * never arrive.
                 */
                if (ArchiveRecoveryRequested &&
                        !XLogRecPtrIsInvalid(ControlFile->backupStartPoint) &&
                        XLogRecPtrIsInvalid(ControlFile->backupEndPoint))
                        ereport(PANIC,
                                        (errmsg("online backup was canceled, recovery cannot continue")));

                /*
                 * If we see a shutdown checkpoint, we know that nothing was running
                 * on the primary at this point. So fake-up an empty running-xacts
                 * record and use that here and now. Recover additional standby state
                 * for prepared transactions.
                 */
                if (standbyState >= STANDBY_INITIALIZED)
                {
                        TransactionId *xids;
                        int                     nxids;
                        TransactionId oldestActiveXID;
                        TransactionId latestCompletedXid;
                        RunningTransactionsData running;

                        oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);

                        /*
                         * Construct a RunningTransactions snapshot representing a shut
                         * down server, with only prepared transactions still alive. We're
                         * never overflowed at this point because all subxids are listed
                         * with their parent prepared transactions.
                         */
                        running.xcnt = nxids;
                        running.subxcnt = 0;
                        running.subxid_overflow = false;
                        running.nextXid = XidFromFullTransactionId(checkPoint.nextXid);
                        running.oldestRunningXid = oldestActiveXID;
                        latestCompletedXid = XidFromFullTransactionId(checkPoint.nextXid);
                        TransactionIdRetreat(latestCompletedXid);
                        Assert(TransactionIdIsNormal(latestCompletedXid));
                        running.latestCompletedXid = latestCompletedXid;
                        running.xids = xids;

                        ProcArrayApplyRecoveryInfo(&running);

                        StandbyRecoverPreparedTransactions();
                }

                /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;
                LWLockRelease(ControlFileLock);

                /* Update shared-memory copy of checkpoint XID/epoch */
                SpinLockAcquire(&XLogCtl->info_lck);
                XLogCtl->ckptFullXid = checkPoint.nextXid;
                SpinLockRelease(&XLogCtl->info_lck);

                /*
                 * We should've already switched to the new TLI before replaying this
                 * record.
                 */
                (void) GetCurrentReplayRecPtr(&replayTLI);
                if (checkPoint.ThisTimeLineID != replayTLI)
                        ereport(PANIC,
                                        (errmsg("unexpected timeline ID %u (should be %u) in shutdown checkpoint record",
                                                        checkPoint.ThisTimeLineID, replayTLI)));

                RecoveryRestartPoint(&checkPoint, record);
        }
        else if (info == XLOG_CHECKPOINT_ONLINE)
        {
                CheckPoint      checkPoint;
                TimeLineID      replayTLI;

                memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                /* In an ONLINE checkpoint, treat the XID counter as a minimum */
                LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                if (FullTransactionIdPrecedes(ShmemVariableCache->nextXid,
                                                                          checkPoint.nextXid))
                        ShmemVariableCache->nextXid = checkPoint.nextXid;
                LWLockRelease(XidGenLock);

                /*
                 * We ignore the nextOid counter in an ONLINE checkpoint, preferring
                 * to track OID assignment through XLOG_NEXTOID records.  The nextOid
                 * counter is from the start of the checkpoint and might well be stale
                 * compared to later XLOG_NEXTOID records.  We could try to take the
                 * maximum of the nextOid counter and our latest value, but since
                 * there's no particular guarantee about the speed with which the OID
                 * counter wraps around, that's a risky thing to do.  In any case,
                 * users of the nextOid counter are required to avoid assignment of
                 * duplicates, so that a somewhat out-of-date value should be safe.
                 */

                /* Handle multixact */
                MultiXactAdvanceNextMXact(checkPoint.nextMulti,
                                                                  checkPoint.nextMultiOffset);

                /*
                 * NB: This may perform multixact truncation when replaying WAL
                 * generated by an older primary.
                 */
                MultiXactAdvanceOldest(checkPoint.oldestMulti,
                                                           checkPoint.oldestMultiDB);
                if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
                                                                  checkPoint.oldestXid))
                        SetTransactionIdLimit(checkPoint.oldestXid,
                                                                  checkPoint.oldestXidDB);
                /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;
                LWLockRelease(ControlFileLock);

                /* Update shared-memory copy of checkpoint XID/epoch */
                SpinLockAcquire(&XLogCtl->info_lck);
                XLogCtl->ckptFullXid = checkPoint.nextXid;
                SpinLockRelease(&XLogCtl->info_lck);

                /* TLI should not change in an on-line checkpoint */
                (void) GetCurrentReplayRecPtr(&replayTLI);
                if (checkPoint.ThisTimeLineID != replayTLI)
                        ereport(PANIC,
                                        (errmsg("unexpected timeline ID %u (should be %u) in online checkpoint record",
                                                        checkPoint.ThisTimeLineID, replayTLI)));

                RecoveryRestartPoint(&checkPoint, record);
        }
        else if (info == XLOG_OVERWRITE_CONTRECORD)
        {
                /* nothing to do here, handled in xlogrecovery_redo() */
        }
        else if (info == XLOG_END_OF_RECOVERY)
        {
                xl_end_of_recovery xlrec;
                TimeLineID      replayTLI;

                memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));

                /*
                 * For Hot Standby, we could treat this like a Shutdown Checkpoint,
                 * but this case is rarer and harder to test, so the benefit doesn't
                 * outweigh the potential extra cost of maintenance.
                 */

                /*
                 * We should've already switched to the new TLI before replaying this
                 * record.
                 */
                (void) GetCurrentReplayRecPtr(&replayTLI);
                if (xlrec.ThisTimeLineID != replayTLI)
                        ereport(PANIC,
                                        (errmsg("unexpected timeline ID %u (should be %u) in end-of-recovery record",
                                                        xlrec.ThisTimeLineID, replayTLI)));
        }
        else if (info == XLOG_NOOP)
        {
                /* nothing to do here */
        }
        else if (info == XLOG_SWITCH)
        {
                /* nothing to do here */
        }
        else if (info == XLOG_RESTORE_POINT)
        {
                /* nothing to do here, handled in xlogrecovery.c */
        }
        else if (info == XLOG_FPI || info == XLOG_FPI_FOR_HINT)
        {
                /*
                 * XLOG_FPI records contain nothing else but one or more block
                 * references. Every block reference must include a full-page image
                 * even if full_page_writes was disabled when the record was generated
                 * - otherwise there would be no point in this record.
                 *
                 * XLOG_FPI_FOR_HINT records are generated when a page needs to be
                 * WAL-logged because of a hint bit update. They are only generated
                 * when checksums and/or wal_log_hints are enabled. They may include
                 * no full-page images if full_page_writes was disabled when they were
                 * generated. In this case there is nothing to do here.
                 *
                 * No recovery conflicts are generated by these generic records - if a
                 * resource manager needs to generate conflicts, it has to define a
                 * separate WAL record type and redo routine.
                 */
                for (uint8 block_id = 0; block_id <= XLogRecMaxBlockId(record); block_id++)
                {
                        Buffer          buffer;

                        if (!XLogRecHasBlockImage(record, block_id))
                        {
                                if (info == XLOG_FPI)
                                        elog(ERROR, "XLOG_FPI record did not contain a full-page image");
                                continue;
                        }

                        if (XLogReadBufferForRedo(record, block_id, &buffer) != BLK_RESTORED)
                                elog(ERROR, "unexpected XLogReadBufferForRedo result when restoring backup block");
                        UnlockReleaseBuffer(buffer);
                }
        }
        else if (info == XLOG_BACKUP_END)
        {
                /* nothing to do here, handled in xlogrecovery_redo() */
        }
        else if (info == XLOG_PARAMETER_CHANGE)
        {
                xl_parameter_change xlrec;

                /* Update our copy of the parameters in pg_control */
                memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_parameter_change));

                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                ControlFile->MaxConnections = xlrec.MaxConnections;
                ControlFile->max_worker_processes = xlrec.max_worker_processes;
                ControlFile->max_wal_senders = xlrec.max_wal_senders;
                ControlFile->max_prepared_xacts = xlrec.max_prepared_xacts;
                ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact;
                ControlFile->wal_level = xlrec.wal_level;
                ControlFile->wal_log_hints = xlrec.wal_log_hints;

                /*
                 * Update minRecoveryPoint to ensure that if recovery is aborted, we
                 * recover back up to this point before allowing hot standby again.
                 * This is important if the max_* settings are decreased, to ensure
                 * you don't run queries against the WAL preceding the change. The
                 * local copies cannot be updated as long as crash recovery is
                 * happening and we expect all the WAL to be replayed.
                 */
                if (InArchiveRecovery)
                {
                        LocalMinRecoveryPoint = ControlFile->minRecoveryPoint;
                        LocalMinRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
                }
                if (LocalMinRecoveryPoint != InvalidXLogRecPtr && LocalMinRecoveryPoint < lsn)
                {
                        TimeLineID      replayTLI;

                        (void) GetCurrentReplayRecPtr(&replayTLI);
                        ControlFile->minRecoveryPoint = lsn;
                        ControlFile->minRecoveryPointTLI = replayTLI;
                }

                CommitTsParameterChange(xlrec.track_commit_timestamp,
                                                                ControlFile->track_commit_timestamp);
                ControlFile->track_commit_timestamp = xlrec.track_commit_timestamp;

                UpdateControlFile();
                LWLockRelease(ControlFileLock);

                /* Check to see if any parameter change gives a problem on recovery */
                CheckRequiredParameterValues();
        }
        else if (info == XLOG_FPW_CHANGE)
        {
                bool            fpw;

                memcpy(&fpw, XLogRecGetData(record), sizeof(bool));

                /*
                 * Update the LSN of the last replayed XLOG_FPW_CHANGE record so that
                 * do_pg_start_backup() and do_pg_stop_backup() can check whether
                 * full_page_writes has been disabled during online backup.
                 */
                if (!fpw)
                {
                        SpinLockAcquire(&XLogCtl->info_lck);
                        if (XLogCtl->lastFpwDisableRecPtr < record->ReadRecPtr)
                                XLogCtl->lastFpwDisableRecPtr = record->ReadRecPtr;
                        SpinLockRelease(&XLogCtl->info_lck);
                }

                /* Keep track of full_page_writes */
                lastFullPageWrites = fpw;
        }
}

/*
 * Return the (possible) sync flag used for opening a file, depending on the
 * value of the GUC wal_sync_method.
 */
static int
get_sync_bit(int method)
{
        int                     o_direct_flag = 0;

        /* If fsync is disabled, never open in sync mode */
        if (!enableFsync)
                return 0;

        /*
         * Optimize writes by bypassing kernel cache with O_DIRECT when using
         * O_SYNC/O_FSYNC and O_DSYNC.  But only if archiving and streaming are
         * disabled, otherwise the archive command or walsender process will read
         * the WAL soon after writing it, which is guaranteed to cause a physical
         * read if we bypassed the kernel cache. We also skip the
         * posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the same
         * reason.
         *
         * Never use O_DIRECT in walreceiver process for similar reasons; the WAL
         * written by walreceiver is normally read by the startup process soon
         * after it's written. Also, walreceiver performs unaligned writes, which
         * don't work with O_DIRECT, so it is required for correctness too.
         */
        if (!XLogIsNeeded() && !AmWalReceiverProcess())
                o_direct_flag = PG_O_DIRECT;

        switch (method)
        {
                        /*
                         * enum values for all sync options are defined even if they are
                         * not supported on the current platform.  But if not, they are
                         * not included in the enum option array, and therefore will never
                         * be seen here.
                         */
                case SYNC_METHOD_FSYNC:
                case SYNC_METHOD_FSYNC_WRITETHROUGH:
                case SYNC_METHOD_FDATASYNC:
                        return 0;
#ifdef OPEN_SYNC_FLAG
                case SYNC_METHOD_OPEN:
                        return OPEN_SYNC_FLAG | o_direct_flag;
#endif
#ifdef OPEN_DATASYNC_FLAG
                case SYNC_METHOD_OPEN_DSYNC:
                        return OPEN_DATASYNC_FLAG | o_direct_flag;
#endif
                default:
                        /* can't happen (unless we are out of sync with option array) */
                        elog(ERROR, "unrecognized wal_sync_method: %d", method);
                        return 0;                       /* silence warning */
        }
}

/*
 * GUC support
 */
void
assign_xlog_sync_method(int new_sync_method, void *extra)
{
        if (sync_method != new_sync_method)
        {
                /*
                 * To ensure that no blocks escape unsynced, force an fsync on the
                 * currently open log segment (if any).  Also, if the open flag is
                 * changing, close the log file so it will be reopened (with new flag
                 * bit) at next use.
                 */
                if (openLogFile >= 0)
                {
                        pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC_METHOD_ASSIGN);
                        if (pg_fsync(openLogFile) != 0)
                        {
                                char            xlogfname[MAXFNAMELEN];
                                int                     save_errno;

                                save_errno = errno;
                                XLogFileName(xlogfname, openLogTLI, openLogSegNo,
                                                         wal_segment_size);
                                errno = save_errno;
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not fsync file \"%s\": %m", xlogfname)));
                        }

                        pgstat_report_wait_end();
                        if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
                                XLogFileClose();
                }
        }
}


/*
 * Issue appropriate kind of fsync (if any) for an XLOG output file.
 *
 * 'fd' is a file descriptor for the XLOG file to be fsync'd.
 * 'segno' is for error reporting purposes.
 */
void
issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
{
        char       *msg = NULL;
        instr_time      start;

        Assert(tli != 0);

        /*
         * Quick exit if fsync is disabled or write() has already synced the WAL
         * file.
         */
        if (!enableFsync ||
                sync_method == SYNC_METHOD_OPEN ||
                sync_method == SYNC_METHOD_OPEN_DSYNC)
                return;

        /* Measure I/O timing to sync the WAL file */
        if (track_wal_io_timing)
                INSTR_TIME_SET_CURRENT(start);

        pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC);
        switch (sync_method)
        {
                case SYNC_METHOD_FSYNC:
                        if (pg_fsync_no_writethrough(fd) != 0)
                                msg = _("could not fsync file \"%s\": %m");
                        break;
#ifdef HAVE_FSYNC_WRITETHROUGH
                case SYNC_METHOD_FSYNC_WRITETHROUGH:
                        if (pg_fsync_writethrough(fd) != 0)
                                msg = _("could not fsync write-through file \"%s\": %m");
                        break;
#endif
#ifdef HAVE_FDATASYNC
                case SYNC_METHOD_FDATASYNC:
                        if (pg_fdatasync(fd) != 0)
                                msg = _("could not fdatasync file \"%s\": %m");
                        break;
#endif
                case SYNC_METHOD_OPEN:
                case SYNC_METHOD_OPEN_DSYNC:
                        /* not reachable */
                        Assert(false);
                        break;
                default:
                        elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
                        break;
        }

        /* PANIC if failed to fsync */
        if (msg)
        {
                char            xlogfname[MAXFNAMELEN];
                int                     save_errno = errno;

                XLogFileName(xlogfname, tli, segno, wal_segment_size);
                errno = save_errno;
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg(msg, xlogfname)));
        }

        pgstat_report_wait_end();

        /*
         * Increment the I/O timing and the number of times WAL files were synced.
         */
        if (track_wal_io_timing)
        {
                instr_time      duration;

                INSTR_TIME_SET_CURRENT(duration);
                INSTR_TIME_SUBTRACT(duration, start);
                WalStats.m_wal_sync_time += INSTR_TIME_GET_MICROSEC(duration);
        }

        WalStats.m_wal_sync++;
}

/*
 * do_pg_start_backup
 *
 * Utility function called at the start of an online backup. It creates the
 * necessary starting checkpoint and constructs the backup label file.
 *
 * There are two kind of backups: exclusive and non-exclusive. An exclusive
 * backup is started with pg_start_backup(), and there can be only one active
 * at a time. The backup and tablespace map files of an exclusive backup are
 * written to $PGDATA/backup_label and $PGDATA/tablespace_map, and they are
 * removed by pg_stop_backup().
 *
 * A non-exclusive backup is used for the streaming base backups (see
 * src/backend/replication/basebackup.c). The difference to exclusive backups
 * is that the backup label and tablespace map files are not written to disk.
 * Instead, their would-be contents are returned in *labelfile and *tblspcmapfile,
 * and the caller is responsible for including them in the backup archive as
 * 'backup_label' and 'tablespace_map'. There can be many non-exclusive backups
 * active at the same time, and they don't conflict with an exclusive backup
 * either.
 *
 * labelfile and tblspcmapfile must be passed as NULL when starting an
 * exclusive backup, and as initially-empty StringInfos for a non-exclusive
 * backup.
 *
 * If "tablespaces" isn't NULL, it receives a list of tablespaceinfo structs
 * describing the cluster's tablespaces.
 *
 * tblspcmapfile is required mainly for tar format in windows as native windows
 * utilities are not able to create symlinks while extracting files from tar.
 * However for consistency, the same is used for all platforms.
 *
 * Returns the minimum WAL location that must be present to restore from this
 * backup, and the corresponding timeline ID in *starttli_p.
 *
 * Every successfully started non-exclusive backup must be stopped by calling
 * do_pg_stop_backup() or do_pg_abort_backup().
 *
 * It is the responsibility of the caller of this function to verify the
 * permissions of the calling user!
 */
XLogRecPtr
do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
                                   StringInfo labelfile, List **tablespaces,
                                   StringInfo tblspcmapfile)
{
        bool            exclusive = (labelfile == NULL);
        bool            backup_started_in_recovery = false;
        XLogRecPtr      checkpointloc;
        XLogRecPtr      startpoint;
        TimeLineID      starttli;
        pg_time_t       stamp_time;
        char            strfbuf[128];
        char            xlogfilename[MAXFNAMELEN];
        XLogSegNo       _logSegNo;
        struct stat stat_buf;
        FILE       *fp;

        backup_started_in_recovery = RecoveryInProgress();

        /*
         * Currently only non-exclusive backup can be taken during recovery.
         */
        if (backup_started_in_recovery && exclusive)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("recovery is in progress"),
                                 errhint("WAL control functions cannot be executed during recovery.")));

        /*
         * During recovery, we don't need to check WAL level. Because, if WAL
         * level is not sufficient, it's impossible to get here during recovery.
         */
        if (!backup_started_in_recovery && !XLogIsNeeded())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("WAL level not sufficient for making an online backup"),
                                 errhint("wal_level must be set to \"replica\" or \"logical\" at server start.")));

        if (strlen(backupidstr) > MAXPGPATH)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("backup label too long (max %d bytes)",
                                                MAXPGPATH)));

        /*
         * Mark backup active in shared memory.  We must do full-page WAL writes
         * during an on-line backup even if not doing so at other times, because
         * it's quite possible for the backup dump to obtain a "torn" (partially
         * written) copy of a database page if it reads the page concurrently with
         * our write to the same page.  This can be fixed as long as the first
         * write to the page in the WAL sequence is a full-page write. Hence, we
         * turn on forcePageWrites and then force a CHECKPOINT, to ensure there
         * are no dirty pages in shared memory that might get dumped while the
         * backup is in progress without having a corresponding WAL record.  (Once
         * the backup is complete, we need not force full-page writes anymore,
         * since we expect that any pages not modified during the backup interval
         * must have been correctly captured by the backup.)
         *
         * Note that forcePageWrites has no effect during an online backup from
         * the standby.
         *
         * We must hold all the insertion locks to change the value of
         * forcePageWrites, to ensure adequate interlocking against
         * XLogInsertRecord().
         */
        WALInsertLockAcquireExclusive();
        if (exclusive)
        {
                /*
                 * At first, mark that we're now starting an exclusive backup, to
                 * ensure that there are no other sessions currently running
                 * pg_start_backup() or pg_stop_backup().
                 */
                if (XLogCtl->Insert.exclusiveBackupState != EXCLUSIVE_BACKUP_NONE)
                {
                        WALInsertLockRelease();
                        ereport(ERROR,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("a backup is already in progress"),
                                         errhint("Run pg_stop_backup() and try again.")));
                }
                XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_STARTING;
        }
        else
                XLogCtl->Insert.nonExclusiveBackups++;
        XLogCtl->Insert.forcePageWrites = true;
        WALInsertLockRelease();

        /* Ensure we release forcePageWrites if fail below */
        PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
        {
                bool            gotUniqueStartpoint = false;
                DIR                *tblspcdir;
                struct dirent *de;
                tablespaceinfo *ti;
                int                     datadirpathlen;

                /*
                 * Force an XLOG file switch before the checkpoint, to ensure that the
                 * WAL segment the checkpoint is written to doesn't contain pages with
                 * old timeline IDs.  That would otherwise happen if you called
                 * pg_start_backup() right after restoring from a PITR archive: the
                 * first WAL segment containing the startup checkpoint has pages in
                 * the beginning with the old timeline ID.  That can cause trouble at
                 * recovery: we won't have a history file covering the old timeline if
                 * pg_wal directory was not included in the base backup and the WAL
                 * archive was cleared too before starting the backup.
                 *
                 * This also ensures that we have emitted a WAL page header that has
                 * XLP_BKP_REMOVABLE off before we emit the checkpoint record.
                 * Therefore, if a WAL archiver (such as pglesslog) is trying to
                 * compress out removable backup blocks, it won't remove any that
                 * occur after this point.
                 *
                 * During recovery, we skip forcing XLOG file switch, which means that
                 * the backup taken during recovery is not available for the special
                 * recovery case described above.
                 */
                if (!backup_started_in_recovery)
                        RequestXLogSwitch(false);

                do
                {
                        bool            checkpointfpw;

                        /*
                         * Force a CHECKPOINT.  Aside from being necessary to prevent torn
                         * page problems, this guarantees that two successive backup runs
                         * will have different checkpoint positions and hence different
                         * history file names, even if nothing happened in between.
                         *
                         * During recovery, establish a restartpoint if possible. We use
                         * the last restartpoint as the backup starting checkpoint. This
                         * means that two successive backup runs can have same checkpoint
                         * positions.
                         *
                         * Since the fact that we are executing do_pg_start_backup()
                         * during recovery means that checkpointer is running, we can use
                         * RequestCheckpoint() to establish a restartpoint.
                         *
                         * We use CHECKPOINT_IMMEDIATE only if requested by user (via
                         * passing fast = true).  Otherwise this can take awhile.
                         */
                        RequestCheckpoint(CHECKPOINT_FORCE | CHECKPOINT_WAIT |
                                                          (fast ? CHECKPOINT_IMMEDIATE : 0));

                        /*
                         * Now we need to fetch the checkpoint record location, and also
                         * its REDO pointer.  The oldest point in WAL that would be needed
                         * to restore starting from the checkpoint is precisely the REDO
                         * pointer.
                         */
                        LWLockAcquire(ControlFileLock, LW_SHARED);
                        checkpointloc = ControlFile->checkPoint;
                        startpoint = ControlFile->checkPointCopy.redo;
                        starttli = ControlFile->checkPointCopy.ThisTimeLineID;
                        checkpointfpw = ControlFile->checkPointCopy.fullPageWrites;
                        LWLockRelease(ControlFileLock);

                        if (backup_started_in_recovery)
                        {
                                XLogRecPtr      recptr;

                                /*
                                 * Check to see if all WAL replayed during online backup
                                 * (i.e., since last restartpoint used as backup starting
                                 * checkpoint) contain full-page writes.
                                 */
                                SpinLockAcquire(&XLogCtl->info_lck);
                                recptr = XLogCtl->lastFpwDisableRecPtr;
                                SpinLockRelease(&XLogCtl->info_lck);

                                if (!checkpointfpw || startpoint <= recptr)
                                        ereport(ERROR,
                                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                                         errmsg("WAL generated with full_page_writes=off was replayed "
                                                                        "since last restartpoint"),
                                                         errhint("This means that the backup being taken on the standby "
                                                                         "is corrupt and should not be used. "
                                                                         "Enable full_page_writes and run CHECKPOINT on the primary, "
                                                                         "and then try an online backup again.")));

                                /*
                                 * During recovery, since we don't use the end-of-backup WAL
                                 * record and don't write the backup history file, the
                                 * starting WAL location doesn't need to be unique. This means
                                 * that two base backups started at the same time might use
                                 * the same checkpoint as starting locations.
                                 */
                                gotUniqueStartpoint = true;
                        }

                        /*
                         * If two base backups are started at the same time (in WAL sender
                         * processes), we need to make sure that they use different
                         * checkpoints as starting locations, because we use the starting
                         * WAL location as a unique identifier for the base backup in the
                         * end-of-backup WAL record and when we write the backup history
                         * file. Perhaps it would be better generate a separate unique ID
                         * for each backup instead of forcing another checkpoint, but
                         * taking a checkpoint right after another is not that expensive
                         * either because only few buffers have been dirtied yet.
                         */
                        WALInsertLockAcquireExclusive();
                        if (XLogCtl->Insert.lastBackupStart < startpoint)
                        {
                                XLogCtl->Insert.lastBackupStart = startpoint;
                                gotUniqueStartpoint = true;
                        }
                        WALInsertLockRelease();
                } while (!gotUniqueStartpoint);

                XLByteToSeg(startpoint, _logSegNo, wal_segment_size);
                XLogFileName(xlogfilename, starttli, _logSegNo, wal_segment_size);

                /*
                 * Construct tablespace_map file.  If caller isn't interested in this,
                 * we make a local StringInfo.
                 */
                if (tblspcmapfile == NULL)
                        tblspcmapfile = makeStringInfo();

                datadirpathlen = strlen(DataDir);

                /* Collect information about all tablespaces */
                tblspcdir = AllocateDir("pg_tblspc");
                while ((de = ReadDir(tblspcdir, "pg_tblspc")) != NULL)
                {
                        char            fullpath[MAXPGPATH + 10];
                        char            linkpath[MAXPGPATH];
                        char       *relpath = NULL;
                        int                     rllen;
                        StringInfoData escapedpath;
                        char       *s;

                        /* Skip anything that doesn't look like a tablespace */
                        if (strspn(de->d_name, "0123456789") != strlen(de->d_name))
                                continue;

                        snprintf(fullpath, sizeof(fullpath), "pg_tblspc/%s", de->d_name);

                        /*
                         * Skip anything that isn't a symlink/junction.  For testing only,
                         * we sometimes use allow_in_place_tablespaces to create
                         * directories directly under pg_tblspc, which would fail below.
                         */
#ifdef WIN32
                        if (!pgwin32_is_junction(fullpath))
                                continue;
#else
                        if (get_dirent_type(fullpath, de, false, ERROR) != PGFILETYPE_LNK)
                                continue;
#endif

#if defined(HAVE_READLINK) || defined(WIN32)
                        rllen = readlink(fullpath, linkpath, sizeof(linkpath));
                        if (rllen < 0)
                        {
                                ereport(WARNING,
                                                (errmsg("could not read symbolic link \"%s\": %m",
                                                                fullpath)));
                                continue;
                        }
                        else if (rllen >= sizeof(linkpath))
                        {
                                ereport(WARNING,
                                                (errmsg("symbolic link \"%s\" target is too long",
                                                                fullpath)));
                                continue;
                        }
                        linkpath[rllen] = '\0';

                        /*
                         * Build a backslash-escaped version of the link path to include
                         * in the tablespace map file.
                         */
                        initStringInfo(&escapedpath);
                        for (s = linkpath; *s; s++)
                        {
                                if (*s == '\n' || *s == '\r' || *s == '\\')
                                        appendStringInfoChar(&escapedpath, '\\');
                                appendStringInfoChar(&escapedpath, *s);
                        }

                        /*
                         * Relpath holds the relative path of the tablespace directory
                         * when it's located within PGDATA, or NULL if it's located
                         * elsewhere.
                         */
                        if (rllen > datadirpathlen &&
                                strncmp(linkpath, DataDir, datadirpathlen) == 0 &&
                                IS_DIR_SEP(linkpath[datadirpathlen]))
                                relpath = linkpath + datadirpathlen + 1;

                        ti = palloc(sizeof(tablespaceinfo));
                        ti->oid = pstrdup(de->d_name);
                        ti->path = pstrdup(linkpath);
                        ti->rpath = relpath ? pstrdup(relpath) : NULL;
                        ti->size = -1;

                        if (tablespaces)
                                *tablespaces = lappend(*tablespaces, ti);

                        appendStringInfo(tblspcmapfile, "%s %s\n",
                                                         ti->oid, escapedpath.data);

                        pfree(escapedpath.data);
#else

                        /*
                         * If the platform does not have symbolic links, it should not be
                         * possible to have tablespaces - clearly somebody else created
                         * them. Warn about it and ignore.
                         */
                        ereport(WARNING,
                                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                         errmsg("tablespaces are not supported on this platform")));
#endif
                }
                FreeDir(tblspcdir);

                /*
                 * Construct backup label file.  If caller isn't interested in this,
                 * we make a local StringInfo.
                 */
                if (labelfile == NULL)
                        labelfile = makeStringInfo();

                /* Use the log timezone here, not the session timezone */
                stamp_time = (pg_time_t) time(NULL);
                pg_strftime(strfbuf, sizeof(strfbuf),
                                        "%Y-%m-%d %H:%M:%S %Z",
                                        pg_localtime(&stamp_time, log_timezone));
                appendStringInfo(labelfile, "START WAL LOCATION: %X/%X (file %s)\n",
                                                 LSN_FORMAT_ARGS(startpoint), xlogfilename);
                appendStringInfo(labelfile, "CHECKPOINT LOCATION: %X/%X\n",
                                                 LSN_FORMAT_ARGS(checkpointloc));
                appendStringInfo(labelfile, "BACKUP METHOD: %s\n",
                                                 exclusive ? "pg_start_backup" : "streamed");
                appendStringInfo(labelfile, "BACKUP FROM: %s\n",
                                                 backup_started_in_recovery ? "standby" : "primary");
                appendStringInfo(labelfile, "START TIME: %s\n", strfbuf);
                appendStringInfo(labelfile, "LABEL: %s\n", backupidstr);
                appendStringInfo(labelfile, "START TIMELINE: %u\n", starttli);

                /*
                 * Okay, write the file, or return its contents to caller.
                 */
                if (exclusive)
                {
                        /*
                         * Check for existing backup label --- implies a backup is already
                         * running.  (XXX given that we checked exclusiveBackupState
                         * above, maybe it would be OK to just unlink any such label
                         * file?)
                         */
                        if (stat(BACKUP_LABEL_FILE, &stat_buf) != 0)
                        {
                                if (errno != ENOENT)
                                        ereport(ERROR,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not stat file \"%s\": %m",
                                                                        BACKUP_LABEL_FILE)));
                        }
                        else
                                ereport(ERROR,
                                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                                 errmsg("a backup is already in progress"),
                                                 errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
                                                                 BACKUP_LABEL_FILE)));

                        fp = AllocateFile(BACKUP_LABEL_FILE, "w");

                        if (!fp)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not create file \"%s\": %m",
                                                                BACKUP_LABEL_FILE)));
                        if (fwrite(labelfile->data, labelfile->len, 1, fp) != 1 ||
                                fflush(fp) != 0 ||
                                pg_fsync(fileno(fp)) != 0 ||
                                ferror(fp) ||
                                FreeFile(fp))
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not write file \"%s\": %m",
                                                                BACKUP_LABEL_FILE)));
                        /* Allocated locally for exclusive backups, so free separately */
                        pfree(labelfile->data);
                        pfree(labelfile);

                        /* Write backup tablespace_map file. */
                        if (tblspcmapfile->len > 0)
                        {
                                if (stat(TABLESPACE_MAP, &stat_buf) != 0)
                                {
                                        if (errno != ENOENT)
                                                ereport(ERROR,
                                                                (errcode_for_file_access(),
                                                                 errmsg("could not stat file \"%s\": %m",
                                                                                TABLESPACE_MAP)));
                                }
                                else
                                        ereport(ERROR,
                                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                                         errmsg("a backup is already in progress"),
                                                         errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
                                                                         TABLESPACE_MAP)));

                                fp = AllocateFile(TABLESPACE_MAP, "w");

                                if (!fp)
                                        ereport(ERROR,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not create file \"%s\": %m",
                                                                        TABLESPACE_MAP)));
                                if (fwrite(tblspcmapfile->data, tblspcmapfile->len, 1, fp) != 1 ||
                                        fflush(fp) != 0 ||
                                        pg_fsync(fileno(fp)) != 0 ||
                                        ferror(fp) ||
                                        FreeFile(fp))
                                        ereport(ERROR,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not write file \"%s\": %m",
                                                                        TABLESPACE_MAP)));
                        }

                        /* Allocated locally for exclusive backups, so free separately */
                        pfree(tblspcmapfile->data);
                        pfree(tblspcmapfile);
                }
        }
        PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));

        /*
         * Mark that start phase has correctly finished for an exclusive backup.
         * Session-level locks are updated as well to reflect that state.
         *
         * Note that CHECK_FOR_INTERRUPTS() must not occur while updating backup
         * counters and session-level lock. Otherwise they can be updated
         * inconsistently, and which might cause do_pg_abort_backup() to fail.
         */
        if (exclusive)
        {
                WALInsertLockAcquireExclusive();
                XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;

                /* Set session-level lock */
                sessionBackupState = SESSION_BACKUP_EXCLUSIVE;
                WALInsertLockRelease();
        }
        else
                sessionBackupState = SESSION_BACKUP_NON_EXCLUSIVE;

        /*
         * We're done.  As a convenience, return the starting WAL location.
         */
        if (starttli_p)
                *starttli_p = starttli;
        return startpoint;
}

/* Error cleanup callback for pg_start_backup */
static void
pg_start_backup_callback(int code, Datum arg)
{
        bool            exclusive = DatumGetBool(arg);

        /* Update backup counters and forcePageWrites on failure */
        WALInsertLockAcquireExclusive();
        if (exclusive)
        {
                Assert(XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_STARTING);
                XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_NONE;
        }
        else
        {
                Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
                XLogCtl->Insert.nonExclusiveBackups--;
        }

        if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
                XLogCtl->Insert.nonExclusiveBackups == 0)
        {
                XLogCtl->Insert.forcePageWrites = false;
        }
        WALInsertLockRelease();
}

/*
 * Error cleanup callback for pg_stop_backup
 */
static void
pg_stop_backup_callback(int code, Datum arg)
{
        bool            exclusive = DatumGetBool(arg);

        /* Update backup status on failure */
        WALInsertLockAcquireExclusive();
        if (exclusive)
        {
                Assert(XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_STOPPING);
                XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;
        }
        WALInsertLockRelease();
}

/*
 * Utility routine to fetch the session-level status of a backup running.
 */
SessionBackupState
get_backup_status(void)
{
        return sessionBackupState;
}

/*
 * do_pg_stop_backup
 *
 * Utility function called at the end of an online backup. It cleans up the
 * backup state and can optionally wait for WAL segments to be archived.
 *
 * If labelfile is NULL, this stops an exclusive backup. Otherwise this stops
 * the non-exclusive backup specified by 'labelfile'.
 *
 * Returns the last WAL location that must be present to restore from this
 * backup, and the corresponding timeline ID in *stoptli_p.
 *
 * It is the responsibility of the caller of this function to verify the
 * permissions of the calling user!
 */
XLogRecPtr
do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
{
        bool            exclusive = (labelfile == NULL);
        bool            backup_started_in_recovery = false;
        XLogRecPtr      startpoint;
        XLogRecPtr      stoppoint;
        TimeLineID      stoptli;
        pg_time_t       stamp_time;
        char            strfbuf[128];
        char            histfilepath[MAXPGPATH];
        char            startxlogfilename[MAXFNAMELEN];
        char            stopxlogfilename[MAXFNAMELEN];
        char            lastxlogfilename[MAXFNAMELEN];
        char            histfilename[MAXFNAMELEN];
        char            backupfrom[20];
        XLogSegNo       _logSegNo;
        FILE       *lfp;
        FILE       *fp;
        char            ch;
        int                     seconds_before_warning;
        int                     waits = 0;
        bool            reported_waiting = false;
        char       *remaining;
        char       *ptr;
        uint32          hi,
                                lo;

        backup_started_in_recovery = RecoveryInProgress();

        /*
         * Currently only non-exclusive backup can be taken during recovery.
         */
        if (backup_started_in_recovery && exclusive)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("recovery is in progress"),
                                 errhint("WAL control functions cannot be executed during recovery.")));

        /*
         * During recovery, we don't need to check WAL level. Because, if WAL
         * level is not sufficient, it's impossible to get here during recovery.
         */
        if (!backup_started_in_recovery && !XLogIsNeeded())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("WAL level not sufficient for making an online backup"),
                                 errhint("wal_level must be set to \"replica\" or \"logical\" at server start.")));

        if (exclusive)
        {
                /*
                 * At first, mark that we're now stopping an exclusive backup, to
                 * ensure that there are no other sessions currently running
                 * pg_start_backup() or pg_stop_backup().
                 */
                WALInsertLockAcquireExclusive();
                if (XLogCtl->Insert.exclusiveBackupState != EXCLUSIVE_BACKUP_IN_PROGRESS)
                {
                        WALInsertLockRelease();
                        ereport(ERROR,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("exclusive backup not in progress")));
                }
                XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_STOPPING;
                WALInsertLockRelease();

                /*
                 * Remove backup_label. In case of failure, the state for an exclusive
                 * backup is switched back to in-progress.
                 */
                PG_ENSURE_ERROR_CLEANUP(pg_stop_backup_callback, (Datum) BoolGetDatum(exclusive));
                {
                        /*
                         * Read the existing label file into memory.
                         */
                        struct stat statbuf;
                        int                     r;

                        if (stat(BACKUP_LABEL_FILE, &statbuf))
                        {
                                /* should not happen per the upper checks */
                                if (errno != ENOENT)
                                        ereport(ERROR,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not stat file \"%s\": %m",
                                                                        BACKUP_LABEL_FILE)));
                                ereport(ERROR,
                                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                                 errmsg("a backup is not in progress")));
                        }

                        lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
                        if (!lfp)
                        {
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not read file \"%s\": %m",
                                                                BACKUP_LABEL_FILE)));
                        }
                        labelfile = palloc(statbuf.st_size + 1);
                        r = fread(labelfile, statbuf.st_size, 1, lfp);
                        labelfile[statbuf.st_size] = '\0';

                        /*
                         * Close and remove the backup label file
                         */
                        if (r != 1 || ferror(lfp) || FreeFile(lfp))
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not read file \"%s\": %m",
                                                                BACKUP_LABEL_FILE)));
                        durable_unlink(BACKUP_LABEL_FILE, ERROR);

                        /*
                         * Remove tablespace_map file if present, it is created only if
                         * there are tablespaces.
                         */
                        durable_unlink(TABLESPACE_MAP, DEBUG1);
                }
                PG_END_ENSURE_ERROR_CLEANUP(pg_stop_backup_callback, (Datum) BoolGetDatum(exclusive));
        }

        /*
         * OK to update backup counters, forcePageWrites and session-level lock.
         *
         * Note that CHECK_FOR_INTERRUPTS() must not occur while updating them.
         * Otherwise they can be updated inconsistently, and which might cause
         * do_pg_abort_backup() to fail.
         */
        WALInsertLockAcquireExclusive();
        if (exclusive)
        {
                XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_NONE;
        }
        else
        {
                /*
                 * The user-visible pg_start/stop_backup() functions that operate on
                 * exclusive backups can be called at any time, but for non-exclusive
                 * backups, it is expected that each do_pg_start_backup() call is
                 * matched by exactly one do_pg_stop_backup() call.
                 */
                Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
                XLogCtl->Insert.nonExclusiveBackups--;
        }

        if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
                XLogCtl->Insert.nonExclusiveBackups == 0)
        {
                XLogCtl->Insert.forcePageWrites = false;
        }

        /*
         * Clean up session-level lock.
         *
         * You might think that WALInsertLockRelease() can be called before
         * cleaning up session-level lock because session-level lock doesn't need
         * to be protected with WAL insertion lock. But since
         * CHECK_FOR_INTERRUPTS() can occur in it, session-level lock must be
         * cleaned up before it.
         */
        sessionBackupState = SESSION_BACKUP_NONE;

        WALInsertLockRelease();

        /*
         * Read and parse the START WAL LOCATION line (this code is pretty crude,
         * but we are not expecting any variability in the file format).
         */
        if (sscanf(labelfile, "START WAL LOCATION: %X/%X (file %24s)%c",
                           &hi, &lo, startxlogfilename,
                           &ch) != 4 || ch != '\n')
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        startpoint = ((uint64) hi) << 32 | lo;
        remaining = strchr(labelfile, '\n') + 1;        /* %n is not portable enough */

        /*
         * Parse the BACKUP FROM line. If we are taking an online backup from the
         * standby, we confirm that the standby has not been promoted during the
         * backup.
         */
        ptr = strstr(remaining, "BACKUP FROM:");
        if (!ptr || sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        if (strcmp(backupfrom, "standby") == 0 && !backup_started_in_recovery)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("the standby was promoted during online backup"),
                                 errhint("This means that the backup being taken is corrupt "
                                                 "and should not be used. "
                                                 "Try taking another online backup.")));

        /*
         * During recovery, we don't write an end-of-backup record. We assume that
         * pg_control was backed up last and its minimum recovery point can be
         * available as the backup end location. Since we don't have an
         * end-of-backup record, we use the pg_control value to check whether
         * we've reached the end of backup when starting recovery from this
         * backup. We have no way of checking if pg_control wasn't backed up last
         * however.
         *
         * We don't force a switch to new WAL file but it is still possible to
         * wait for all the required files to be archived if waitforarchive is
         * true. This is okay if we use the backup to start a standby and fetch
         * the missing WAL using streaming replication. But in the case of an
         * archive recovery, a user should set waitforarchive to true and wait for
         * them to be archived to ensure that all the required files are
         * available.
         *
         * We return the current minimum recovery point as the backup end
         * location. Note that it can be greater than the exact backup end
         * location if the minimum recovery point is updated after the backup of
         * pg_control. This is harmless for current uses.
         *
         * XXX currently a backup history file is for informational and debug
         * purposes only. It's not essential for an online backup. Furthermore,
         * even if it's created, it will not be archived during recovery because
         * an archiver is not invoked. So it doesn't seem worthwhile to write a
         * backup history file during recovery.
         */
        if (backup_started_in_recovery)
        {
                XLogRecPtr      recptr;

                /*
                 * Check to see if all WAL replayed during online backup contain
                 * full-page writes.
                 */
                SpinLockAcquire(&XLogCtl->info_lck);
                recptr = XLogCtl->lastFpwDisableRecPtr;
                SpinLockRelease(&XLogCtl->info_lck);

                if (startpoint <= recptr)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("WAL generated with full_page_writes=off was replayed "
                                                        "during online backup"),
                                         errhint("This means that the backup being taken on the standby "
                                                         "is corrupt and should not be used. "
                                                         "Enable full_page_writes and run CHECKPOINT on the primary, "
                                                         "and then try an online backup again.")));


                LWLockAcquire(ControlFileLock, LW_SHARED);
                stoppoint = ControlFile->minRecoveryPoint;
                stoptli = ControlFile->minRecoveryPointTLI;
                LWLockRelease(ControlFileLock);
        }
        else
        {
                /*
                 * Write the backup-end xlog record
                 */
                XLogBeginInsert();
                XLogRegisterData((char *) (&startpoint), sizeof(startpoint));
                stoppoint = XLogInsert(RM_XLOG_ID, XLOG_BACKUP_END);

                /*
                 * Given that we're not in recovery, InsertTimeLineID is set and can't
                 * change, so we can read it without a lock.
                 */
                stoptli = XLogCtl->InsertTimeLineID;

                /*
                 * Force a switch to a new xlog segment file, so that the backup is
                 * valid as soon as archiver moves out the current segment file.
                 */
                RequestXLogSwitch(false);

                XLByteToPrevSeg(stoppoint, _logSegNo, wal_segment_size);
                XLogFileName(stopxlogfilename, stoptli, _logSegNo, wal_segment_size);

                /* Use the log timezone here, not the session timezone */
                stamp_time = (pg_time_t) time(NULL);
                pg_strftime(strfbuf, sizeof(strfbuf),
                                        "%Y-%m-%d %H:%M:%S %Z",
                                        pg_localtime(&stamp_time, log_timezone));

                /*
                 * Write the backup history file
                 */
                XLByteToSeg(startpoint, _logSegNo, wal_segment_size);
                BackupHistoryFilePath(histfilepath, stoptli, _logSegNo,
                                                          startpoint, wal_segment_size);
                fp = AllocateFile(histfilepath, "w");
                if (!fp)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not create file \"%s\": %m",
                                                        histfilepath)));
                fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
                                LSN_FORMAT_ARGS(startpoint), startxlogfilename);
                fprintf(fp, "STOP WAL LOCATION: %X/%X (file %s)\n",
                                LSN_FORMAT_ARGS(stoppoint), stopxlogfilename);

                /*
                 * Transfer remaining lines including label and start timeline to
                 * history file.
                 */
                fprintf(fp, "%s", remaining);
                fprintf(fp, "STOP TIME: %s\n", strfbuf);
                fprintf(fp, "STOP TIMELINE: %u\n", stoptli);
                if (fflush(fp) || ferror(fp) || FreeFile(fp))
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not write file \"%s\": %m",
                                                        histfilepath)));

                /*
                 * Clean out any no-longer-needed history files.  As a side effect,
                 * this will post a .ready file for the newly created history file,
                 * notifying the archiver that history file may be archived
                 * immediately.
                 */
                CleanupBackupHistory();
        }

        /*
         * If archiving is enabled, wait for all the required WAL files to be
         * archived before returning. If archiving isn't enabled, the required WAL
         * needs to be transported via streaming replication (hopefully with
         * wal_keep_size set high enough), or some more exotic mechanism like
         * polling and copying files from pg_wal with script. We have no knowledge
         * of those mechanisms, so it's up to the user to ensure that he gets all
         * the required WAL.
         *
         * We wait until both the last WAL file filled during backup and the
         * history file have been archived, and assume that the alphabetic sorting
         * property of the WAL files ensures any earlier WAL files are safely
         * archived as well.
         *
         * We wait forever, since archive_command is supposed to work and we
         * assume the admin wanted his backup to work completely. If you don't
         * wish to wait, then either waitforarchive should be passed in as false,
         * or you can set statement_timeout.  Also, some notices are issued to
         * clue in anyone who might be doing this interactively.
         */

        if (waitforarchive &&
                ((!backup_started_in_recovery && XLogArchivingActive()) ||
                 (backup_started_in_recovery && XLogArchivingAlways())))
        {
                XLByteToPrevSeg(stoppoint, _logSegNo, wal_segment_size);
                XLogFileName(lastxlogfilename, stoptli, _logSegNo, wal_segment_size);

                XLByteToSeg(startpoint, _logSegNo, wal_segment_size);
                BackupHistoryFileName(histfilename, stoptli, _logSegNo,
                                                          startpoint, wal_segment_size);

                seconds_before_warning = 60;
                waits = 0;

                while (XLogArchiveIsBusy(lastxlogfilename) ||
                           XLogArchiveIsBusy(histfilename))
                {
                        CHECK_FOR_INTERRUPTS();

                        if (!reported_waiting && waits > 5)
                        {
                                ereport(NOTICE,
                                                (errmsg("base backup done, waiting for required WAL segments to be archived")));
                                reported_waiting = true;
                        }

                        (void) WaitLatch(MyLatch,
                                                         WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                                                         1000L,
                                                         WAIT_EVENT_BACKUP_WAIT_WAL_ARCHIVE);
                        ResetLatch(MyLatch);

                        if (++waits >= seconds_before_warning)
                        {
                                seconds_before_warning *= 2;    /* This wraps in >10 years... */
                                ereport(WARNING,
                                                (errmsg("still waiting for all required WAL segments to be archived (%d seconds elapsed)",
                                                                waits),
                                                 errhint("Check that your archive_command is executing properly.  "
                                                                 "You can safely cancel this backup, "
                                                                 "but the database backup will not be usable without all the WAL segments.")));
                        }
                }

                ereport(NOTICE,
                                (errmsg("all required WAL segments have been archived")));
        }
        else if (waitforarchive)
                ereport(NOTICE,
                                (errmsg("WAL archiving is not enabled; you must ensure that all required WAL segments are copied through other means to complete the backup")));

        /*
         * We're done.  As a convenience, return the ending WAL location.
         */
        if (stoptli_p)
                *stoptli_p = stoptli;
        return stoppoint;
}


/*
 * do_pg_abort_backup: abort a running backup
 *
 * This does just the most basic steps of do_pg_stop_backup(), by taking the
 * system out of backup mode, thus making it a lot more safe to call from
 * an error handler.
 *
 * The caller can pass 'arg' as 'true' or 'false' to control whether a warning
 * is emitted.
 *
 * NB: This is only for aborting a non-exclusive backup that doesn't write
 * backup_label. A backup started with pg_start_backup() needs to be finished
 * with pg_stop_backup().
 *
 * NB: This gets used as a before_shmem_exit handler, hence the odd-looking
 * signature.
 */
void
do_pg_abort_backup(int code, Datum arg)
{
        bool            emit_warning = DatumGetBool(arg);

        /*
         * Quick exit if session is not keeping around a non-exclusive backup
         * already started.
         */
        if (sessionBackupState != SESSION_BACKUP_NON_EXCLUSIVE)
                return;

        WALInsertLockAcquireExclusive();
        Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
        XLogCtl->Insert.nonExclusiveBackups--;

        if (XLogCtl->Insert.exclusiveBackupState == EXCLUSIVE_BACKUP_NONE &&
                XLogCtl->Insert.nonExclusiveBackups == 0)
        {
                XLogCtl->Insert.forcePageWrites = false;
        }
        WALInsertLockRelease();

        if (emit_warning)
                ereport(WARNING,
                                (errmsg("aborting backup due to backend exiting before pg_stop_backup was called")));
}

/*
 * Register a handler that will warn about unterminated backups at end of
 * session, unless this has already been done.
 */
void
register_persistent_abort_backup_handler(void)
{
        static bool already_done = false;

        if (already_done)
                return;
        before_shmem_exit(do_pg_abort_backup, DatumGetBool(true));
        already_done = true;
}

/*
 * Get latest WAL insert pointer
 */
XLogRecPtr
GetXLogInsertRecPtr(void)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        uint64          current_bytepos;

        SpinLockAcquire(&Insert->insertpos_lck);
        current_bytepos = Insert->CurrBytePos;
        SpinLockRelease(&Insert->insertpos_lck);

        return XLogBytePosToRecPtr(current_bytepos);
}

/*
 * Get latest WAL write pointer
 */
XLogRecPtr
GetXLogWriteRecPtr(void)
{
        SpinLockAcquire(&XLogCtl->info_lck);
        LogwrtResult = XLogCtl->LogwrtResult;
        SpinLockRelease(&XLogCtl->info_lck);

        return LogwrtResult.Write;
}

/*
 * Returns the redo pointer of the last checkpoint or restartpoint. This is
 * the oldest point in WAL that we still need, if we have to restart recovery.
 */
void
GetOldestRestartPoint(XLogRecPtr *oldrecptr, TimeLineID *oldtli)
{
        LWLockAcquire(ControlFileLock, LW_SHARED);
        *oldrecptr = ControlFile->checkPointCopy.redo;
        *oldtli = ControlFile->checkPointCopy.ThisTimeLineID;
        LWLockRelease(ControlFileLock);
}

/*
 * BackupInProgress: check if online backup mode is active
 *
 * This is done by checking for existence of the "backup_label" file.
 */
bool
BackupInProgress(void)
{
        struct stat stat_buf;

        return (stat(BACKUP_LABEL_FILE, &stat_buf) == 0);
}

/*
 * CancelBackup: rename the "backup_label" and "tablespace_map"
 *                               files to cancel backup mode
 *
 * If the "backup_label" file exists, it will be renamed to "backup_label.old".
 * Similarly, if the "tablespace_map" file exists, it will be renamed to
 * "tablespace_map.old".
 *
 * Note that this will render an online backup in progress
 * useless. To correctly finish an online backup, pg_stop_backup must be
 * called.
 */
void
CancelBackup(void)
{
        struct stat stat_buf;

        /* if the backup_label file is not there, return */
        if (stat(BACKUP_LABEL_FILE, &stat_buf) < 0)
                return;

        /* remove leftover file from previously canceled backup if it exists */
        unlink(BACKUP_LABEL_OLD);

        if (durable_rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD, DEBUG1) != 0)
        {
                ereport(WARNING,
                                (errcode_for_file_access(),
                                 errmsg("online backup mode was not canceled"),
                                 errdetail("File \"%s\" could not be renamed to \"%s\": %m.",
                                                   BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
                return;
        }

        /* if the tablespace_map file is not there, return */
        if (stat(TABLESPACE_MAP, &stat_buf) < 0)
        {
                ereport(LOG,
                                (errmsg("online backup mode canceled"),
                                 errdetail("File \"%s\" was renamed to \"%s\".",
                                                   BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
                return;
        }

        /* remove leftover file from previously canceled backup if it exists */
        unlink(TABLESPACE_MAP_OLD);

        if (durable_rename(TABLESPACE_MAP, TABLESPACE_MAP_OLD, DEBUG1) == 0)
        {
                ereport(LOG,
                                (errmsg("online backup mode canceled"),
                                 errdetail("Files \"%s\" and \"%s\" were renamed to "
                                                   "\"%s\" and \"%s\", respectively.",
                                                   BACKUP_LABEL_FILE, TABLESPACE_MAP,
                                                   BACKUP_LABEL_OLD, TABLESPACE_MAP_OLD)));
        }
        else
        {
                ereport(WARNING,
                                (errcode_for_file_access(),
                                 errmsg("online backup mode canceled"),
                                 errdetail("File \"%s\" was renamed to \"%s\", but "
                                                   "file \"%s\" could not be renamed to \"%s\": %m.",
                                                   BACKUP_LABEL_FILE, BACKUP_LABEL_OLD,
                                                   TABLESPACE_MAP, TABLESPACE_MAP_OLD)));
        }
}

/* Thin wrapper around ShutdownWalRcv(). */
void
XLogShutdownWalRcv(void)
{
        ShutdownWalRcv();

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        XLogCtl->InstallXLogFileSegmentActive = false;
        LWLockRelease(ControlFileLock);
}

/* Enable WAL file recycling and preallocation. */
void
SetInstallXLogFileSegmentActive(void)
{
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        XLogCtl->InstallXLogFileSegmentActive = true;
        LWLockRelease(ControlFileLock);
}

bool
IsInstallXLogFileSegmentActive(void)
{
        bool            result;

        LWLockAcquire(ControlFileLock, LW_SHARED);
        result = XLogCtl->InstallXLogFileSegmentActive;
        LWLockRelease(ControlFileLock);

        return result;
}

/*
 * Update the WalWriterSleeping flag.
 */
void
SetWalWriterSleeping(bool sleeping)
{
        SpinLockAcquire(&XLogCtl->info_lck);
        XLogCtl->WalWriterSleeping = sleeping;
        SpinLockRelease(&XLogCtl->info_lck);
}