If pg_stop_backup() is called just after switching to a new xlog file,

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

/*-------------------------------------------------------------------------
 *
 * xlog.c
 *              PostgreSQL transaction log manager
 *
 *
 * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <ctype.h>
#include <signal.h>
#include <time.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <unistd.h>

#include "access/clog.h"
#include "access/multixact.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogutils.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/pmsignal.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/ps_status.h"


/* File path names (all relative to $PGDATA) */
#define BACKUP_LABEL_FILE               "backup_label"
#define BACKUP_LABEL_OLD                "backup_label.old"
#define RECOVERY_COMMAND_FILE   "recovery.conf"
#define RECOVERY_COMMAND_DONE   "recovery.done"


/* User-settable parameters */
int                     CheckPointSegments = 3;
int                     XLOGbuffers = 8;
int                     XLogArchiveTimeout = 0;
bool            XLogArchiveMode = false;
char       *XLogArchiveCommand = NULL;
bool            fullPageWrites = true;
bool            log_checkpoints = false;
int             sync_method = DEFAULT_SYNC_METHOD;

#ifdef WAL_DEBUG
bool            XLOG_DEBUG = false;
#endif

/*
 * XLOGfileslop is the maximum number of preallocated future XLOG segments.
 * When we are done with an old XLOG segment file, we will recycle it as a
 * future XLOG segment as long as there aren't already XLOGfileslop future
 * segments; else we'll delete it.  This could be made a separate GUC
 * variable, but at present I think it's sufficient to hardwire it as
 * 2*CheckPointSegments+1.      Under normal conditions, a checkpoint will free
 * no more than 2*CheckPointSegments log segments, and we want to recycle all
 * of them; the +1 allows boundary cases to happen without wasting a
 * delete/create-segment cycle.
 */
#define XLOGfileslop    (2*CheckPointSegments + 1)

/*
 * 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}
};

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

/*
 * ThisTimeLineID will be same in all backends --- it identifies current
 * WAL timeline for the database system.
 */
TimeLineID      ThisTimeLineID = 0;

/* Are we doing recovery from XLOG? */
bool            InRecovery = false;

/* Are we recovering using offline XLOG archives? */
static bool InArchiveRecovery = false;

/* Was the last xlog file restored from archive, or local? */
static bool restoredFromArchive = false;

/* options taken from recovery.conf */
static char *recoveryRestoreCommand = NULL;
static bool recoveryTarget = false;
static bool recoveryTargetExact = false;
static bool recoveryTargetInclusive = true;
static bool recoveryLogRestartpoints = false;
static TransactionId recoveryTargetXid;
static TimestampTz recoveryTargetTime;
static TimestampTz recoveryLastXTime = 0;

/* if recoveryStopsHere returns true, it saves actual stop xid/time here */
static TransactionId recoveryStopXid;
static TimestampTz recoveryStopTime;
static bool recoveryStopAfter;

/*
 * During normal operation, the only timeline we care about is ThisTimeLineID.
 * During recovery, however, things are more complicated.  To simplify life
 * for rmgr code, we keep ThisTimeLineID set to the "current" timeline as we
 * scan through the WAL history (that is, it is the line that was active when
 * the currently-scanned WAL record was generated).  We also need these
 * timeline values:
 *
 * recoveryTargetTLI: the desired timeline that we want to end in.
 *
 * expectedTLIs: an integer list of recoveryTargetTLI and the TLIs of
 * its known parents, newest first (so recoveryTargetTLI is always the
 * first list member).  Only these TLIs are expected to be seen in the WAL
 * segments we read, and indeed only these TLIs will be considered as
 * candidate WAL files to open at all.
 *
 * curFileTLI: the TLI appearing in the name of the current input WAL file.
 * (This is not necessarily the same as ThisTimeLineID, because we could
 * be scanning data that was copied from an ancestor timeline when the current
 * file was created.)  During a sequential scan we do not allow this value
 * to decrease.
 */
static TimeLineID recoveryTargetTLI;
static List *expectedTLIs;
static TimeLineID curFileTLI;

/*
 * 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.
 */
static XLogRecPtr ProcLastRecPtr = {0, 0};

XLogRecPtr      XactLastRecEnd = {0, 0};

/*
 * 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 the Insert lock).  See XLogInsert for details.  We are also allowed
 * to update from XLogCtl->Insert.RedoRecPtr if we hold the info_lck;
 * see GetRedoRecPtr.  A freshly spawned backend obtains the value during
 * InitXLOGAccess.
 */
static XLogRecPtr RedoRecPtr;

/*----------
 * 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.
 *
 * We do a lot of pushups to minimize the amount of access to lockable
 * shared memory values.  There are actually three shared-memory copies of
 * LogwrtResult, plus one unshared copy in each backend.  Here's how it works:
 *              XLogCtl->LogwrtResult is protected by info_lck
 *              XLogCtl->Write.LogwrtResult is protected by WALWriteLock
 *              XLogCtl->Insert.LogwrtResult is protected by WALInsertLock
 * One must hold the associated lock to read or write any of these, but
 * of course no lock is needed to read/write the unshared LogwrtResult.
 *
 * XLogCtl->LogwrtResult and XLogCtl->Write.LogwrtResult are both "always
 * right", since both are updated by a write or flush operation before
 * it releases WALWriteLock.  The point of keeping XLogCtl->Write.LogwrtResult
 * is that it can be examined/modified by code that already holds WALWriteLock
 * without needing to grab info_lck as well.
 *
 * XLogCtl->Insert.LogwrtResult may lag behind the reality of the other two,
 * but is updated when convenient.      Again, it exists for the convenience of
 * code that is already holding WALInsertLock but not the other locks.
 *
 * The unshared LogwrtResult may lag behind any or all of these, and again
 * 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.
 *
 * Note that this all works because the request and result positions can only
 * advance forward, never back up, and so we can easily determine which of two
 * values is "more up to date".
 *
 * 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:
 *
 * WALInsertLock: must be held to insert a record into the WAL buffers.
 *
 * 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.
 *
 * CheckpointLock: must be held to do a checkpoint (ensures only one
 * checkpointer at a time; currently, with all checkpoints done by the
 * bgwriter, this is just pro forma).
 *
 *----------
 */

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;

/*
 * Shared state data for XLogInsert.
 */
typedef struct XLogCtlInsert
{
        XLogwrtResult LogwrtResult; /* a recent value of LogwrtResult */
        XLogRecPtr      PrevRecord;             /* start of previously-inserted record */
        int                     curridx;                /* current block index in cache */
        XLogPageHeader currpage;        /* points to header of block in cache */
        char       *currpos;            /* current insertion point in cache */
        XLogRecPtr      RedoRecPtr;             /* current redo point for insertions */
        bool            forcePageWrites;        /* forcing full-page writes for PITR? */
} XLogCtlInsert;

/*
 * Shared state data for XLogWrite/XLogFlush.
 */
typedef struct XLogCtlWrite
{
        XLogwrtResult LogwrtResult; /* current value of LogwrtResult */
        int                     curridx;                /* cache index of next block to write */
        pg_time_t       lastSegSwitchTime;              /* time of last xlog segment switch */
} XLogCtlWrite;

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

        /* Protected by info_lck: */
        XLogwrtRqst LogwrtRqst;
        XLogwrtResult LogwrtResult;
        uint32          ckptXidEpoch;   /* nextXID & epoch of latest checkpoint */
        TransactionId ckptXid;
        XLogRecPtr      asyncCommitLSN; /* LSN of newest async commit */

        /* Protected by WALWriteLock: */
        XLogCtlWrite Write;

        /*
         * These values do not change after startup, although the pointed-to pages
         * and xlblocks values certainly do.  Permission to read/write the pages
         * and xlblocks values depends on WALInsertLock and WALWriteLock.
         */
        char       *pages;                      /* buffers for unwritten XLOG pages */
        XLogRecPtr *xlblocks;           /* 1st byte ptr-s + XLOG_BLCKSZ */
        int                     XLogCacheBlck;  /* highest allocated xlog buffer index */
        TimeLineID      ThisTimeLineID;

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

static XLogCtlData *XLogCtl = NULL;

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

/*
 * Macros for managing XLogInsert state.  In most cases, the calling routine
 * has local copies of XLogCtl->Insert and/or XLogCtl->Insert->curridx,
 * so these are passed as parameters instead of being fetched via XLogCtl.
 */

/* Free space remaining in the current xlog page buffer */
#define INSERT_FREESPACE(Insert)  \
        (XLOG_BLCKSZ - ((Insert)->currpos - (char *) (Insert)->currpage))

/* Construct XLogRecPtr value for current insertion point */
#define INSERT_RECPTR(recptr,Insert,curridx)  \
        ( \
          (recptr).xlogid = XLogCtl->xlblocks[curridx].xlogid, \
          (recptr).xrecoff = \
                XLogCtl->xlblocks[curridx].xrecoff - INSERT_FREESPACE(Insert) \
        )

#define PrevBufIdx(idx)         \
                (((idx) == 0) ? XLogCtl->XLogCacheBlck : ((idx) - 1))

#define NextBufIdx(idx)         \
                (((idx) == XLogCtl->XLogCacheBlck) ? 0 : ((idx) + 1))

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

/*
 * openLogFile is -1 or a kernel FD for an open log file segment.
 * When it's open, openLogOff is the current seek offset in the file.
 * openLogId/openLogSeg identify the segment.  These variables are only
 * used to write the XLOG, and so will normally refer to the active segment.
 */
static int      openLogFile = -1;
static uint32 openLogId = 0;
static uint32 openLogSeg = 0;
static uint32 openLogOff = 0;

/*
 * These variables are used similarly to the ones above, but for reading
 * the XLOG.  Note, however, that readOff generally represents the offset
 * of the page just read, not the seek position of the FD itself, which
 * will be just past that page.
 */
static int      readFile = -1;
static uint32 readId = 0;
static uint32 readSeg = 0;
static uint32 readOff = 0;

/* Buffer for currently read page (XLOG_BLCKSZ bytes) */
static char *readBuf = NULL;

/* Buffer for current ReadRecord result (expandable) */
static char *readRecordBuf = NULL;
static uint32 readRecordBufSize = 0;

/* State information for XLOG reading */
static XLogRecPtr ReadRecPtr;   /* start of last record read */
static XLogRecPtr EndRecPtr;    /* end+1 of last record read */
static XLogRecord *nextRecord = NULL;
static TimeLineID lastPageTLI = 0;

static bool InRedo = false;


static void XLogArchiveNotify(const char *xlog);
static void XLogArchiveNotifySeg(uint32 log, uint32 seg);
static bool XLogArchiveCheckDone(const char *xlog);
static bool XLogArchiveIsBusy(const char *xlog);
static void XLogArchiveCleanup(const char *xlog);
static void readRecoveryCommandFile(void);
static void exitArchiveRecovery(TimeLineID endTLI,
                                        uint32 endLogId, uint32 endLogSeg);
static bool recoveryStopsHere(XLogRecord *record, bool *includeThis);
static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);

static bool XLogCheckBuffer(XLogRecData *rdata, bool doPageWrites,
                                XLogRecPtr *lsn, BkpBlock *bkpb);
static bool AdvanceXLInsertBuffer(bool new_segment);
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch);
static int XLogFileInit(uint32 log, uint32 seg,
                         bool *use_existent, bool use_lock);
static bool InstallXLogFileSegment(uint32 *log, uint32 *seg, char *tmppath,
                                           bool find_free, int *max_advance,
                                           bool use_lock);
static int      XLogFileOpen(uint32 log, uint32 seg);
static int      XLogFileRead(uint32 log, uint32 seg, int emode);
static void XLogFileClose(void);
static bool RestoreArchivedFile(char *path, const char *xlogfname,
                                        const char *recovername, off_t expectedSize);
static void PreallocXlogFiles(XLogRecPtr endptr);
static void RemoveOldXlogFiles(uint32 log, uint32 seg, XLogRecPtr endptr);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
static XLogRecord *ReadRecord(XLogRecPtr *RecPtr, int emode);
static bool ValidXLOGHeader(XLogPageHeader hdr, int emode);
static XLogRecord *ReadCheckpointRecord(XLogRecPtr RecPtr, int whichChkpt);
static List *readTimeLineHistory(TimeLineID targetTLI);
static bool existsTimeLineHistory(TimeLineID probeTLI);
static TimeLineID findNewestTimeLine(TimeLineID startTLI);
static void writeTimeLineHistory(TimeLineID newTLI, TimeLineID parentTLI,
                                         TimeLineID endTLI,
                                         uint32 endLogId, uint32 endLogSeg);
static void WriteControlFile(void);
static void ReadControlFile(void);
static char *str_time(pg_time_t tnow);
#ifdef WAL_DEBUG
static void xlog_outrec(StringInfo buf, XLogRecord *record);
#endif
static void issue_xlog_fsync(void);
static void pg_start_backup_callback(int code, Datum arg);
static bool read_backup_label(XLogRecPtr *checkPointLoc,
                                  XLogRecPtr *minRecoveryLoc);
static void rm_redo_error_callback(void *arg);
static int get_sync_bit(int method);


/*
 * Insert an XLOG record having the specified RMID and info bytes,
 * with the body of the record being the data chunk(s) described by
 * the rdata chain (see xlog.h for notes about rdata).
 *
 * 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".)
 *
 * NB: this routine feels free to scribble on the XLogRecData structs,
 * though not on the data they reference.  This is OK since the XLogRecData
 * structs are always just temporaries in the calling code.
 */
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        XLogRecord *record;
        XLogContRecord *contrecord;
        XLogRecPtr      RecPtr;
        XLogRecPtr      WriteRqst;
        uint32          freespace;
        int                     curridx;
        XLogRecData *rdt;
        Buffer          dtbuf[XLR_MAX_BKP_BLOCKS];
        bool            dtbuf_bkp[XLR_MAX_BKP_BLOCKS];
        BkpBlock        dtbuf_xlg[XLR_MAX_BKP_BLOCKS];
        XLogRecPtr      dtbuf_lsn[XLR_MAX_BKP_BLOCKS];
        XLogRecData dtbuf_rdt1[XLR_MAX_BKP_BLOCKS];
        XLogRecData dtbuf_rdt2[XLR_MAX_BKP_BLOCKS];
        XLogRecData dtbuf_rdt3[XLR_MAX_BKP_BLOCKS];
        pg_crc32        rdata_crc;
        uint32          len,
                                write_len;
        unsigned        i;
        bool            updrqst;
        bool            doPageWrites;
        bool            isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);

        /* info's high bits are reserved for use by me */
        if (info & XLR_INFO_MASK)
                elog(PANIC, "invalid xlog info mask %02X", info);

        /*
         * In bootstrap mode, we don't actually log anything but XLOG resources;
         * return a phony record pointer.
         */
        if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
        {
                RecPtr.xlogid = 0;
                RecPtr.xrecoff = SizeOfXLogLongPHD;             /* start of 1st chkpt record */
                return RecPtr;
        }

        /*
         * Here we scan the rdata chain, determine which buffers must be backed
         * up, and compute the CRC values for the data.  Note that the record
         * header isn't added into the CRC initially since we don't know the final
         * length or info bits quite yet.  Thus, the CRC will represent the CRC of
         * the whole record in the order "rdata, then backup blocks, then record
         * header".
         *
         * We may have to loop back to here if a race condition is detected below.
         * We could prevent the race by doing all this work while holding the
         * insert lock, but it seems better to avoid doing CRC calculations while
         * holding the lock.  This means we have to be careful about modifying the
         * rdata chain until we know we aren't going to loop back again.  The only
         * change we allow ourselves to make earlier is to set rdt->data = NULL in
         * chain items we have decided we will have to back up the whole buffer
         * for.  This is OK because we will certainly decide the same thing again
         * for those items if we do it over; doing it here saves an extra pass
         * over the chain later.
         */
begin:;
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                dtbuf[i] = InvalidBuffer;
                dtbuf_bkp[i] = false;
        }

        /*
         * Decide if we need to do full-page writes in this XLOG record: true if
         * full_page_writes is on or we have a PITR request for it.  Since we
         * don't yet have the insert lock, forcePageWrites could change under us,
         * but we'll recheck it once we have the lock.
         */
        doPageWrites = fullPageWrites || Insert->forcePageWrites;

        INIT_CRC32(rdata_crc);
        len = 0;
        for (rdt = rdata;;)
        {
                if (rdt->buffer == InvalidBuffer)
                {
                        /* Simple data, just include it */
                        len += rdt->len;
                        COMP_CRC32(rdata_crc, rdt->data, rdt->len);
                }
                else
                {
                        /* Find info for buffer */
                        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
                        {
                                if (rdt->buffer == dtbuf[i])
                                {
                                        /* Buffer already referenced by earlier chain item */
                                        if (dtbuf_bkp[i])
                                                rdt->data = NULL;
                                        else if (rdt->data)
                                        {
                                                len += rdt->len;
                                                COMP_CRC32(rdata_crc, rdt->data, rdt->len);
                                        }
                                        break;
                                }
                                if (dtbuf[i] == InvalidBuffer)
                                {
                                        /* OK, put it in this slot */
                                        dtbuf[i] = rdt->buffer;
                                        if (XLogCheckBuffer(rdt, doPageWrites,
                                                                                &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                                        {
                                                dtbuf_bkp[i] = true;
                                                rdt->data = NULL;
                                        }
                                        else if (rdt->data)
                                        {
                                                len += rdt->len;
                                                COMP_CRC32(rdata_crc, rdt->data, rdt->len);
                                        }
                                        break;
                                }
                        }
                        if (i >= XLR_MAX_BKP_BLOCKS)
                                elog(PANIC, "can backup at most %d blocks per xlog record",
                                         XLR_MAX_BKP_BLOCKS);
                }
                /* Break out of loop when rdt points to last chain item */
                if (rdt->next == NULL)
                        break;
                rdt = rdt->next;
        }

        /*
         * Now add the backup block headers and data into the CRC
         */
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                if (dtbuf_bkp[i])
                {
                        BkpBlock   *bkpb = &(dtbuf_xlg[i]);
                        char       *page;

                        COMP_CRC32(rdata_crc,
                                           (char *) bkpb,
                                           sizeof(BkpBlock));
                        page = (char *) BufferGetBlock(dtbuf[i]);
                        if (bkpb->hole_length == 0)
                        {
                                COMP_CRC32(rdata_crc,
                                                   page,
                                                   BLCKSZ);
                        }
                        else
                        {
                                /* must skip the hole */
                                COMP_CRC32(rdata_crc,
                                                   page,
                                                   bkpb->hole_offset);
                                COMP_CRC32(rdata_crc,
                                                   page + (bkpb->hole_offset + bkpb->hole_length),
                                                   BLCKSZ - (bkpb->hole_offset + bkpb->hole_length));
                        }
                }
        }

        /*
         * NOTE: We disallow len == 0 because it provides a useful bit of extra
         * error checking in ReadRecord.  This means that all callers of
         * XLogInsert must supply at least some not-in-a-buffer data.  However, we
         * make an exception for XLOG SWITCH records because we don't want them to
         * ever cross a segment boundary.
         */
        if (len == 0 && !isLogSwitch)
                elog(PANIC, "invalid xlog record length %u", len);

        START_CRIT_SECTION();

        /* Now wait to get insert lock */
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);

        /*
         * Check to see if my RedoRecPtr is out of date.  If so, may have to go
         * back and recompute everything.  This can only happen just after a
         * checkpoint, so it's better to be slow in this case and fast otherwise.
         *
         * 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 (!XLByteEQ(RedoRecPtr, Insert->RedoRecPtr))
        {
                Assert(XLByteLT(RedoRecPtr, Insert->RedoRecPtr));
                RedoRecPtr = Insert->RedoRecPtr;

                if (doPageWrites)
                {
                        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
                        {
                                if (dtbuf[i] == InvalidBuffer)
                                        continue;
                                if (dtbuf_bkp[i] == false &&
                                        XLByteLE(dtbuf_lsn[i], RedoRecPtr))
                                {
                                        /*
                                         * Oops, this buffer now needs to be backed up, but we
                                         * didn't think so above.  Start over.
                                         */
                                        LWLockRelease(WALInsertLock);
                                        END_CRIT_SECTION();
                                        goto begin;
                                }
                        }
                }
        }

        /*
         * Also check to see if forcePageWrites was just turned on; if we weren't
         * already doing full-page writes then go back and recompute. (If it was
         * just turned off, we could recompute the record without full pages, but
         * we choose not to bother.)
         */
        if (Insert->forcePageWrites && !doPageWrites)
        {
                /* Oops, must redo it with full-page data */
                LWLockRelease(WALInsertLock);
                END_CRIT_SECTION();
                goto begin;
        }

        /*
         * Make additional rdata chain entries for the backup blocks, so that we
         * don't need to special-case them in the write loop.  Note that we have
         * now irrevocably changed the input rdata chain.  At the exit of this
         * loop, write_len includes the backup block data.
         *
         * Also set the appropriate info bits to show which buffers were backed
         * up. The i'th XLR_SET_BKP_BLOCK bit corresponds to the i'th distinct
         * buffer value (ignoring InvalidBuffer) appearing in the rdata chain.
         */
        write_len = len;
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                BkpBlock   *bkpb;
                char       *page;

                if (!dtbuf_bkp[i])
                        continue;

                info |= XLR_SET_BKP_BLOCK(i);

                bkpb = &(dtbuf_xlg[i]);
                page = (char *) BufferGetBlock(dtbuf[i]);

                rdt->next = &(dtbuf_rdt1[i]);
                rdt = rdt->next;

                rdt->data = (char *) bkpb;
                rdt->len = sizeof(BkpBlock);
                write_len += sizeof(BkpBlock);

                rdt->next = &(dtbuf_rdt2[i]);
                rdt = rdt->next;

                if (bkpb->hole_length == 0)
                {
                        rdt->data = page;
                        rdt->len = BLCKSZ;
                        write_len += BLCKSZ;
                        rdt->next = NULL;
                }
                else
                {
                        /* must skip the hole */
                        rdt->data = page;
                        rdt->len = bkpb->hole_offset;
                        write_len += bkpb->hole_offset;

                        rdt->next = &(dtbuf_rdt3[i]);
                        rdt = rdt->next;

                        rdt->data = page + (bkpb->hole_offset + bkpb->hole_length);
                        rdt->len = BLCKSZ - (bkpb->hole_offset + bkpb->hole_length);
                        write_len += rdt->len;
                        rdt->next = NULL;
                }
        }

        /*
         * If we backed up any full blocks and online backup is not in progress,
         * mark the backup blocks as removable.  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.
         *
         * Note: we could just set the flag whenever !forcePageWrites, but
         * defining it like this leaves the info bit free for some potential other
         * use in records without any backup blocks.
         */
        if ((info & XLR_BKP_BLOCK_MASK) && !Insert->forcePageWrites)
                info |= XLR_BKP_REMOVABLE;

        /*
         * If there isn't enough space on the current XLOG page for a record
         * header, advance to the next page (leaving the unused space as zeroes).
         */
        updrqst = false;
        freespace = INSERT_FREESPACE(Insert);
        if (freespace < SizeOfXLogRecord)
        {
                updrqst = AdvanceXLInsertBuffer(false);
                freespace = INSERT_FREESPACE(Insert);
        }

        /* Compute record's XLOG location */
        curridx = Insert->curridx;
        INSERT_RECPTR(RecPtr, Insert, curridx);

        /*
         * If the record is an XLOG_SWITCH, and we are exactly at the start of a
         * segment, we need not insert it (and don't want to because we'd like
         * consecutive switch requests to be no-ops).  Instead, make sure
         * everything is written and flushed through the end of the prior segment,
         * and return the prior segment's end address.
         */
        if (isLogSwitch &&
                (RecPtr.xrecoff % XLogSegSize) == SizeOfXLogLongPHD)
        {
                /* We can release insert lock immediately */
                LWLockRelease(WALInsertLock);

                RecPtr.xrecoff -= SizeOfXLogLongPHD;
                if (RecPtr.xrecoff == 0)
                {
                        /* crossing a logid boundary */
                        RecPtr.xlogid -= 1;
                        RecPtr.xrecoff = XLogFileSize;
                }

                LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
                LogwrtResult = XLogCtl->Write.LogwrtResult;
                if (!XLByteLE(RecPtr, LogwrtResult.Flush))
                {
                        XLogwrtRqst FlushRqst;

                        FlushRqst.Write = RecPtr;
                        FlushRqst.Flush = RecPtr;
                        XLogWrite(FlushRqst, false, false);
                }
                LWLockRelease(WALWriteLock);

                END_CRIT_SECTION();

                return RecPtr;
        }

        /* Insert record header */

        record = (XLogRecord *) Insert->currpos;
        record->xl_prev = Insert->PrevRecord;
        record->xl_xid = GetCurrentTransactionIdIfAny();
        record->xl_tot_len = SizeOfXLogRecord + write_len;
        record->xl_len = len;           /* doesn't include backup blocks */
        record->xl_info = info;
        record->xl_rmid = rmid;

        /* Now we can finish computing the record's CRC */
        COMP_CRC32(rdata_crc, (char *) record + sizeof(pg_crc32),
                           SizeOfXLogRecord - sizeof(pg_crc32));
        FIN_CRC32(rdata_crc);
        record->xl_crc = rdata_crc;

#ifdef WAL_DEBUG
        if (XLOG_DEBUG)
        {
                StringInfoData buf;

                initStringInfo(&buf);
                appendStringInfo(&buf, "INSERT @ %X/%X: ",
                                                 RecPtr.xlogid, RecPtr.xrecoff);
                xlog_outrec(&buf, record);
                if (rdata->data != NULL)
                {
                        appendStringInfo(&buf, " - ");
                        RmgrTable[record->xl_rmid].rm_desc(&buf, record->xl_info, rdata->data);
                }
                elog(LOG, "%s", buf.data);
                pfree(buf.data);
        }
#endif

        /* Record begin of record in appropriate places */
        ProcLastRecPtr = RecPtr;
        Insert->PrevRecord = RecPtr;

        Insert->currpos += SizeOfXLogRecord;
        freespace -= SizeOfXLogRecord;

        /*
         * Append the data, including backup blocks if any
         */
        while (write_len)
        {
                while (rdata->data == NULL)
                        rdata = rdata->next;

                if (freespace > 0)
                {
                        if (rdata->len > freespace)
                        {
                                memcpy(Insert->currpos, rdata->data, freespace);
                                rdata->data += freespace;
                                rdata->len -= freespace;
                                write_len -= freespace;
                        }
                        else
                        {
                                memcpy(Insert->currpos, rdata->data, rdata->len);
                                freespace -= rdata->len;
                                write_len -= rdata->len;
                                Insert->currpos += rdata->len;
                                rdata = rdata->next;
                                continue;
                        }
                }

                /* Use next buffer */
                updrqst = AdvanceXLInsertBuffer(false);
                curridx = Insert->curridx;
                /* Insert cont-record header */
                Insert->currpage->xlp_info |= XLP_FIRST_IS_CONTRECORD;
                contrecord = (XLogContRecord *) Insert->currpos;
                contrecord->xl_rem_len = write_len;
                Insert->currpos += SizeOfXLogContRecord;
                freespace = INSERT_FREESPACE(Insert);
        }

        /* Ensure next record will be properly aligned */
        Insert->currpos = (char *) Insert->currpage +
                MAXALIGN(Insert->currpos - (char *) Insert->currpage);
        freespace = INSERT_FREESPACE(Insert);

        /*
         * The recptr I return is the beginning of the *next* record. This will be
         * stored as LSN for changed data pages...
         */
        INSERT_RECPTR(RecPtr, Insert, curridx);

        /*
         * If the record is an XLOG_SWITCH, we must now write and flush all the
         * existing data, and then forcibly advance to the start of the next
         * segment.  It's not good to do this I/O while holding the insert lock,
         * but there seems too much risk of confusion if we try to release the
         * lock sooner.  Fortunately xlog switch needn't be a high-performance
         * operation anyway...
         */
        if (isLogSwitch)
        {
                XLogCtlWrite *Write = &XLogCtl->Write;
                XLogwrtRqst FlushRqst;
                XLogRecPtr      OldSegEnd;

                LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);

                /*
                 * Flush through the end of the page containing XLOG_SWITCH, and
                 * perform end-of-segment actions (eg, notifying archiver).
                 */
                WriteRqst = XLogCtl->xlblocks[curridx];
                FlushRqst.Write = WriteRqst;
                FlushRqst.Flush = WriteRqst;
                XLogWrite(FlushRqst, false, true);

                /* Set up the next buffer as first page of next segment */
                /* Note: AdvanceXLInsertBuffer cannot need to do I/O here */
                (void) AdvanceXLInsertBuffer(true);

                /* There should be no unwritten data */
                curridx = Insert->curridx;
                Assert(curridx == Write->curridx);

                /* Compute end address of old segment */
                OldSegEnd = XLogCtl->xlblocks[curridx];
                OldSegEnd.xrecoff -= XLOG_BLCKSZ;
                if (OldSegEnd.xrecoff == 0)
                {
                        /* crossing a logid boundary */
                        OldSegEnd.xlogid -= 1;
                        OldSegEnd.xrecoff = XLogFileSize;
                }

                /* Make it look like we've written and synced all of old segment */
                LogwrtResult.Write = OldSegEnd;
                LogwrtResult.Flush = OldSegEnd;

                /*
                 * Update shared-memory status --- this code should match XLogWrite
                 */
                {
                        /* use volatile pointer to prevent code rearrangement */
                        volatile XLogCtlData *xlogctl = XLogCtl;

                        SpinLockAcquire(&xlogctl->info_lck);
                        xlogctl->LogwrtResult = LogwrtResult;
                        if (XLByteLT(xlogctl->LogwrtRqst.Write, LogwrtResult.Write))
                                xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
                        if (XLByteLT(xlogctl->LogwrtRqst.Flush, LogwrtResult.Flush))
                                xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
                        SpinLockRelease(&xlogctl->info_lck);
                }

                Write->LogwrtResult = LogwrtResult;

                LWLockRelease(WALWriteLock);

                updrqst = false;                /* done already */
        }
        else
        {
                /* normal case, ie not xlog switch */

                /* Need to update shared LogwrtRqst if some block was filled up */
                if (freespace < SizeOfXLogRecord)
                {
                        /* curridx is filled and available for writing out */
                        updrqst = true;
                }
                else
                {
                        /* if updrqst already set, write through end of previous buf */
                        curridx = PrevBufIdx(curridx);
                }
                WriteRqst = XLogCtl->xlblocks[curridx];
        }

        LWLockRelease(WALInsertLock);

        if (updrqst)
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                /* advance global request to include new block(s) */
                if (XLByteLT(xlogctl->LogwrtRqst.Write, WriteRqst))
                        xlogctl->LogwrtRqst.Write = WriteRqst;
                /* update local result copy while I have the chance */
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);
        }

        XactLastRecEnd = RecPtr;

        END_CRIT_SECTION();

        return RecPtr;
}

/*
 * Determine whether the buffer referenced by an XLogRecData item has to
 * be backed up, and if so fill a BkpBlock struct for it.  In any case
 * save the buffer's LSN at *lsn.
 */
static bool
XLogCheckBuffer(XLogRecData *rdata, bool doPageWrites,
                                XLogRecPtr *lsn, BkpBlock *bkpb)
{
        Page            page;

        page = BufferGetPage(rdata->buffer);

        /*
         * XXX We assume page LSN is first data on *every* page that can be passed
         * to XLogInsert, whether it otherwise has the standard page layout or
         * not.
         */
        *lsn = PageGetLSN(page);

        if (doPageWrites &&
                XLByteLE(PageGetLSN(page), RedoRecPtr))
        {
                /*
                 * The page needs to be backed up, so set up *bkpb
                 */
                BufferGetTag(rdata->buffer, &bkpb->node, &bkpb->fork, &bkpb->block);

                if (rdata->buffer_std)
                {
                        /* Assume we can omit data between pd_lower and pd_upper */
                        uint16          lower = ((PageHeader) page)->pd_lower;
                        uint16          upper = ((PageHeader) page)->pd_upper;

                        if (lower >= SizeOfPageHeaderData &&
                                upper > lower &&
                                upper <= BLCKSZ)
                        {
                                bkpb->hole_offset = lower;
                                bkpb->hole_length = upper - lower;
                        }
                        else
                        {
                                /* No "hole" to compress out */
                                bkpb->hole_offset = 0;
                                bkpb->hole_length = 0;
                        }
                }
                else
                {
                        /* Not a standard page header, don't try to eliminate "hole" */
                        bkpb->hole_offset = 0;
                        bkpb->hole_length = 0;
                }

                return true;                    /* buffer requires backup */
        }

        return false;                           /* buffer does not need to be backed up */
}

/*
 * XLogArchiveNotify
 *
 * Create an archive notification file
 *
 * The name of the notification file is the message that will be picked up
 * by the archiver, e.g. we write 0000000100000001000000C6.ready
 * and the archiver then knows to archive XLOGDIR/0000000100000001000000C6,
 * then when complete, rename it to 0000000100000001000000C6.done
 */
static void
XLogArchiveNotify(const char *xlog)
{
        char            archiveStatusPath[MAXPGPATH];
        FILE       *fd;

        /* insert an otherwise empty file called <XLOG>.ready */
        StatusFilePath(archiveStatusPath, xlog, ".ready");
        fd = AllocateFile(archiveStatusPath, "w");
        if (fd == NULL)
        {
                ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not create archive status file \"%s\": %m",
                                                archiveStatusPath)));
                return;
        }
        if (FreeFile(fd))
        {
                ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not write archive status file \"%s\": %m",
                                                archiveStatusPath)));
                return;
        }

        /* Notify archiver that it's got something to do */
        if (IsUnderPostmaster)
                SendPostmasterSignal(PMSIGNAL_WAKEN_ARCHIVER);
}

/*
 * Convenience routine to notify using log/seg representation of filename
 */
static void
XLogArchiveNotifySeg(uint32 log, uint32 seg)
{
        char            xlog[MAXFNAMELEN];

        XLogFileName(xlog, ThisTimeLineID, log, seg);
        XLogArchiveNotify(xlog);
}

/*
 * XLogArchiveCheckDone
 *
 * This is called when we are ready to delete or recycle an old XLOG segment
 * file or backup history file.  If it is okay to delete it then return true.
 * If it is not time to delete it, make sure a .ready file exists, and return
 * false.
 *
 * If <XLOG>.done exists, then return true; else if <XLOG>.ready exists,
 * then return false; else create <XLOG>.ready and return false.
 *
 * The reason we do things this way is so that if the original attempt to
 * create <XLOG>.ready fails, we'll retry during subsequent checkpoints.
 */
static bool
XLogArchiveCheckDone(const char *xlog)
{
        char            archiveStatusPath[MAXPGPATH];
        struct stat stat_buf;

        /* Always deletable if archiving is off */
        if (!XLogArchivingActive())
                return true;

        /* First check for .done --- this means archiver is done with it */
        StatusFilePath(archiveStatusPath, xlog, ".done");
        if (stat(archiveStatusPath, &stat_buf) == 0)
                return true;

        /* check for .ready --- this means archiver is still busy with it */
        StatusFilePath(archiveStatusPath, xlog, ".ready");
        if (stat(archiveStatusPath, &stat_buf) == 0)
                return false;

        /* Race condition --- maybe archiver just finished, so recheck */
        StatusFilePath(archiveStatusPath, xlog, ".done");
        if (stat(archiveStatusPath, &stat_buf) == 0)
                return true;

        /* Retry creation of the .ready file */
        XLogArchiveNotify(xlog);
        return false;
}

/*
 * XLogArchiveIsBusy
 *
 * Check to see if an XLOG segment file is still unarchived.
 * This is almost but not quite the inverse of XLogArchiveCheckDone: in
 * the first place we aren't chartered to recreate the .ready file, and
 * in the second place we should consider that if the file is already gone
 * then it's not busy.  (This check is needed to handle the race condition
 * that a checkpoint already deleted the no-longer-needed file.)
 */
static bool
XLogArchiveIsBusy(const char *xlog)
{
        char            archiveStatusPath[MAXPGPATH];
        struct stat stat_buf;

        /* First check for .done --- this means archiver is done with it */
        StatusFilePath(archiveStatusPath, xlog, ".done");
        if (stat(archiveStatusPath, &stat_buf) == 0)
                return false;

        /* check for .ready --- this means archiver is still busy with it */
        StatusFilePath(archiveStatusPath, xlog, ".ready");
        if (stat(archiveStatusPath, &stat_buf) == 0)
                return true;

        /* Race condition --- maybe archiver just finished, so recheck */
        StatusFilePath(archiveStatusPath, xlog, ".done");
        if (stat(archiveStatusPath, &stat_buf) == 0)
                return false;

        /*
         * Check to see if the WAL file has been removed by checkpoint,
         * which implies it has already been archived, and explains why we
         * can't see a status file for it.
         */
        snprintf(archiveStatusPath, MAXPGPATH, XLOGDIR "/%s", xlog);
        if (stat(archiveStatusPath, &stat_buf) != 0 &&
                errno == ENOENT)
                return false;

        return true;
}

/*
 * XLogArchiveCleanup
 *
 * Cleanup archive notification file(s) for a particular xlog segment
 */
static void
XLogArchiveCleanup(const char *xlog)
{
        char            archiveStatusPath[MAXPGPATH];

        /* Remove the .done file */
        StatusFilePath(archiveStatusPath, xlog, ".done");
        unlink(archiveStatusPath);
        /* should we complain about failure? */

        /* Remove the .ready file if present --- normally it shouldn't be */
        StatusFilePath(archiveStatusPath, xlog, ".ready");
        unlink(archiveStatusPath);
        /* should we complain about failure? */
}

/*
 * Advance the Insert state to the next buffer page, writing out the next
 * buffer if it still contains unwritten data.
 *
 * If new_segment is TRUE then we set up the next buffer page as the first
 * page of the next xlog segment file, possibly but not usually the next
 * consecutive file page.
 *
 * The global LogwrtRqst.Write pointer needs to be advanced to include the
 * just-filled page.  If we can do this for free (without an extra lock),
 * we do so here.  Otherwise the caller must do it.  We return TRUE if the
 * request update still needs to be done, FALSE if we did it internally.
 *
 * Must be called with WALInsertLock held.
 */
static bool
AdvanceXLInsertBuffer(bool new_segment)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        XLogCtlWrite *Write = &XLogCtl->Write;
        int                     nextidx = NextBufIdx(Insert->curridx);
        bool            update_needed = true;
        XLogRecPtr      OldPageRqstPtr;
        XLogwrtRqst WriteRqst;
        XLogRecPtr      NewPageEndPtr;
        XLogPageHeader NewPage;

        /* Use Insert->LogwrtResult copy if it's more fresh */
        if (XLByteLT(LogwrtResult.Write, Insert->LogwrtResult.Write))
                LogwrtResult = Insert->LogwrtResult;

        /*
         * 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 (!XLByteLE(OldPageRqstPtr, LogwrtResult.Write))
        {
                /* nope, got work to do... */
                XLogRecPtr      FinishedPageRqstPtr;

                FinishedPageRqstPtr = XLogCtl->xlblocks[Insert->curridx];

                /* Before waiting, get info_lck and update LogwrtResult */
                {
                        /* use volatile pointer to prevent code rearrangement */
                        volatile XLogCtlData *xlogctl = XLogCtl;

                        SpinLockAcquire(&xlogctl->info_lck);
                        if (XLByteLT(xlogctl->LogwrtRqst.Write, FinishedPageRqstPtr))
                                xlogctl->LogwrtRqst.Write = FinishedPageRqstPtr;
                        LogwrtResult = xlogctl->LogwrtResult;
                        SpinLockRelease(&xlogctl->info_lck);
                }

                update_needed = false;  /* Did the shared-request update */

                if (XLByteLE(OldPageRqstPtr, LogwrtResult.Write))
                {
                        /* OK, someone wrote it already */
                        Insert->LogwrtResult = LogwrtResult;
                }
                else
                {
                        /* Must acquire write lock */
                        LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
                        LogwrtResult = Write->LogwrtResult;
                        if (XLByteLE(OldPageRqstPtr, LogwrtResult.Write))
                        {
                                /* OK, someone wrote it already */
                                LWLockRelease(WALWriteLock);
                                Insert->LogwrtResult = LogwrtResult;
                        }
                        else
                        {
                                /*
                                 * Have to write buffers while holding insert lock. This is
                                 * not good, so only write as much as we absolutely must.
                                 */
                                WriteRqst.Write = OldPageRqstPtr;
                                WriteRqst.Flush.xlogid = 0;
                                WriteRqst.Flush.xrecoff = 0;
                                XLogWrite(WriteRqst, false, false);
                                LWLockRelease(WALWriteLock);
                                Insert->LogwrtResult = LogwrtResult;
                        }
                }
        }

        /*
         * Now the next buffer slot is free and we can set it up to be the next
         * output page.
         */
        NewPageEndPtr = XLogCtl->xlblocks[Insert->curridx];

        if (new_segment)
        {
                /* force it to a segment start point */
                NewPageEndPtr.xrecoff += XLogSegSize - 1;
                NewPageEndPtr.xrecoff -= NewPageEndPtr.xrecoff % XLogSegSize;
        }

        if (NewPageEndPtr.xrecoff >= XLogFileSize)
        {
                /* crossing a logid boundary */
                NewPageEndPtr.xlogid += 1;
                NewPageEndPtr.xrecoff = XLOG_BLCKSZ;
        }
        else
                NewPageEndPtr.xrecoff += XLOG_BLCKSZ;
        XLogCtl->xlblocks[nextidx] = NewPageEndPtr;
        NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);

        Insert->curridx = nextidx;
        Insert->currpage = NewPage;

        Insert->currpos = ((char *) NewPage) +SizeOfXLogShortPHD;

        /*
         * 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 = ThisTimeLineID;
        NewPage   ->xlp_pageaddr.xlogid = NewPageEndPtr.xlogid;
        NewPage   ->xlp_pageaddr.xrecoff = NewPageEndPtr.xrecoff - XLOG_BLCKSZ;

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

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

                Insert->currpos = ((char *) NewPage) +SizeOfXLogLongPHD;
        }

        return update_needed;
}

/*
 * Check whether we've consumed enough xlog space that a checkpoint is needed.
 *
 * Caller must have just finished filling the open log file (so that
 * openLogId/openLogSeg are valid).  We measure the distance from RedoRecPtr
 * to the open log file and see if that exceeds CheckPointSegments.
 *
 * Note: it is caller's responsibility that RedoRecPtr is up-to-date.
 */
static bool
XLogCheckpointNeeded(void)
{
        /*
         * A straight computation of segment number could overflow 32 bits. Rather
         * than assuming we have working 64-bit arithmetic, we compare the
         * highest-order bits separately, and force a checkpoint immediately when
         * they change.
         */
        uint32          old_segno,
                                new_segno;
        uint32          old_highbits,
                                new_highbits;

        old_segno = (RedoRecPtr.xlogid % XLogSegSize) * XLogSegsPerFile +
                (RedoRecPtr.xrecoff / XLogSegSize);
        old_highbits = RedoRecPtr.xlogid / XLogSegSize;
        new_segno = (openLogId % XLogSegSize) * XLogSegsPerFile + openLogSeg;
        new_highbits = openLogId / XLogSegSize;
        if (new_highbits != old_highbits ||
                new_segno >= old_segno + (uint32) (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.
 *
 * If xlog_switch == TRUE, we are intending an xlog segment switch, so
 * perform end-of-segment actions after writing the last page, even if
 * it's not physically the end of its segment.  (NB: this will work properly
 * only if caller specifies WriteRqst == page-end and flexible == false,
 * and there is some data to write.)
 *
 * Must be called with WALWriteLock held.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
{
        XLogCtlWrite *Write = &XLogCtl->Write;
        bool            ispartialpage;
        bool            last_iteration;
        bool            finishing_seg;
        bool            use_existent;
        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 = Write->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.  We advance Write->curridx only after successfully
         * writing pages.  (Right now, this refinement is useless since we are
         * going to PANIC if any error occurs anyway; but someday it may come in
         * useful.)
         */
        curridx = Write->curridx;

        while (XLByteLT(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.
                 */
                if (!XLByteLT(LogwrtResult.Write, XLogCtl->xlblocks[curridx]))
                        elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
                                 LogwrtResult.Write.xlogid, LogwrtResult.Write.xrecoff,
                                 XLogCtl->xlblocks[curridx].xlogid,
                                 XLogCtl->xlblocks[curridx].xrecoff);

                /* Advance LogwrtResult.Write to end of current buffer page */
                LogwrtResult.Write = XLogCtl->xlblocks[curridx];
                ispartialpage = XLByteLT(WriteRqst.Write, LogwrtResult.Write);

                if (!XLByteInPrevSeg(LogwrtResult.Write, openLogId, openLogSeg))
                {
                        /*
                         * 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, openLogId, openLogSeg);

                        /* create/use new log file */
                        use_existent = true;
                        openLogFile = XLogFileInit(openLogId, openLogSeg,
                                                                           &use_existent, true);
                        openLogOff = 0;
                }

                /* Make sure we have the current logfile open */
                if (openLogFile < 0)
                {
                        XLByteToPrevSeg(LogwrtResult.Write, openLogId, openLogSeg);
                        openLogFile = XLogFileOpen(openLogId, openLogSeg);
                        openLogOff = 0;
                }

                /* Add current page to the set of pending pages-to-dump */
                if (npages == 0)
                {
                        /* first of group */
                        startidx = curridx;
                        startoffset = (LogwrtResult.Write.xrecoff - XLOG_BLCKSZ) % XLogSegSize;
                }
                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 = !XLByteLT(LogwrtResult.Write, WriteRqst.Write);

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

                if (last_iteration ||
                        curridx == XLogCtl->XLogCacheBlck ||
                        finishing_seg)
                {
                        char       *from;
                        Size            nbytes;

                        /* Need to seek in the file? */
                        if (openLogOff != startoffset)
                        {
                                if (lseek(openLogFile, (off_t) startoffset, SEEK_SET) < 0)
                                        ereport(PANIC,
                                                        (errcode_for_file_access(),
                                                         errmsg("could not seek in log file %u, "
                                                                        "segment %u to offset %u: %m",
                                                                        openLogId, openLogSeg, startoffset)));
                                openLogOff = startoffset;
                        }

                        /* OK to write the page(s) */
                        from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
                        nbytes = npages * (Size) XLOG_BLCKSZ;
                        errno = 0;
                        if (write(openLogFile, from, nbytes) != nbytes)
                        {
                                /* if write didn't set errno, assume no disk space */
                                if (errno == 0)
                                        errno = ENOSPC;
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not write to log file %u, segment %u "
                                                                "at offset %u, length %lu: %m",
                                                                openLogId, openLogSeg,
                                                                openLogOff, (unsigned long) nbytes)));
                        }

                        /* Update state for write */
                        openLogOff += nbytes;
                        Write->curridx = ispartialpage ? curridx : NextBufIdx(curridx);
                        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.
                         *
                         * We also do this if this is the last page written for an xlog
                         * switch.
                         *
                         * 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 || (xlog_switch && last_iteration))
                        {
                                issue_xlog_fsync();
                                LogwrtResult.Flush = LogwrtResult.Write;                /* end of page */

                                if (XLogArchivingActive())
                                        XLogArchiveNotifySeg(openLogId, openLogSeg);

                                Write->lastSegSwitchTime = (pg_time_t) time(NULL);

                                /*
                                 * Signal bgwriter to start 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())
                                {
                                        (void) GetRedoRecPtr();
                                        if (XLogCheckpointNeeded())
                                                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);
        Assert(curridx == Write->curridx);

        /*
         * If asked to flush, do so
         */
        if (XLByteLT(LogwrtResult.Flush, WriteRqst.Flush) &&
                XLByteLT(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, openLogId, openLogSeg))
                                XLogFileClose();
                        if (openLogFile < 0)
                        {
                                XLByteToPrevSeg(LogwrtResult.Write, openLogId, openLogSeg);
                                openLogFile = XLogFileOpen(openLogId, openLogSeg);
                                openLogOff = 0;
                        }
                        issue_xlog_fsync();
                }
                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.
         */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->LogwrtResult = LogwrtResult;
                if (XLByteLT(xlogctl->LogwrtRqst.Write, LogwrtResult.Write))
                        xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
                if (XLByteLT(xlogctl->LogwrtRqst.Flush, LogwrtResult.Flush))
                        xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
                SpinLockRelease(&xlogctl->info_lck);
        }

        Write->LogwrtResult = LogwrtResult;
}

/*
 * Record the LSN for an asynchronous transaction commit.
 * (This should not be called for aborts, nor for synchronous commits.)
 */
void
XLogSetAsyncCommitLSN(XLogRecPtr asyncCommitLSN)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        if (XLByteLT(xlogctl->asyncCommitLSN, asyncCommitLSN))
                xlogctl->asyncCommitLSN = asyncCommitLSN;
        SpinLockRelease(&xlogctl->info_lck);
}

/*
 * 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;

        /* Disabled during REDO */
        if (InRedo)
                return;

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

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

        /* read LogwrtResult and update local state */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                if (XLByteLT(WriteRqstPtr, xlogctl->LogwrtRqst.Write))
                        WriteRqstPtr = xlogctl->LogwrtRqst.Write;
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /* done already? */
        if (!XLByteLE(record, LogwrtResult.Flush))
        {
                /* now wait for the write lock */
                LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
                LogwrtResult = XLogCtl->Write.LogwrtResult;
                if (!XLByteLE(record, LogwrtResult.Flush))
                {
                        /* try to write/flush later additions to XLOG as well */
                        if (LWLockConditionalAcquire(WALInsertLock, LW_EXCLUSIVE))
                        {
                                XLogCtlInsert *Insert = &XLogCtl->Insert;
                                uint32          freespace = INSERT_FREESPACE(Insert);

                                if (freespace < SizeOfXLogRecord)               /* buffer is full */
                                        WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
                                else
                                {
                                        WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
                                        WriteRqstPtr.xrecoff -= freespace;
                                }
                                LWLockRelease(WALInsertLock);
                                WriteRqst.Write = WriteRqstPtr;
                                WriteRqst.Flush = WriteRqstPtr;
                        }
                        else
                        {
                                WriteRqst.Write = WriteRqstPtr;
                                WriteRqst.Flush = record;
                        }
                        XLogWrite(WriteRqst, false, false);
                }
                LWLockRelease(WALWriteLock);
        }

        END_CRIT_SECTION();

        /*
         * 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 has actually
         * happened in the field several times with 7.1 releases. Note that we
         * cannot get here while InRedo is true, but if the bad page is brought in
         * and marked dirty during recovery then CreateCheckPoint will try to
         * flush it at the end of recovery.)
         *
         * The current approach is to ERROR under normal conditions, but only
         * WARNING during recovery, so that the system can be brought up even if
         * there's a corrupt 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 (XLByteLT(LogwrtResult.Flush, record))
                elog(InRecovery ? WARNING : ERROR,
                "xlog flush request %X/%X is not satisfied --- flushed only to %X/%X",
                         record.xlogid, record.xrecoff,
                         LogwrtResult.Flush.xlogid, LogwrtResult.Flush.xrecoff);
}

/*
 * Flush xlog, but without specifying exactly where to flush to.
 *
 * We normally flush only completed blocks; but if there is nothing to do on
 * that basis, we check for unflushed async commits in the current incomplete
 * block, and flush through the latest one of those.  Thus, if async commits
 * are not being used, we will flush complete blocks only.      We can guarantee
 * that async commits reach disk after at most three cycles; normally only
 * one or two.  (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.
 */
void
XLogBackgroundFlush(void)
{
        XLogRecPtr      WriteRqstPtr;
        bool            flexible = true;

        /* read LogwrtResult and update local state */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                LogwrtResult = xlogctl->LogwrtResult;
                WriteRqstPtr = xlogctl->LogwrtRqst.Write;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /* back off to last completed page boundary */
        WriteRqstPtr.xrecoff -= WriteRqstPtr.xrecoff % XLOG_BLCKSZ;

        /* if we have already flushed that far, consider async commit records */
        if (XLByteLE(WriteRqstPtr, LogwrtResult.Flush))
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                WriteRqstPtr = xlogctl->asyncCommitLSN;
                SpinLockRelease(&xlogctl->info_lck);
                flexible = false;               /* ensure it all gets written */
        }

        /* Done if already known flushed */
        if (XLByteLE(WriteRqstPtr, LogwrtResult.Flush))
                return;

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

        START_CRIT_SECTION();

        /* now wait for the write lock */
        LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
        LogwrtResult = XLogCtl->Write.LogwrtResult;
        if (!XLByteLE(WriteRqstPtr, LogwrtResult.Flush))
        {
                XLogwrtRqst WriteRqst;

                WriteRqst.Write = WriteRqstPtr;
                WriteRqst.Flush = WriteRqstPtr;
                XLogWrite(WriteRqst, flexible, false);
        }
        LWLockRelease(WALWriteLock);

        END_CRIT_SECTION();
}

/*
 * Flush any previous asynchronously-committed transactions' commit records.
 *
 * NOTE: it is unwise to assume that this provides any strong guarantees.
 * In particular, because of the inexact LSN bookkeeping used by clog.c,
 * we cannot assume that hint bits will be settable for these transactions.
 */
void
XLogAsyncCommitFlush(void)
{
        XLogRecPtr      WriteRqstPtr;

        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        WriteRqstPtr = xlogctl->asyncCommitLSN;
        SpinLockRelease(&xlogctl->info_lck);

        XLogFlush(WriteRqstPtr);
}

/*
 * 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)
{
        /* Quick exit if already known flushed */
        if (XLByteLE(record, LogwrtResult.Flush))
                return false;

        /* read LogwrtResult and update local state */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /* check again */
        if (XLByteLE(record, LogwrtResult.Flush))
                return false;

        return true;
}

/*
 * Create a new XLOG file segment, or open a pre-existing one.
 *
 * log, seg: identify segment to be created/opened.
 *
 * *use_existent: if TRUE, OK to use a pre-existing file (else, any
 * pre-existing file will be deleted).  On return, TRUE if a pre-existing
 * file was used.
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * 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.
 */
static int
XLogFileInit(uint32 log, uint32 seg,
                         bool *use_existent, bool use_lock)
{
        char            path[MAXPGPATH];
        char            tmppath[MAXPGPATH];
        char       *zbuffer;
        uint32          installed_log;
        uint32          installed_seg;
        int                     max_advance;
        int                     fd;
        int                     nbytes;

        XLogFilePath(path, ThisTimeLineID, log, seg);

        /*
         * Try to use existent file (checkpoint maker may have created it already)
         */
        if (*use_existent)
        {
                fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                                                   S_IRUSR | S_IWUSR);
                if (fd < 0)
                {
                        if (errno != ENOENT)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                                                                path, log, seg)));
                }
                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,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m", tmppath)));

        /*
         * Zero-fill the file.  We have to 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.
         *
         * Note: palloc zbuffer, instead of just using a local char array, to
         * ensure it is reasonably well-aligned; this may save a few cycles
         * transferring data to the kernel.
         */
        zbuffer = (char *) palloc0(XLOG_BLCKSZ);
        for (nbytes = 0; nbytes < XLogSegSize; nbytes += XLOG_BLCKSZ)
        {
                errno = 0;
                if ((int) write(fd, zbuffer, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
                {
                        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)));
                }
        }
        pfree(zbuffer);

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

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

        /*
         * Now move the segment into place with its final name.
         *
         * If caller didn't want to use a pre-existing file, get rid of any
         * pre-existing file.  Otherwise, 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_log = log;
        installed_seg = seg;
        max_advance = XLOGfileslop;
        if (!InstallXLogFileSegment(&installed_log, &installed_seg, tmppath,
                                                                *use_existent, &max_advance,
                                                                use_lock))
        {
                /* No need for any more future segments... */
                unlink(tmppath);
        }

        elog(DEBUG2, "done creating and filling new WAL file");

        /* Set flag to tell caller there was no existent file */
        *use_existent = false;

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

        return fd;
}

/*
 * Create a new XLOG file segment by copying a pre-existing one.
 *
 * log, seg: identify segment to be created.
 *
 * srcTLI, srclog, srcseg: identify segment to be copied (could be from
 *              a different timeline)
 *
 * 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(uint32 log, uint32 seg,
                         TimeLineID srcTLI, uint32 srclog, uint32 srcseg)
{
        char            path[MAXPGPATH];
        char            tmppath[MAXPGPATH];
        char            buffer[XLOG_BLCKSZ];
        int                     srcfd;
        int                     fd;
        int                     nbytes;

        /*
         * Open the source file
         */
        XLogFilePath(path, srcTLI, srclog, srcseg);
        srcfd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
        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 = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        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 < XLogSegSize; nbytes += sizeof(buffer))
        {
                errno = 0;
                if ((int) read(srcfd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
                {
                        if (errno != 0)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not read file \"%s\": %m", path)));
                        else
                                ereport(ERROR,
                                                (errmsg("not enough data in file \"%s\"", path)));
                }
                errno = 0;
                if ((int) write(fd, buffer, 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)));
                }
        }

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

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

        close(srcfd);

        /*
         * Now move the segment into place with its final name.
         */
        if (!InstallXLogFileSegment(&log, &seg, tmppath, false, NULL, false))
                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.
 *
 * *log, *seg: identify segment to install as (or first possible target).
 * When find_free is TRUE, these are 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 log/seg
 * number at or after the passed numbers.  If FALSE, install the new segment
 * exactly where specified, deleting any existing segment file there.
 *
 * *max_advance: maximum number of log/seg slots to advance past the starting
 * point.  Fail if no free slot is found in this range.  On return, reduced
 * by the number of slots skipped over.  (Irrelevant, and may be NULL,
 * when find_free is FALSE.)
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns TRUE if file installed, FALSE if not installed because of
 * exceeding max_advance limit.  On Windows, we also return FALSE if we
 * can't rename the file into place because someone's got it open.
 * (Any other kind of failure causes ereport().)
 */
static bool
InstallXLogFileSegment(uint32 *log, uint32 *seg, char *tmppath,
                                           bool find_free, int *max_advance,
                                           bool use_lock)
{
        char            path[MAXPGPATH];
        struct stat stat_buf;

        XLogFilePath(path, ThisTimeLineID, *log, *seg);

        /*
         * We want to be sure that only one process does this at a time.
         */
        if (use_lock)
                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

        if (!find_free)
        {
                /* Force installation: get rid of any pre-existing segment file */
                unlink(path);
        }
        else
        {
                /* Find a free slot to put it in */
                while (stat(path, &stat_buf) == 0)
                {
                        if (*max_advance <= 0)
                        {
                                /* Failed to find a free slot within specified range */
                                if (use_lock)
                                        LWLockRelease(ControlFileLock);
                                return false;
                        }
                        NextLogSeg(*log, *seg);
                        (*max_advance)--;
                        XLogFilePath(path, ThisTimeLineID, *log, *seg);
                }
        }

        /*
         * Prefer link() to rename() here just to be really sure that we don't
         * overwrite an existing logfile.  However, there shouldn't be one, so
         * rename() is an acceptable substitute except for the truly paranoid.
         */
#if HAVE_WORKING_LINK
        if (link(tmppath, path) < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not link file \"%s\" to \"%s\" (initialization of log file %u, segment %u): %m",
                                                tmppath, path, *log, *seg)));
        unlink(tmppath);
#else
        if (rename(tmppath, path) < 0)
        {
#ifdef WIN32
#if !defined(__CYGWIN__)
                if (GetLastError() == ERROR_ACCESS_DENIED)
#else
                if (errno == EACCES)
#endif
                {
                        if (use_lock)
                                LWLockRelease(ControlFileLock);
                        return false;
                }
#endif   /* WIN32 */

                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not rename file \"%s\" to \"%s\" (initialization of log file %u, segment %u): %m",
                                                tmppath, path, *log, *seg)));
        }
#endif

        if (use_lock)
                LWLockRelease(ControlFileLock);

        return true;
}

/*
 * Open a pre-existing logfile segment for writing.
 */
static int
XLogFileOpen(uint32 log, uint32 seg)
{
        char            path[MAXPGPATH];
        int                     fd;

        XLogFilePath(path, ThisTimeLineID, log, seg);

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

        return fd;
}

/*
 * Open a logfile segment for reading (during recovery).
 */
static int
XLogFileRead(uint32 log, uint32 seg, int emode)
{
        char            path[MAXPGPATH];
        char            xlogfname[MAXFNAMELEN];
        char            activitymsg[MAXFNAMELEN + 16];
        ListCell   *cell;
        int                     fd;

        /*
         * Loop looking for a suitable timeline ID: we might need to read any of
         * the timelines listed in expectedTLIs.
         *
         * We expect curFileTLI on entry to be the TLI of the preceding file in
         * sequence, or 0 if there was no predecessor.  We do not allow curFileTLI
         * to go backwards; this prevents us from picking up the wrong file when a
         * parent timeline extends to higher segment numbers than the child we
         * want to read.
         */
        foreach(cell, expectedTLIs)
        {
                TimeLineID      tli = (TimeLineID) lfirst_int(cell);

                if (tli < curFileTLI)
                        break;                          /* don't bother looking at too-old TLIs */

                XLogFileName(xlogfname, tli, log, seg);

                if (InArchiveRecovery)
                {
                        /* Report recovery progress in PS display */
                        snprintf(activitymsg, sizeof(activitymsg), "waiting for %s",
                                         xlogfname);
                        set_ps_display(activitymsg, false);

                        restoredFromArchive = RestoreArchivedFile(path, xlogfname,
                                                                                                          "RECOVERYXLOG",
                                                                                                          XLogSegSize);
                }
                else
                        XLogFilePath(path, tli, log, seg);

                fd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
                if (fd >= 0)
                {
                        /* Success! */
                        curFileTLI = tli;

                        /* Report recovery progress in PS display */
                        snprintf(activitymsg, sizeof(activitymsg), "recovering %s",
                                         xlogfname);
                        set_ps_display(activitymsg, false);

                        return fd;
                }
                if (errno != ENOENT)    /* unexpected failure? */
                        ereport(PANIC,
                                        (errcode_for_file_access(),
                        errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                                   path, log, seg)));
        }

        /* Couldn't find it.  For simplicity, complain about front timeline */
        XLogFilePath(path, recoveryTargetTLI, log, seg);
        errno = ENOENT;
        ereport(emode,
                        (errcode_for_file_access(),
                   errmsg("could not open file \"%s\" (log file %u, segment %u): %m",
                                  path, log, seg)));
        return -1;
}

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

        /*
         * posix_fadvise is problematic on many platforms: on older x86 Linux it
         * just dumps core, and there are reports of problems on PPC platforms as
         * well.  The following is therefore disabled for the time being. We could
         * consider some kind of configure test to see if it's safe to use, but
         * since we lack hard evidence that there's any useful performance gain to
         * be had, spending time on that seems unprofitable for now.
         */
#ifdef NOT_USED

        /*
         * WAL segment files will not be re-read in normal operation, so we advise
         * OS to release any cached pages.      But do not do so if WAL archiving is
         * active, because archiver process could use the cache to read the WAL
         * segment.
         *
         * While O_DIRECT works for O_SYNC, posix_fadvise() works for fsync() and
         * O_SYNC, and some platforms only have posix_fadvise().
         */
#if defined(HAVE_DECL_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
        if (!XLogArchivingActive())
                posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
#endif
#endif   /* NOT_USED */

        if (close(openLogFile))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close log file %u, segment %u: %m",
                                                openLogId, openLogSeg)));
        openLogFile = -1;
}

/*
 * Attempt to retrieve the specified file from off-line archival storage.
 * If successful, fill "path" with its complete path (note that this will be
 * a temp file name that doesn't follow the normal naming convention), and
 * return TRUE.
 *
 * If not successful, fill "path" with the name of the normal on-line file
 * (which may or may not actually exist, but we'll try to use it), and return
 * FALSE.
 *
 * For fixed-size files, the caller may pass the expected size as an
 * additional crosscheck on successful recovery.  If the file size is not
 * known, set expectedSize = 0.
 */
static bool
RestoreArchivedFile(char *path, const char *xlogfname,
                                        const char *recovername, off_t expectedSize)
{
        char            xlogpath[MAXPGPATH];
        char            xlogRestoreCmd[MAXPGPATH];
        char            lastRestartPointFname[MAXPGPATH];
        char       *dp;
        char       *endp;
        const char *sp;
        int                     rc;
        bool            signaled;
        struct stat stat_buf;
        uint32          restartLog;
        uint32          restartSeg;

        /*
         * When doing archive recovery, we always prefer an archived log file even
         * if a file of the same name exists in XLOGDIR.  The reason is that the
         * file in XLOGDIR could be an old, un-filled or partly-filled version
         * that was copied and restored as part of backing up $PGDATA.
         *
         * We could try to optimize this slightly by checking the local copy
         * lastchange timestamp against the archived copy, but we have no API to
         * do this, nor can we guarantee that the lastchange timestamp was
         * preserved correctly when we copied to archive. Our aim is robustness,
         * so we elect not to do this.
         *
         * If we cannot obtain the log file from the archive, however, we will try
         * to use the XLOGDIR file if it exists.  This is so that we can make use
         * of log segments that weren't yet transferred to the archive.
         *
         * Notice that we don't actually overwrite any files when we copy back
         * from archive because the recoveryRestoreCommand may inadvertently
         * restore inappropriate xlogs, or they may be corrupt, so we may wish to
         * fallback to the segments remaining in current XLOGDIR later. The
         * copy-from-archive filename is always the same, ensuring that we don't
         * run out of disk space on long recoveries.
         */
        snprintf(xlogpath, MAXPGPATH, XLOGDIR "/%s", recovername);

        /*
         * Make sure there is no existing file named recovername.
         */
        if (stat(xlogpath, &stat_buf) != 0)
        {
                if (errno != ENOENT)
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not stat file \"%s\": %m",
                                                        xlogpath)));
        }
        else
        {
                if (unlink(xlogpath) != 0)
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not remove file \"%s\": %m",
                                                        xlogpath)));
        }

        /*
         * Calculate the archive file cutoff point for use during log shipping
         * replication. All files earlier than this point can be deleted
         * from the archive, though there is no requirement to do so.
         *
         * We initialise this with the filename of an InvalidXLogRecPtr, which
         * will prevent the deletion of any WAL files from the archive
         * because of the alphabetic sorting property of WAL filenames.
         *
         * Once we have successfully located the redo pointer of the checkpoint
         * from which we start recovery we never request a file prior to the redo
         * pointer of the last restartpoint. When redo begins we know that we
         * have successfully located it, so there is no need for additional
         * status flags to signify the point when we can begin deleting WAL files
         * from the archive.
         */
        if (InRedo)
        {
                XLByteToSeg(ControlFile->checkPointCopy.redo,
                                        restartLog, restartSeg);
                XLogFileName(lastRestartPointFname,
                                         ControlFile->checkPointCopy.ThisTimeLineID,
                                         restartLog, restartSeg);
                /* we shouldn't need anything earlier than last restart point */
                Assert(strcmp(lastRestartPointFname, xlogfname) <= 0);
        }
        else
                XLogFileName(lastRestartPointFname, 0, 0, 0);

        /*
         * construct the command to be executed
         */
        dp = xlogRestoreCmd;
        endp = xlogRestoreCmd + MAXPGPATH - 1;
        *endp = '\0';

        for (sp = recoveryRestoreCommand; *sp; sp++)
        {
                if (*sp == '%')
                {
                        switch (sp[1])
                        {
                                case 'p':
                                        /* %p: relative path of target file */
                                        sp++;
                                        StrNCpy(dp, xlogpath, endp - dp);
                                        make_native_path(dp);
                                        dp += strlen(dp);
                                        break;
                                case 'f':
                                        /* %f: filename of desired file */
                                        sp++;
                                        StrNCpy(dp, xlogfname, endp - dp);
                                        dp += strlen(dp);
                                        break;
                                case 'r':
                                        /* %r: filename of last restartpoint */
                                        sp++;
                                        StrNCpy(dp, lastRestartPointFname, endp - dp);
                                        dp += strlen(dp);
                                        break;
                                case '%':
                                        /* convert %% to a single % */
                                        sp++;
                                        if (dp < endp)
                                                *dp++ = *sp;
                                        break;
                                default:
                                        /* otherwise treat the % as not special */
                                        if (dp < endp)
                                                *dp++ = *sp;
                                        break;
                        }
                }
                else
                {
                        if (dp < endp)
                                *dp++ = *sp;
                }
        }
        *dp = '\0';

        ereport(DEBUG3,
                        (errmsg_internal("executing restore command \"%s\"",
                                                         xlogRestoreCmd)));

        /*
         * Copy xlog from archival storage to XLOGDIR
         */
        rc = system(xlogRestoreCmd);
        if (rc == 0)
        {
                /*
                 * command apparently succeeded, but let's make sure the file is
                 * really there now and has the correct size.
                 *
                 * XXX I made wrong-size a fatal error to ensure the DBA would notice
                 * it, but is that too strong?  We could try to plow ahead with a
                 * local copy of the file ... but the problem is that there probably
                 * isn't one, and we'd incorrectly conclude we've reached the end of
                 * WAL and we're done recovering ...
                 */
                if (stat(xlogpath, &stat_buf) == 0)
                {
                        if (expectedSize > 0 && stat_buf.st_size != expectedSize)
                                ereport(FATAL,
                                                (errmsg("archive file \"%s\" has wrong size: %lu instead of %lu",
                                                                xlogfname,
                                                                (unsigned long) stat_buf.st_size,
                                                                (unsigned long) expectedSize)));
                        else
                        {
                                ereport(LOG,
                                                (errmsg("restored log file \"%s\" from archive",
                                                                xlogfname)));
                                strcpy(path, xlogpath);
                                return true;
                        }
                }
                else
                {
                        /* stat failed */
                        if (errno != ENOENT)
                                ereport(FATAL,
                                                (errcode_for_file_access(),
                                                 errmsg("could not stat file \"%s\": %m",
                                                                xlogpath)));
                }
        }

        /*
         * Remember, we rollforward UNTIL the restore fails so failure here is
         * just part of the process... that makes it difficult to determine
         * whether the restore failed because there isn't an archive to restore,
         * or because the administrator has specified the restore program
         * incorrectly.  We have to assume the former.
         *
         * However, if the failure was due to any sort of signal, it's best to
         * punt and abort recovery.  (If we "return false" here, upper levels will
         * assume that recovery is complete and start up the database!) It's
         * essential to abort on child SIGINT and SIGQUIT, because per spec
         * system() ignores SIGINT and SIGQUIT while waiting; if we see one of
         * those it's a good bet we should have gotten it too.  Aborting on other
         * signals such as SIGTERM seems a good idea as well.
         *
         * Per the Single Unix Spec, shells report exit status > 128 when a called
         * command died on a signal.  Also, 126 and 127 are used to report
         * problems such as an unfindable command; treat those as fatal errors
         * too.
         */
        signaled = WIFSIGNALED(rc) || WEXITSTATUS(rc) > 125;

        ereport(signaled ? FATAL : DEBUG2,
                (errmsg("could not restore file \"%s\" from archive: return code %d",
                                xlogfname, rc)));

        /*
         * if an archived file is not available, there might still be a version of
         * this file in XLOGDIR, so return that as the filename to open.
         *
         * In many recovery scenarios we expect this to fail also, but if so that
         * just means we've reached the end of WAL.
         */
        snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlogfname);
        return false;
}

/*
 * 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.
 */
static void
PreallocXlogFiles(XLogRecPtr endptr)
{
        uint32          _logId;
        uint32          _logSeg;
        int                     lf;
        bool            use_existent;

        XLByteToPrevSeg(endptr, _logId, _logSeg);
        if ((endptr.xrecoff - 1) % XLogSegSize >=
                (uint32) (0.75 * XLogSegSize))
        {
                NextLogSeg(_logId, _logSeg);
                use_existent = true;
                lf = XLogFileInit(_logId, _logSeg, &use_existent, true);
                close(lf);
                if (!use_existent)
                        CheckpointStats.ckpt_segs_added++;
        }
}

/*
 * Recycle or remove all log files older or equal to passed log/seg#
 *
 * endptr is current (or recent) end of xlog; this is used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 */
static void
RemoveOldXlogFiles(uint32 log, uint32 seg, XLogRecPtr endptr)
{
        uint32          endlogId;
        uint32          endlogSeg;
        int                     max_advance;
        DIR                *xldir;
        struct dirent *xlde;
        char            lastoff[MAXFNAMELEN];
        char            path[MAXPGPATH];

        /*
         * Initialize info about where to try to recycle to.  We allow recycling
         * segments up to XLOGfileslop segments beyond the current XLOG location.
         */
        XLByteToPrevSeg(endptr, endlogId, endlogSeg);
        max_advance = XLOGfileslop;

        xldir = AllocateDir(XLOGDIR);
        if (xldir == NULL)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open transaction log directory \"%s\": %m",
                                                XLOGDIR)));

        XLogFileName(lastoff, ThisTimeLineID, log, seg);

        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                /*
                 * 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 (strlen(xlde->d_name) == 24 &&
                        strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
                        strcmp(xlde->d_name + 8, lastoff + 8) <= 0)
                {
                        if (XLogArchiveCheckDone(xlde->d_name))
                        {
                                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);

                                /*
                                 * Before deleting the file, see if it can be recycled as a
                                 * future log segment.
                                 */
                                if (InstallXLogFileSegment(&endlogId, &endlogSeg, path,
                                                                                   true, &max_advance,
                                                                                   true))
                                {
                                        ereport(DEBUG2,
                                                        (errmsg("recycled transaction log file \"%s\"",
                                                                        xlde->d_name)));
                                        CheckpointStats.ckpt_segs_recycled++;
                                        /* Needn't recheck that slot on future iterations */
                                        if (max_advance > 0)
                                        {
                                                NextLogSeg(endlogId, endlogSeg);
                                                max_advance--;
                                        }
                                }
                                else
                                {
                                        /* No need for any more future segments... */
                                        ereport(DEBUG2,
                                                        (errmsg("removing transaction log file \"%s\"",
                                                                        xlde->d_name)));
                                        unlink(path);
                                        CheckpointStats.ckpt_segs_removed++;
                                }

                                XLogArchiveCleanup(xlde->d_name);
                        }
                }
        }

        FreeDir(xldir);
}

/*
 * Verify whether pg_xlog and pg_xlog/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_xlog from the copy.
 *
 * We could also recreate pg_xlog if it doesn't exist, but a deliberate
 * policy decision was made not to.  It is fairly common for pg_xlog 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_xlog; 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 (mkdir(path, 0700) < 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];

        xldir = AllocateDir(XLOGDIR);
        if (xldir == NULL)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open transaction log directory \"%s\": %m",
                                                XLOGDIR)));

        while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
        {
                if (strlen(xlde->d_name) > 24 &&
                        strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
                        strcmp(xlde->d_name + strlen(xlde->d_name) - strlen(".backup"),
                                   ".backup") == 0)
                {
                        if (XLogArchiveCheckDone(xlde->d_name))
                        {
                                ereport(DEBUG2,
                                (errmsg("removing transaction log backup history file \"%s\"",
                                                xlde->d_name)));
                                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);
                                unlink(path);
                                XLogArchiveCleanup(xlde->d_name);
                        }
                }
        }

        FreeDir(xldir);
}

/*
 * Restore the backup blocks present in an XLOG record, if any.
 *
 * We assume all of the record has been read into memory at *record.
 *
 * Note: when a backup block is available in XLOG, we restore it
 * unconditionally, even if the page in the database appears newer.
 * This is to protect ourselves against database pages that were partially
 * or incorrectly written during a crash.  We assume that the XLOG data
 * must be good because it has passed a CRC check, while the database
 * page might not be.  This will force us to replay all subsequent
 * modifications of the page that appear in XLOG, rather than possibly
 * ignoring them as already applied, but that's not a huge drawback.
 */
static void
RestoreBkpBlocks(XLogRecord *record, XLogRecPtr lsn)
{
        Buffer          buffer;
        Page            page;
        BkpBlock        bkpb;
        char       *blk;
        int                     i;

        blk = (char *) XLogRecGetData(record) + record->xl_len;
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                if (!(record->xl_info & XLR_SET_BKP_BLOCK(i)))
                        continue;

                memcpy(&bkpb, blk, sizeof(BkpBlock));
                blk += sizeof(BkpBlock);

                buffer = XLogReadBufferExtended(bkpb.node, bkpb.fork, bkpb.block,
                                                                                RBM_ZERO);
                Assert(BufferIsValid(buffer));
                page = (Page) BufferGetPage(buffer);

                if (bkpb.hole_length == 0)
                {
                        memcpy((char *) page, blk, BLCKSZ);
                }
                else
                {
                        /* must zero-fill the hole */
                        MemSet((char *) page, 0, BLCKSZ);
                        memcpy((char *) page, blk, bkpb.hole_offset);
                        memcpy((char *) page + (bkpb.hole_offset + bkpb.hole_length),
                                   blk + bkpb.hole_offset,
                                   BLCKSZ - (bkpb.hole_offset + bkpb.hole_length));
                }

                PageSetLSN(page, lsn);
                PageSetTLI(page, ThisTimeLineID);
                MarkBufferDirty(buffer);
                UnlockReleaseBuffer(buffer);

                blk += BLCKSZ - bkpb.hole_length;
        }
}

/*
 * CRC-check an XLOG record.  We do not believe the contents of an XLOG
 * record (other than to the minimal extent of computing the amount of
 * data to read in) until we've checked the CRCs.
 *
 * We assume all of the record has been read into memory at *record.
 */
static bool
RecordIsValid(XLogRecord *record, XLogRecPtr recptr, int emode)
{
        pg_crc32        crc;
        int                     i;
        uint32          len = record->xl_len;
        BkpBlock        bkpb;
        char       *blk;

        /* First the rmgr data */
        INIT_CRC32(crc);
        COMP_CRC32(crc, XLogRecGetData(record), len);

        /* Add in the backup blocks, if any */
        blk = (char *) XLogRecGetData(record) + len;
        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                uint32          blen;

                if (!(record->xl_info & XLR_SET_BKP_BLOCK(i)))
                        continue;

                memcpy(&bkpb, blk, sizeof(BkpBlock));
                if (bkpb.hole_offset + bkpb.hole_length > BLCKSZ)
                {
                        ereport(emode,
                                        (errmsg("incorrect hole size in record at %X/%X",
                                                        recptr.xlogid, recptr.xrecoff)));
                        return false;
                }
                blen = sizeof(BkpBlock) + BLCKSZ - bkpb.hole_length;
                COMP_CRC32(crc, blk, blen);
                blk += blen;
        }

        /* Check that xl_tot_len agrees with our calculation */
        if (blk != (char *) record + record->xl_tot_len)
        {
                ereport(emode,
                                (errmsg("incorrect total length in record at %X/%X",
                                                recptr.xlogid, recptr.xrecoff)));
                return false;
        }

        /* Finally include the record header */
        COMP_CRC32(crc, (char *) record + sizeof(pg_crc32),
                           SizeOfXLogRecord - sizeof(pg_crc32));
        FIN_CRC32(crc);

        if (!EQ_CRC32(record->xl_crc, crc))
        {
                ereport(emode,
                (errmsg("incorrect resource manager data checksum in record at %X/%X",
                                recptr.xlogid, recptr.xrecoff)));
                return false;
        }

        return true;
}

/*
 * Attempt to read an XLOG record.
 *
 * If RecPtr is not NULL, try to read a record at that position.  Otherwise
 * try to read a record just after the last one previously read.
 *
 * If no valid record is available, returns NULL, or fails if emode is PANIC.
 * (emode must be either PANIC or LOG.)
 *
 * The record is copied into readRecordBuf, so that on successful return,
 * the returned record pointer always points there.
 */
static XLogRecord *
ReadRecord(XLogRecPtr *RecPtr, int emode)
{
        XLogRecord *record;
        char       *buffer;
        XLogRecPtr      tmpRecPtr = EndRecPtr;
        bool            randAccess = false;
        uint32          len,
                                total_len;
        uint32          targetPageOff;
        uint32          targetRecOff;
        uint32          pageHeaderSize;

        if (readBuf == NULL)
        {
                /*
                 * First time through, permanently allocate readBuf.  We do it this
                 * way, rather than just making a static array, for two reasons: (1)
                 * no need to waste the storage in most instantiations of the backend;
                 * (2) a static char array isn't guaranteed to have any particular
                 * alignment, whereas malloc() will provide MAXALIGN'd storage.
                 */
                readBuf = (char *) malloc(XLOG_BLCKSZ);
                Assert(readBuf != NULL);
        }

        if (RecPtr == NULL)
        {
                RecPtr = &tmpRecPtr;
                /* fast case if next record is on same page */
                if (nextRecord != NULL)
                {
                        record = nextRecord;
                        goto got_record;
                }
                /* align old recptr to next page */
                if (tmpRecPtr.xrecoff % XLOG_BLCKSZ != 0)
                        tmpRecPtr.xrecoff += (XLOG_BLCKSZ - tmpRecPtr.xrecoff % XLOG_BLCKSZ);
                if (tmpRecPtr.xrecoff >= XLogFileSize)
                {
                        (tmpRecPtr.xlogid)++;
                        tmpRecPtr.xrecoff = 0;
                }
                /* We will account for page header size below */
        }
        else
        {
                if (!XRecOffIsValid(RecPtr->xrecoff))
                        ereport(PANIC,
                                        (errmsg("invalid record offset at %X/%X",
                                                        RecPtr->xlogid, RecPtr->xrecoff)));

                /*
                 * Since we are going to a random position in WAL, forget any prior
                 * state about what timeline we were in, and allow it to be any
                 * timeline in expectedTLIs.  We also set a flag to allow curFileTLI
                 * to go backwards (but we can't reset that variable right here, since
                 * we might not change files at all).
                 */
                lastPageTLI = 0;                /* see comment in ValidXLOGHeader */
                randAccess = true;              /* allow curFileTLI to go backwards too */
        }

        if (readFile >= 0 && !XLByteInSeg(*RecPtr, readId, readSeg))
        {
                close(readFile);
                readFile = -1;
        }
        XLByteToSeg(*RecPtr, readId, readSeg);
        if (readFile < 0)
        {
                /* Now it's okay to reset curFileTLI if random fetch */
                if (randAccess)
                        curFileTLI = 0;

                readFile = XLogFileRead(readId, readSeg, emode);
                if (readFile < 0)
                        goto next_record_is_invalid;

                /*
                 * Whenever switching to a new WAL segment, we read the first page of
                 * the file and validate its header, even if that's not where the
                 * target record is.  This is so that we can check the additional
                 * identification info that is present in the first page's "long"
                 * header.
                 */
                readOff = 0;
                if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
                {
                        ereport(emode,
                                        (errcode_for_file_access(),
                                         errmsg("could not read from log file %u, segment %u, offset %u: %m",
                                                        readId, readSeg, readOff)));
                        goto next_record_is_invalid;
                }
                if (!ValidXLOGHeader((XLogPageHeader) readBuf, emode))
                        goto next_record_is_invalid;
        }

        targetPageOff = ((RecPtr->xrecoff % XLogSegSize) / XLOG_BLCKSZ) * XLOG_BLCKSZ;
        if (readOff != targetPageOff)
        {
                readOff = targetPageOff;
                if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
                {
                        ereport(emode,
                                        (errcode_for_file_access(),
                                         errmsg("could not seek in log file %u, segment %u to offset %u: %m",
                                                        readId, readSeg, readOff)));
                        goto next_record_is_invalid;
                }
                if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
                {
                        ereport(emode,
                                        (errcode_for_file_access(),
                                         errmsg("could not read from log file %u, segment %u, offset %u: %m",
                                                        readId, readSeg, readOff)));
                        goto next_record_is_invalid;
                }
                if (!ValidXLOGHeader((XLogPageHeader) readBuf, emode))
                        goto next_record_is_invalid;
        }
        pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
        targetRecOff = RecPtr->xrecoff % XLOG_BLCKSZ;
        if (targetRecOff == 0)
        {
                /*
                 * Can only get here in the continuing-from-prev-page case, because
                 * XRecOffIsValid eliminated the zero-page-offset case otherwise. Need
                 * to skip over the new page's header.
                 */
                tmpRecPtr.xrecoff += pageHeaderSize;
                targetRecOff = pageHeaderSize;
        }
        else if (targetRecOff < pageHeaderSize)
        {
                ereport(emode,
                                (errmsg("invalid record offset at %X/%X",
                                                RecPtr->xlogid, RecPtr->xrecoff)));
                goto next_record_is_invalid;
        }
        if ((((XLogPageHeader) readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) &&
                targetRecOff == pageHeaderSize)
        {
                ereport(emode,
                                (errmsg("contrecord is requested by %X/%X",
                                                RecPtr->xlogid, RecPtr->xrecoff)));
                goto next_record_is_invalid;
        }
        record = (XLogRecord *) ((char *) readBuf + RecPtr->xrecoff % XLOG_BLCKSZ);

got_record:;

        /*
         * xl_len == 0 is bad data for everything except XLOG SWITCH, where it is
         * required.
         */
        if (record->xl_rmid == RM_XLOG_ID && record->xl_info == XLOG_SWITCH)
        {
                if (record->xl_len != 0)
                {
                        ereport(emode,
                                        (errmsg("invalid xlog switch record at %X/%X",
                                                        RecPtr->xlogid, RecPtr->xrecoff)));
                        goto next_record_is_invalid;
                }
        }
        else if (record->xl_len == 0)
        {
                ereport(emode,
                                (errmsg("record with zero length at %X/%X",
                                                RecPtr->xlogid, RecPtr->xrecoff)));
                goto next_record_is_invalid;
        }
        if (record->xl_tot_len < SizeOfXLogRecord + record->xl_len ||
                record->xl_tot_len > SizeOfXLogRecord + record->xl_len +
                XLR_MAX_BKP_BLOCKS * (sizeof(BkpBlock) + BLCKSZ))
        {
                ereport(emode,
                                (errmsg("invalid record length at %X/%X",
                                                RecPtr->xlogid, RecPtr->xrecoff)));
                goto next_record_is_invalid;
        }
        if (record->xl_rmid > RM_MAX_ID)
        {
                ereport(emode,
                                (errmsg("invalid resource manager ID %u at %X/%X",
                                                record->xl_rmid, RecPtr->xlogid, RecPtr->xrecoff)));
                goto next_record_is_invalid;
        }
        if (randAccess)
        {
                /*
                 * We can't exactly verify the prev-link, but surely it should be less
                 * than the record's own address.
                 */
                if (!XLByteLT(record->xl_prev, *RecPtr))
                {
                        ereport(emode,
                                        (errmsg("record with incorrect prev-link %X/%X at %X/%X",
                                                        record->xl_prev.xlogid, record->xl_prev.xrecoff,
                                                        RecPtr->xlogid, RecPtr->xrecoff)));
                        goto next_record_is_invalid;
                }
        }
        else
        {
                /*
                 * Record's prev-link should exactly match our previous location. This
                 * check guards against torn WAL pages where a stale but valid-looking
                 * WAL record starts on a sector boundary.
                 */
                if (!XLByteEQ(record->xl_prev, ReadRecPtr))
                {
                        ereport(emode,
                                        (errmsg("record with incorrect prev-link %X/%X at %X/%X",
                                                        record->xl_prev.xlogid, record->xl_prev.xrecoff,
                                                        RecPtr->xlogid, RecPtr->xrecoff)));
                        goto next_record_is_invalid;
                }
        }

        /*
         * Allocate or enlarge readRecordBuf as needed.  To avoid useless small
         * increases, round its size to a multiple of XLOG_BLCKSZ, and make sure
         * it's at least 4*Max(BLCKSZ, XLOG_BLCKSZ) to start with.  (That is
         * enough for all "normal" records, but very large commit or abort records
         * might need more space.)
         */
        total_len = record->xl_tot_len;
        if (total_len > readRecordBufSize)
        {
                uint32          newSize = total_len;

                newSize += XLOG_BLCKSZ - (newSize % XLOG_BLCKSZ);
                newSize = Max(newSize, 4 * Max(BLCKSZ, XLOG_BLCKSZ));
                if (readRecordBuf)
                        free(readRecordBuf);
                readRecordBuf = (char *) malloc(newSize);
                if (!readRecordBuf)
                {
                        readRecordBufSize = 0;
                        /* We treat this as a "bogus data" condition */
                        ereport(emode,
                                        (errmsg("record length %u at %X/%X too long",
                                                        total_len, RecPtr->xlogid, RecPtr->xrecoff)));
                        goto next_record_is_invalid;
                }
                readRecordBufSize = newSize;
        }

        buffer = readRecordBuf;
        nextRecord = NULL;
        len = XLOG_BLCKSZ - RecPtr->xrecoff % XLOG_BLCKSZ;
        if (total_len > len)
        {
                /* Need to reassemble record */
                XLogContRecord *contrecord;
                uint32          gotlen = len;

                memcpy(buffer, record, len);
                record = (XLogRecord *) buffer;
                buffer += len;
                for (;;)
                {
                        readOff += XLOG_BLCKSZ;
                        if (readOff >= XLogSegSize)
                        {
                                close(readFile);
                                readFile = -1;
                                NextLogSeg(readId, readSeg);
                                readFile = XLogFileRead(readId, readSeg, emode);
                                if (readFile < 0)
                                        goto next_record_is_invalid;
                                readOff = 0;
                        }
                        if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
                        {
                                ereport(emode,
                                                (errcode_for_file_access(),
                                                 errmsg("could not read from log file %u, segment %u, offset %u: %m",
                                                                readId, readSeg, readOff)));
                                goto next_record_is_invalid;
                        }
                        if (!ValidXLOGHeader((XLogPageHeader) readBuf, emode))
                                goto next_record_is_invalid;
                        if (!(((XLogPageHeader) readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD))
                        {
                                ereport(emode,
                                                (errmsg("there is no contrecord flag in log file %u, segment %u, offset %u",
                                                                readId, readSeg, readOff)));
                                goto next_record_is_invalid;
                        }
                        pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
                        contrecord = (XLogContRecord *) ((char *) readBuf + pageHeaderSize);
                        if (contrecord->xl_rem_len == 0 ||
                                total_len != (contrecord->xl_rem_len + gotlen))
                        {
                                ereport(emode,
                                                (errmsg("invalid contrecord length %u in log file %u, segment %u, offset %u",
                                                                contrecord->xl_rem_len,
                                                                readId, readSeg, readOff)));
                                goto next_record_is_invalid;
                        }
                        len = XLOG_BLCKSZ - pageHeaderSize - SizeOfXLogContRecord;
                        if (contrecord->xl_rem_len > len)
                        {
                                memcpy(buffer, (char *) contrecord + SizeOfXLogContRecord, len);
                                gotlen += len;
                                buffer += len;
                                continue;
                        }
                        memcpy(buffer, (char *) contrecord + SizeOfXLogContRecord,
                                   contrecord->xl_rem_len);
                        break;
                }
                if (!RecordIsValid(record, *RecPtr, emode))
                        goto next_record_is_invalid;
                pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) readBuf);
                if (XLOG_BLCKSZ - SizeOfXLogRecord >= pageHeaderSize +
                        MAXALIGN(SizeOfXLogContRecord + contrecord->xl_rem_len))
                {
                        nextRecord = (XLogRecord *) ((char *) contrecord +
                                        MAXALIGN(SizeOfXLogContRecord + contrecord->xl_rem_len));
                }
                EndRecPtr.xlogid = readId;
                EndRecPtr.xrecoff = readSeg * XLogSegSize + readOff +
                        pageHeaderSize +
                        MAXALIGN(SizeOfXLogContRecord + contrecord->xl_rem_len);
                ReadRecPtr = *RecPtr;
                /* needn't worry about XLOG SWITCH, it can't cross page boundaries */
                return record;
        }

        /* Record does not cross a page boundary */
        if (!RecordIsValid(record, *RecPtr, emode))
                goto next_record_is_invalid;
        if (XLOG_BLCKSZ - SizeOfXLogRecord >= RecPtr->xrecoff % XLOG_BLCKSZ +
                MAXALIGN(total_len))
                nextRecord = (XLogRecord *) ((char *) record + MAXALIGN(total_len));
        EndRecPtr.xlogid = RecPtr->xlogid;
        EndRecPtr.xrecoff = RecPtr->xrecoff + MAXALIGN(total_len);
        ReadRecPtr = *RecPtr;
        memcpy(buffer, record, total_len);

        /*
         * Special processing if it's an XLOG SWITCH record
         */
        if (record->xl_rmid == RM_XLOG_ID && record->xl_info == XLOG_SWITCH)
        {
                /* Pretend it extends to end of segment */
                EndRecPtr.xrecoff += XLogSegSize - 1;
                EndRecPtr.xrecoff -= EndRecPtr.xrecoff % XLogSegSize;
                nextRecord = NULL;              /* definitely not on same page */

                /*
                 * Pretend that readBuf contains the last page of the segment. This is
                 * just to avoid Assert failure in StartupXLOG if XLOG ends with this
                 * segment.
                 */
                readOff = XLogSegSize - XLOG_BLCKSZ;
        }
        return (XLogRecord *) buffer;

next_record_is_invalid:;
        if (readFile >= 0)
        {
                close(readFile);
                readFile = -1;
        }
        nextRecord = NULL;
        return NULL;
}

/*
 * Check whether the xlog header of a page just read in looks valid.
 *
 * This is just a convenience subroutine to avoid duplicated code in
 * ReadRecord.  It's not intended for use from anywhere else.
 */
static bool
ValidXLOGHeader(XLogPageHeader hdr, int emode)
{
        XLogRecPtr      recaddr;

        if (hdr->xlp_magic != XLOG_PAGE_MAGIC)
        {
                ereport(emode,
                                (errmsg("invalid magic number %04X in log file %u, segment %u, offset %u",
                                                hdr->xlp_magic, readId, readSeg, readOff)));
                return false;
        }
        if ((hdr->xlp_info & ~XLP_ALL_FLAGS) != 0)
        {
                ereport(emode,
                                (errmsg("invalid info bits %04X in log file %u, segment %u, offset %u",
                                                hdr->xlp_info, readId, readSeg, readOff)));
                return false;
        }
        if (hdr->xlp_info & XLP_LONG_HEADER)
        {
                XLogLongPageHeader longhdr = (XLogLongPageHeader) hdr;

                if (longhdr->xlp_sysid != ControlFile->system_identifier)
                {
                        char            fhdrident_str[32];
                        char            sysident_str[32];

                        /*
                         * Format sysids separately to keep platform-dependent format code
                         * out of the translatable message string.
                         */
                        snprintf(fhdrident_str, sizeof(fhdrident_str), UINT64_FORMAT,
                                         longhdr->xlp_sysid);
                        snprintf(sysident_str, sizeof(sysident_str), UINT64_FORMAT,
                                         ControlFile->system_identifier);
                        ereport(emode,
                                        (errmsg("WAL file is from different system"),
                                         errdetail("WAL file SYSID is %s, pg_control SYSID is %s",
                                                           fhdrident_str, sysident_str)));
                        return false;
                }
                if (longhdr->xlp_seg_size != XLogSegSize)
                {
                        ereport(emode,
                                        (errmsg("WAL file is from different system"),
                                         errdetail("Incorrect XLOG_SEG_SIZE in page header.")));
                        return false;
                }
                if (longhdr->xlp_xlog_blcksz != XLOG_BLCKSZ)
                {
                        ereport(emode,
                                        (errmsg("WAL file is from different system"),
                                         errdetail("Incorrect XLOG_BLCKSZ in page header.")));
                        return false;
                }
        }
        else if (readOff == 0)
        {
                /* hmm, first page of file doesn't have a long header? */
                ereport(emode,
                                (errmsg("invalid info bits %04X in log file %u, segment %u, offset %u",
                                                hdr->xlp_info, readId, readSeg, readOff)));
                return false;
        }

        recaddr.xlogid = readId;
        recaddr.xrecoff = readSeg * XLogSegSize + readOff;
        if (!XLByteEQ(hdr->xlp_pageaddr, recaddr))
        {
                ereport(emode,
                                (errmsg("unexpected pageaddr %X/%X in log file %u, segment %u, offset %u",
                                                hdr->xlp_pageaddr.xlogid, hdr->xlp_pageaddr.xrecoff,
                                                readId, readSeg, readOff)));
                return false;
        }

        /*
         * Check page TLI is one of the expected values.
         */
        if (!list_member_int(expectedTLIs, (int) hdr->xlp_tli))
        {
                ereport(emode,
                                (errmsg("unexpected timeline ID %u in log file %u, segment %u, offset %u",
                                                hdr->xlp_tli,
                                                readId, readSeg, readOff)));
                return false;
        }

        /*
         * Since child timelines are always assigned a TLI greater than their
         * immediate parent's TLI, we should never see TLI go backwards across
         * successive pages of a consistent WAL sequence.
         *
         * Of course this check should only be applied when advancing sequentially
         * across pages; therefore ReadRecord resets lastPageTLI to zero when
         * going to a random page.
         */
        if (hdr->xlp_tli < lastPageTLI)
        {
                ereport(emode,
                                (errmsg("out-of-sequence timeline ID %u (after %u) in log file %u, segment %u, offset %u",
                                                hdr->xlp_tli, lastPageTLI,
                                                readId, readSeg, readOff)));
                return false;
        }
        lastPageTLI = hdr->xlp_tli;
        return true;
}

/*
 * Try to read a timeline's history file.
 *
 * If successful, return the list of component TLIs (the given TLI followed by
 * its ancestor TLIs).  If we can't find the history file, assume that the
 * timeline has no parents, and return a list of just the specified timeline
 * ID.
 */
static List *
readTimeLineHistory(TimeLineID targetTLI)
{
        List       *result;
        char            path[MAXPGPATH];
        char            histfname[MAXFNAMELEN];
        char            fline[MAXPGPATH];
        FILE       *fd;

        if (InArchiveRecovery)
        {
                TLHistoryFileName(histfname, targetTLI);
                RestoreArchivedFile(path, histfname, "RECOVERYHISTORY", 0);
        }
        else
                TLHistoryFilePath(path, targetTLI);

        fd = AllocateFile(path, "r");
        if (fd == NULL)
        {
                if (errno != ENOENT)
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not open file \"%s\": %m", path)));
                /* Not there, so assume no parents */
                return list_make1_int((int) targetTLI);
        }

        result = NIL;

        /*
         * Parse the file...
         */
        while (fgets(fline, sizeof(fline), fd) != NULL)
        {
                /* skip leading whitespace and check for # comment */
                char       *ptr;
                char       *endptr;
                TimeLineID      tli;

                for (ptr = fline; *ptr; ptr++)
                {
                        if (!isspace((unsigned char) *ptr))
                                break;
                }
                if (*ptr == '\0' || *ptr == '#')
                        continue;

                /* expect a numeric timeline ID as first field of line */
                tli = (TimeLineID) strtoul(ptr, &endptr, 0);
                if (endptr == ptr)
                        ereport(FATAL,
                                        (errmsg("syntax error in history file: %s", fline),
                                         errhint("Expected a numeric timeline ID.")));

                if (result &&
                        tli <= (TimeLineID) linitial_int(result))
                        ereport(FATAL,
                                        (errmsg("invalid data in history file: %s", fline),
                                   errhint("Timeline IDs must be in increasing sequence.")));

                /* Build list with newest item first */
                result = lcons_int((int) tli, result);

                /* we ignore the remainder of each line */
        }

        FreeFile(fd);

        if (result &&
                targetTLI <= (TimeLineID) linitial_int(result))
                ereport(FATAL,
                                (errmsg("invalid data in history file \"%s\"", path),
                        errhint("Timeline IDs must be less than child timeline's ID.")));

        result = lcons_int((int) targetTLI, result);

        ereport(DEBUG3,
                        (errmsg_internal("history of timeline %u is %s",
                                                         targetTLI, nodeToString(result))));

        return result;
}

/*
 * Probe whether a timeline history file exists for the given timeline ID
 */
static bool
existsTimeLineHistory(TimeLineID probeTLI)
{
        char            path[MAXPGPATH];
        char            histfname[MAXFNAMELEN];
        FILE       *fd;

        if (InArchiveRecovery)
        {
                TLHistoryFileName(histfname, probeTLI);
                RestoreArchivedFile(path, histfname, "RECOVERYHISTORY", 0);
        }
        else
                TLHistoryFilePath(path, probeTLI);

        fd = AllocateFile(path, "r");
        if (fd != NULL)
        {
                FreeFile(fd);
                return true;
        }
        else
        {
                if (errno != ENOENT)
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not open file \"%s\": %m", path)));
                return false;
        }
}

/*
 * Find the newest existing timeline, assuming that startTLI exists.
 *
 * Note: while this is somewhat heuristic, it does positively guarantee
 * that (result + 1) is not a known timeline, and therefore it should
 * be safe to assign that ID to a new timeline.
 */
static TimeLineID
findNewestTimeLine(TimeLineID startTLI)
{
        TimeLineID      newestTLI;
        TimeLineID      probeTLI;

        /*
         * The algorithm is just to probe for the existence of timeline history
         * files.  XXX is it useful to allow gaps in the sequence?
         */
        newestTLI = startTLI;

        for (probeTLI = startTLI + 1;; probeTLI++)
        {
                if (existsTimeLineHistory(probeTLI))
                {
                        newestTLI = probeTLI;           /* probeTLI exists */
                }
                else
                {
                        /* doesn't exist, assume we're done */
                        break;
                }
        }

        return newestTLI;
}

/*
 * Create a new timeline history file.
 *
 *      newTLI: ID of the new timeline
 *      parentTLI: ID of its immediate parent
 *      endTLI et al: ID of the last used WAL file, for annotation purposes
 *
 * 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
writeTimeLineHistory(TimeLineID newTLI, TimeLineID parentTLI,
                                         TimeLineID endTLI, uint32 endLogId, uint32 endLogSeg)
{
        char            path[MAXPGPATH];
        char            tmppath[MAXPGPATH];
        char            histfname[MAXFNAMELEN];
        char            xlogfname[MAXFNAMELEN];
        char            buffer[BLCKSZ];
        int                     srcfd;
        int                     fd;
        int                     nbytes;

        Assert(newTLI > parentTLI); /* else bad selection of newTLI */

        /*
         * Write 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 = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create file \"%s\": %m", tmppath)));

        /*
         * If a history file exists for the parent, copy it verbatim
         */
        if (InArchiveRecovery)
        {
                TLHistoryFileName(histfname, parentTLI);
                RestoreArchivedFile(path, histfname, "RECOVERYHISTORY", 0);
        }
        else
                TLHistoryFilePath(path, parentTLI);

        srcfd = BasicOpenFile(path, O_RDONLY, 0);
        if (srcfd < 0)
        {
                if (errno != ENOENT)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not open file \"%s\": %m", path)));
                /* Not there, so assume parent has no parents */
        }
        else
        {
                for (;;)
                {
                        errno = 0;
                        nbytes = (int) read(srcfd, buffer, sizeof(buffer));
                        if (nbytes < 0 || errno != 0)
                                ereport(ERROR,
                                                (errcode_for_file_access(),
                                                 errmsg("could not read file \"%s\": %m", path)));
                        if (nbytes == 0)
                                break;
                        errno = 0;
                        if ((int) write(fd, buffer, nbytes) != nbytes)
                        {
                                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)));
                        }
                }
                close(srcfd);
        }

        /*
         * Append one line with the details of this timeline split.
         *
         * If we did have a parent file, insert an extra newline just in case the
         * parent file failed to end with one.
         */
        XLogFileName(xlogfname, endTLI, endLogId, endLogSeg);

        snprintf(buffer, sizeof(buffer),
                         "%s%u\t%s\t%s transaction %u at %s\n",
                         (srcfd < 0) ? "" : "\n",
                         parentTLI,
                         xlogfname,
                         recoveryStopAfter ? "after" : "before",
                         recoveryStopXid,
                         timestamptz_to_str(recoveryStopTime));

        nbytes = strlen(buffer);
        errno = 0;
        if ((int) write(fd, buffer, nbytes) != nbytes)
        {
                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)));
        }

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

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


        /*
         * Now move the completed history file into place with its final name.
         */
        TLHistoryFilePath(path, newTLI);

        /*
         * Prefer link() to rename() here just to be really sure that we don't
         * overwrite an existing logfile.  However, there shouldn't be one, so
         * rename() is an acceptable substitute except for the truly paranoid.
         */
#if HAVE_WORKING_LINK
        if (link(tmppath, path) < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not link file \"%s\" to \"%s\": %m",
                                                tmppath, path)));
        unlink(tmppath);
#else
        if (rename(tmppath, path) < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                tmppath, path)));
#endif

        /* The history file can be archived immediately. */
        TLHistoryFileName(histfname, newTLI);
        XLogArchiveNotify(histfname);
}

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

        /*
         * 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 = XLOG_SEG_SIZE;

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

        ControlFile->toast_max_chunk_size = TOAST_MAX_CHUNK_SIZE;

#ifdef HAVE_INT64_TIMESTAMP
        ControlFile->enableIntTimes = true;
#else
        ControlFile->enableIntTimes = false;
#endif
        ControlFile->float4ByVal = FLOAT4PASSBYVAL;
        ControlFile->float8ByVal = FLOAT8PASSBYVAL;

        /* Contents are protected with a CRC */
        INIT_CRC32(ControlFile->crc);
        COMP_CRC32(ControlFile->crc,
                           (char *) ControlFile,
                           offsetof(ControlFileData, crc));
        FIN_CRC32(ControlFile->crc);

        /*
         * We write out PG_CONTROL_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".
         */
        if (sizeof(ControlFileData) > PG_CONTROL_SIZE)
                elog(PANIC, "sizeof(ControlFileData) is larger than PG_CONTROL_SIZE; fix either one");

        memset(buffer, 0, PG_CONTROL_SIZE);
        memcpy(buffer, ControlFile, sizeof(ControlFileData));

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

        errno = 0;
        if (write(fd, buffer, PG_CONTROL_SIZE) != PG_CONTROL_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 control file: %m")));
        }

        if (pg_fsync(fd) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync control file: %m")));

        if (close(fd))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close control file: %m")));
}

static void
ReadControlFile(void)
{
        pg_crc32        crc;
        int                     fd;

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

        if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not read from control file: %m")));

        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_CRC32(crc);
        COMP_CRC32(crc,
                           (char *) ControlFile,
                           offsetof(ControlFileData, crc));
        FIN_CRC32(crc);

        if (!EQ_CRC32(crc, ControlFile->crc))
                ereport(FATAL,
                                (errmsg("incorrect checksum in control file")));

        /*
         * Do compatibility checking immediately.  We do this here for 2 reasons:
         *
         * (1) if the database isn't compatible with the backend executable, we
         * want to abort before we can possibly do any damage;
         *
         * (2) this code is executed in the postmaster, so the setlocale() will
         * propagate to forked backends, which aren't going to read this file for
         * themselves.  (These locale settings are considered critical
         * compatibility items because they can affect sort order of indexes.)
         */
        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->xlog_seg_size != XLOG_SEG_SIZE)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with XLOG_SEG_SIZE %d,"
                                           " but the server was compiled with XLOG_SEG_SIZE %d.",
                                                   ControlFile->xlog_seg_size, XLOG_SEG_SIZE),
                                 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.")));

#ifdef HAVE_INT64_TIMESTAMP
        if (ControlFile->enableIntTimes != true)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized without HAVE_INT64_TIMESTAMP"
                                  " but the server was compiled with HAVE_INT64_TIMESTAMP."),
                                 errhint("It looks like you need to recompile or initdb.")));
#else
        if (ControlFile->enableIntTimes != false)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with HAVE_INT64_TIMESTAMP"
                           " but the server was compiled without HAVE_INT64_TIMESTAMP."),
                                 errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT4_BYVAL
        if (ControlFile->float4ByVal != true)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized without USE_FLOAT4_BYVAL"
                                                   " but the server was compiled with USE_FLOAT4_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#else
        if (ControlFile->float4ByVal != false)
                ereport(FATAL,
                                (errmsg("database files are incompatible with server"),
                                 errdetail("The database cluster was initialized with USE_FLOAT4_BYVAL"
                                                   " but the server was compiled without USE_FLOAT4_BYVAL."),
                                 errhint("It looks like you need to recompile or initdb.")));
#endif

#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
}

void
UpdateControlFile(void)
{
        int                     fd;

        INIT_CRC32(ControlFile->crc);
        COMP_CRC32(ControlFile->crc,
                           (char *) ControlFile,
                           offsetof(ControlFileData, crc));
        FIN_CRC32(ControlFile->crc);

        fd = BasicOpenFile(XLOG_CONTROL_FILE,
                                           O_RDWR | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not open control file \"%s\": %m",
                                                XLOG_CONTROL_FILE)));

        errno = 0;
        if (write(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
        {
                /* 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 control file: %m")));
        }

        if (pg_fsync(fd) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync control file: %m")));

        if (close(fd))
                ereport(PANIC,
                                (errcode_for_file_access(),
                                 errmsg("could not close control file: %m")));
}

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

        /* XLogCtl */
        size = sizeof(XLogCtlData);
        /* xlblocks array */
        size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
        /* extra alignment padding for XLOG I/O buffers */
        size = add_size(size, ALIGNOF_XLOG_BUFFER);
        /* 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;

        ControlFile = (ControlFileData *)
                ShmemInitStruct("Control File", sizeof(ControlFileData), &foundCFile);
        XLogCtl = (XLogCtlData *)
                ShmemInitStruct("XLOG Ctl", XLOGShmemSize(), &foundXLog);

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

        memset(XLogCtl, 0, sizeof(XLogCtlData));

        /*
         * 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;

        /*
         * Align the start of the page buffers to an ALIGNOF_XLOG_BUFFER boundary.
         */
        allocptr = (char *) TYPEALIGN(ALIGNOF_XLOG_BUFFER, 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->Insert.currpage = (XLogPageHeader) (XLogCtl->pages);
        SpinLockInit(&XLogCtl->info_lck);

        /*
         * If we are not in bootstrap mode, pg_control should already exist. Read
         * and validate it immediately (see comments in ReadControlFile() for the
         * reasons why).
         */
        if (!IsBootstrapProcessingMode())
                ReadControlFile();
}

/*
 * 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;
        bool            use_existent;
        uint64          sysidentifier;
        struct timeval tv;
        pg_crc32        crc;

        /*
         * 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 is the XOR
         * of tv_sec and tv_usec.  This is to ensure that we don't lose uniqueness
         * unnecessarily if "uint64" is really only 32 bits wide.  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 |= (uint32) (tv.tv_sec | tv.tv_usec);

        /* First timeline ID is always 1 */
        ThisTimeLineID = 1;

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

        /* Set up information for the initial checkpoint record */
        checkPoint.redo.xlogid = 0;
        checkPoint.redo.xrecoff = SizeOfXLogLongPHD;
        checkPoint.ThisTimeLineID = ThisTimeLineID;
        checkPoint.nextXidEpoch = 0;
        checkPoint.nextXid = FirstNormalTransactionId;
        checkPoint.nextOid = FirstBootstrapObjectId;
        checkPoint.nextMulti = FirstMultiXactId;
        checkPoint.nextMultiOffset = 0;
        checkPoint.time = (pg_time_t) time(NULL);

        ShmemVariableCache->nextXid = checkPoint.nextXid;
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);

        /* Set up the XLOG page header */
        page->xlp_magic = XLOG_PAGE_MAGIC;
        page->xlp_info = XLP_LONG_HEADER;
        page->xlp_tli = ThisTimeLineID;
        page->xlp_pageaddr.xlogid = 0;
        page->xlp_pageaddr.xrecoff = 0;
        longpage = (XLogLongPageHeader) page;
        longpage->xlp_sysid = sysidentifier;
        longpage->xlp_seg_size = XLogSegSize;
        longpage->xlp_xlog_blcksz = XLOG_BLCKSZ;

        /* Insert the initial checkpoint record */
        record = (XLogRecord *) ((char *) page + SizeOfXLogLongPHD);
        record->xl_prev.xlogid = 0;
        record->xl_prev.xrecoff = 0;
        record->xl_xid = InvalidTransactionId;
        record->xl_tot_len = SizeOfXLogRecord + sizeof(checkPoint);
        record->xl_len = sizeof(checkPoint);
        record->xl_info = XLOG_CHECKPOINT_SHUTDOWN;
        record->xl_rmid = RM_XLOG_ID;
        memcpy(XLogRecGetData(record), &checkPoint, sizeof(checkPoint));

        INIT_CRC32(crc);
        COMP_CRC32(crc, &checkPoint, sizeof(checkPoint));
        COMP_CRC32(crc, (char *) record + sizeof(pg_crc32),
                           SizeOfXLogRecord - sizeof(pg_crc32));
        FIN_CRC32(crc);
        record->xl_crc = crc;

        /* Create first XLOG segment file */
        use_existent = false;
        openLogFile = XLogFileInit(0, 0, &use_existent, false);

        /* Write the first page with the initial record */
        errno = 0;
        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 transaction log file: %m")));
        }

        if (pg_fsync(openLogFile) != 0)
                ereport(PANIC,
                                (errcode_for_file_access(),
                          errmsg("could not fsync bootstrap transaction log file: %m")));

        if (close(openLogFile))
                ereport(PANIC,
                                (errcode_for_file_access(),
                          errmsg("could not close bootstrap transaction log file: %m")));

        openLogFile = -1;

        /* Now create pg_control */

        memset(ControlFile, 0, sizeof(ControlFileData));
        /* Initialize pg_control status fields */
        ControlFile->system_identifier = sysidentifier;
        ControlFile->state = DB_SHUTDOWNED;
        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();
        BootStrapSUBTRANS();
        BootStrapMultiXact();

        pfree(buffer);
}

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;
}

/*
 * See if there is a recovery command file (recovery.conf), and if so
 * read in parameters for archive recovery.
 *
 * XXX longer term intention is to expand this to
 * cater for additional parameters and controls
 * possibly use a flex lexer similar to the GUC one
 */
static void
readRecoveryCommandFile(void)
{
        FILE       *fd;
        char            cmdline[MAXPGPATH];
        TimeLineID      rtli = 0;
        bool            rtliGiven = false;
        bool            syntaxError = false;

        fd = AllocateFile(RECOVERY_COMMAND_FILE, "r");
        if (fd == NULL)
        {
                if (errno == ENOENT)
                        return;                         /* not there, so no archive recovery */
                ereport(FATAL,
                                (errcode_for_file_access(),
                                 errmsg("could not open recovery command file \"%s\": %m",
                                                RECOVERY_COMMAND_FILE)));
        }

        ereport(LOG,
                        (errmsg("starting archive recovery")));

        /*
         * Parse the file...
         */
        while (fgets(cmdline, sizeof(cmdline), fd) != NULL)
        {
                /* skip leading whitespace and check for # comment */
                char       *ptr;
                char       *tok1;
                char       *tok2;

                for (ptr = cmdline; *ptr; ptr++)
                {
                        if (!isspace((unsigned char) *ptr))
                                break;
                }
                if (*ptr == '\0' || *ptr == '#')
                        continue;

                /* identify the quoted parameter value */
                tok1 = strtok(ptr, "'");
                if (!tok1)
                {
                        syntaxError = true;
                        break;
                }
                tok2 = strtok(NULL, "'");
                if (!tok2)
                {
                        syntaxError = true;
                        break;
                }
                /* reparse to get just the parameter name */
                tok1 = strtok(ptr, " \t=");
                if (!tok1)
                {
                        syntaxError = true;
                        break;
                }

                if (strcmp(tok1, "restore_command") == 0)
                {
                        recoveryRestoreCommand = pstrdup(tok2);
                        ereport(LOG,
                                        (errmsg("restore_command = '%s'",
                                                        recoveryRestoreCommand)));
                }
                else if (strcmp(tok1, "recovery_target_timeline") == 0)
                {
                        rtliGiven = true;
                        if (strcmp(tok2, "latest") == 0)
                                rtli = 0;
                        else
                        {
                                errno = 0;
                                rtli = (TimeLineID) strtoul(tok2, NULL, 0);
                                if (errno == EINVAL || errno == ERANGE)
                                        ereport(FATAL,
                                                        (errmsg("recovery_target_timeline is not a valid number: \"%s\"",
                                                                        tok2)));
                        }
                        if (rtli)
                                ereport(LOG,
                                                (errmsg("recovery_target_timeline = %u", rtli)));
                        else
                                ereport(LOG,
                                                (errmsg("recovery_target_timeline = latest")));
                }
                else if (strcmp(tok1, "recovery_target_xid") == 0)
                {
                        errno = 0;
                        recoveryTargetXid = (TransactionId) strtoul(tok2, NULL, 0);
                        if (errno == EINVAL || errno == ERANGE)
                                ereport(FATAL,
                                 (errmsg("recovery_target_xid is not a valid number: \"%s\"",
                                                 tok2)));
                        ereport(LOG,
                                        (errmsg("recovery_target_xid = %u",
                                                        recoveryTargetXid)));
                        recoveryTarget = true;
                        recoveryTargetExact = true;
                }
                else if (strcmp(tok1, "recovery_target_time") == 0)
                {
                        /*
                         * if recovery_target_xid specified, then this overrides
                         * recovery_target_time
                         */
                        if (recoveryTargetExact)
                                continue;
                        recoveryTarget = true;
                        recoveryTargetExact = false;

                        /*
                         * Convert the time string given by the user to TimestampTz form.
                         */
                        recoveryTargetTime =
                                DatumGetTimestampTz(DirectFunctionCall3(timestamptz_in,
                                                                                                                CStringGetDatum(tok2),
                                                                                                ObjectIdGetDatum(InvalidOid),
                                                                                                                Int32GetDatum(-1)));
                        ereport(LOG,
                                        (errmsg("recovery_target_time = '%s'",
                                                        timestamptz_to_str(recoveryTargetTime))));
                }
                else if (strcmp(tok1, "recovery_target_inclusive") == 0)
                {
                        /*
                         * does nothing if a recovery_target is not also set
                         */
                        if (!parse_bool(tok2, &recoveryTargetInclusive))
                                  ereport(ERROR,
                                                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                          errmsg("parameter \"recovery_target_inclusive\" requires a Boolean value")));
                        ereport(LOG,
                                        (errmsg("recovery_target_inclusive = %s", tok2)));
                }
                else if (strcmp(tok1, "log_restartpoints") == 0)
                {
                        /*
                         * does nothing if a recovery_target is not also set
                         */
                        if (!parse_bool(tok2, &recoveryLogRestartpoints))
                                  ereport(ERROR,
                                                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                          errmsg("parameter \"log_restartpoints\" requires a Boolean value")));
                        ereport(LOG,
                                        (errmsg("log_restartpoints = %s", tok2)));
                }
                else
                        ereport(FATAL,
                                        (errmsg("unrecognized recovery parameter \"%s\"",
                                                        tok1)));
        }

        FreeFile(fd);

        if (syntaxError)
                ereport(FATAL,
                                (errmsg("syntax error in recovery command file: %s",
                                                cmdline),
                          errhint("Lines should have the format parameter = 'value'.")));

        /* Check that required parameters were supplied */
        if (recoveryRestoreCommand == NULL)
                ereport(FATAL,
                                (errmsg("recovery command file \"%s\" did not specify restore_command",
                                                RECOVERY_COMMAND_FILE)));

        /* Enable fetching from archive recovery area */
        InArchiveRecovery = true;

        /*
         * If user specified recovery_target_timeline, validate it or compute the
         * "latest" value.      We can't do this until after we've gotten the restore
         * command and set InArchiveRecovery, because we need to fetch timeline
         * history files from the archive.
         */
        if (rtliGiven)
        {
                if (rtli)
                {
                        /* Timeline 1 does not have a history file, all else should */
                        if (rtli != 1 && !existsTimeLineHistory(rtli))
                                ereport(FATAL,
                                                (errmsg("recovery target timeline %u does not exist",
                                                                rtli)));
                        recoveryTargetTLI = rtli;
                }
                else
                {
                        /* We start the "latest" search from pg_control's timeline */
                        recoveryTargetTLI = findNewestTimeLine(recoveryTargetTLI);
                }
        }
}

/*
 * Exit archive-recovery state
 */
static void
exitArchiveRecovery(TimeLineID endTLI, uint32 endLogId, uint32 endLogSeg)
{
        char            recoveryPath[MAXPGPATH];
        char            xlogpath[MAXPGPATH];

        /*
         * We are no longer in archive recovery state.
         */
        InArchiveRecovery = false;

        /*
         * We should have the ending log segment currently open.  Verify, and then
         * close it (to avoid problems on Windows with trying to rename or delete
         * an open file).
         */
        Assert(readFile >= 0);
        Assert(readId == endLogId);
        Assert(readSeg == endLogSeg);

        close(readFile);
        readFile = -1;

        /*
         * If the segment was fetched from archival storage, we want to replace
         * the existing xlog segment (if any) with the archival version.  This is
         * because whatever is in XLOGDIR is very possibly older than what we have
         * from the archives, since it could have come from restoring a PGDATA
         * backup.      In any case, the archival version certainly is more
         * descriptive of what our current database state is, because that is what
         * we replayed from.
         *
         * Note that if we are establishing a new timeline, ThisTimeLineID is
         * already set to the new value, and so we will create a new file instead
         * of overwriting any existing file.  (This is, in fact, always the case
         * at present.)
         */
        snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYXLOG");
        XLogFilePath(xlogpath, ThisTimeLineID, endLogId, endLogSeg);

        if (restoredFromArchive)
        {
                ereport(DEBUG3,
                                (errmsg_internal("moving last restored xlog to \"%s\"",
                                                                 xlogpath)));
                unlink(xlogpath);               /* might or might not exist */
                if (rename(recoveryPath, xlogpath) != 0)
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                        recoveryPath, xlogpath)));
                /* XXX might we need to fix permissions on the file? */
        }
        else
        {
                /*
                 * If the latest segment is not archival, but there's still a
                 * RECOVERYXLOG laying about, get rid of it.
                 */
                unlink(recoveryPath);   /* ignore any error */

                /*
                 * If we are establishing a new timeline, we have to copy data from
                 * the last WAL segment of the old timeline to create a starting WAL
                 * segment for the new timeline.
                 */
                if (endTLI != ThisTimeLineID)
                        XLogFileCopy(endLogId, endLogSeg,
                                                 endTLI, endLogId, endLogSeg);
        }

        /*
         * Let's just make real sure there are not .ready or .done flags posted
         * for the new segment.
         */
        XLogFileName(xlogpath, ThisTimeLineID, endLogId, endLogSeg);
        XLogArchiveCleanup(xlogpath);

        /* Get rid of any remaining recovered timeline-history file, too */
        snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYHISTORY");
        unlink(recoveryPath);           /* ignore any error */

        /*
         * Rename the config file out of the way, so that we don't accidentally
         * re-enter archive recovery mode in a subsequent crash.
         */
        unlink(RECOVERY_COMMAND_DONE);
        if (rename(RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE) != 0)
                ereport(FATAL,
                                (errcode_for_file_access(),
                                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE)));

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

/*
 * For point-in-time recovery, this function decides whether we want to
 * stop applying the XLOG at or after the current record.
 *
 * Returns TRUE if we are stopping, FALSE otherwise.  On TRUE return,
 * *includeThis is set TRUE if we should apply this record before stopping.
 *
 * We also track the timestamp of the latest applied COMMIT/ABORT record
 * in recoveryLastXTime, for logging purposes.
 * Also, some information is saved in recoveryStopXid et al for use in
 * annotating the new timeline's history file.
 */
static bool
recoveryStopsHere(XLogRecord *record, bool *includeThis)
{
        bool            stopsHere;
        uint8           record_info;
        TimestampTz recordXtime;

        /* We only consider stopping at COMMIT or ABORT records */
        if (record->xl_rmid != RM_XACT_ID)
                return false;
        record_info = record->xl_info & ~XLR_INFO_MASK;
        if (record_info == XLOG_XACT_COMMIT)
        {
                xl_xact_commit *recordXactCommitData;

                recordXactCommitData = (xl_xact_commit *) XLogRecGetData(record);
                recordXtime = recordXactCommitData->xact_time;
        }
        else if (record_info == XLOG_XACT_ABORT)
        {
                xl_xact_abort *recordXactAbortData;

                recordXactAbortData = (xl_xact_abort *) XLogRecGetData(record);
                recordXtime = recordXactAbortData->xact_time;
        }
        else
                return false;

        /* Do we have a PITR target at all? */
        if (!recoveryTarget)
        {
                recoveryLastXTime = recordXtime;
                return false;
        }

        if (recoveryTargetExact)
        {
                /*
                 * there can be only one transaction end record with this exact
                 * transactionid
                 *
                 * when testing for an xid, we MUST test for equality only, since
                 * transactions are numbered in the order they start, not the order
                 * they complete. A higher numbered xid will complete before you about
                 * 50% of the time...
                 */
                stopsHere = (record->xl_xid == recoveryTargetXid);
                if (stopsHere)
                        *includeThis = recoveryTargetInclusive;
        }
        else
        {
                /*
                 * there can be many transactions that share the same commit time, so
                 * we stop after the last one, if we are inclusive, or stop at the
                 * first one if we are exclusive
                 */
                if (recoveryTargetInclusive)
                        stopsHere = (recordXtime > recoveryTargetTime);
                else
                        stopsHere = (recordXtime >= recoveryTargetTime);
                if (stopsHere)
                        *includeThis = false;
        }

        if (stopsHere)
        {
                recoveryStopXid = record->xl_xid;
                recoveryStopTime = recordXtime;
                recoveryStopAfter = *includeThis;

                if (record_info == XLOG_XACT_COMMIT)
                {
                        if (recoveryStopAfter)
                                ereport(LOG,
                                                (errmsg("recovery stopping after commit of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                        else
                                ereport(LOG,
                                                (errmsg("recovery stopping before commit of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                }
                else
                {
                        if (recoveryStopAfter)
                                ereport(LOG,
                                                (errmsg("recovery stopping after abort of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                        else
                                ereport(LOG,
                                                (errmsg("recovery stopping before abort of transaction %u, time %s",
                                                                recoveryStopXid,
                                                                timestamptz_to_str(recoveryStopTime))));
                }

                if (recoveryStopAfter)
                        recoveryLastXTime = recordXtime;
        }
        else
                recoveryLastXTime = recordXtime;

        return stopsHere;
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
        XLogCtlInsert *Insert;
        CheckPoint      checkPoint;
        bool            wasShutdown;
        bool            reachedStopPoint = false;
        bool            haveBackupLabel = false;
        XLogRecPtr      RecPtr,
                                LastRec,
                                checkPointLoc,
                                minRecoveryLoc,
                                EndOfLog;
        uint32          endLogId;
        uint32          endLogSeg;
        XLogRecord *record;
        uint32          freespace;
        TransactionId oldestActiveXID;

        /*
         * Read control file and check XLOG status looks valid.
         *
         * Note: in most control paths, *ControlFile is already valid and we need
         * not do ReadControlFile() here, but might as well do it to be sure.
         */
        ReadControlFile();

        if (ControlFile->state < DB_SHUTDOWNED ||
                ControlFile->state > DB_IN_PRODUCTION ||
                !XRecOffIsValid(ControlFile->checkPoint.xrecoff))
                ereport(FATAL,
                                (errmsg("control file contains invalid data")));

        if (ControlFile->state == DB_SHUTDOWNED)
                ereport(LOG,
                                (errmsg("database system was shut down at %s",
                                                str_time(ControlFile->time))));
        else if (ControlFile->state == DB_SHUTDOWNING)
                ereport(LOG,
                                (errmsg("database system shutdown was interrupted; last known up at %s",
                                                str_time(ControlFile->time))));
        else if (ControlFile->state == 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.")));
        else if (ControlFile->state == 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.")));
        else if (ControlFile->state == DB_IN_PRODUCTION)
                ereport(LOG,
                          (errmsg("database system was interrupted; last known up at %s",
                                          str_time(ControlFile->time))));

        /* 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_xlog and pg_xlog/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();

        /*
         * Initialize on the assumption we want to recover to the same timeline
         * that's active according to pg_control.
         */
        recoveryTargetTLI = ControlFile->checkPointCopy.ThisTimeLineID;

        /*
         * Check for recovery control file, and if so set up state for offline
         * recovery
         */
        readRecoveryCommandFile();

        /* Now we can determine the list of expected TLIs */
        expectedTLIs = readTimeLineHistory(recoveryTargetTLI);

        /*
         * If pg_control's timeline is not in expectedTLIs, then we cannot
         * proceed: the backup is not part of the history of the requested
         * timeline.
         */
        if (!list_member_int(expectedTLIs,
                                                 (int) ControlFile->checkPointCopy.ThisTimeLineID))
                ereport(FATAL,
                                (errmsg("requested timeline %u is not a child of database system timeline %u",
                                                recoveryTargetTLI,
                                                ControlFile->checkPointCopy.ThisTimeLineID)));

        if (read_backup_label(&checkPointLoc, &minRecoveryLoc))
        {
                /*
                 * When a backup_label file is present, we want to roll forward from
                 * the checkpoint it identifies, rather than using pg_control.
                 */
                record = ReadCheckpointRecord(checkPointLoc, 0);
                if (record != NULL)
                {
                        ereport(DEBUG1,
                                        (errmsg("checkpoint record is at %X/%X",
                                                        checkPointLoc.xlogid, checkPointLoc.xrecoff)));
                        InRecovery = true;      /* force recovery even if SHUTDOWNED */
                }
                else
                {
                        ereport(PANIC,
                                        (errmsg("could not locate required checkpoint record"),
                                         errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
                }
                /* set flag to delete it later */
                haveBackupLabel = true;
        }
        else
        {
                /*
                 * Get the last valid checkpoint record.  If the latest one according
                 * to pg_control is broken, try the next-to-last one.
                 */
                checkPointLoc = ControlFile->checkPoint;
                record = ReadCheckpointRecord(checkPointLoc, 1);
                if (record != NULL)
                {
                        ereport(DEBUG1,
                                        (errmsg("checkpoint record is at %X/%X",
                                                        checkPointLoc.xlogid, checkPointLoc.xrecoff)));
                }
                else
                {
                        checkPointLoc = ControlFile->prevCheckPoint;
                        record = ReadCheckpointRecord(checkPointLoc, 2);
                        if (record != NULL)
                        {
                                ereport(LOG,
                                                (errmsg("using previous checkpoint record at %X/%X",
                                                          checkPointLoc.xlogid, checkPointLoc.xrecoff)));
                                InRecovery = true;              /* force recovery even if SHUTDOWNED */
                        }
                        else
                                ereport(PANIC,
                                         (errmsg("could not locate a valid checkpoint record")));
                }
        }

        LastRec = RecPtr = checkPointLoc;
        memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
        wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);

        ereport(DEBUG1,
                        (errmsg("redo record is at %X/%X; shutdown %s",
                                        checkPoint.redo.xlogid, checkPoint.redo.xrecoff,
                                        wasShutdown ? "TRUE" : "FALSE")));
        ereport(DEBUG1,
                        (errmsg("next transaction ID: %u/%u; next OID: %u",
                                        checkPoint.nextXidEpoch, checkPoint.nextXid,
                                        checkPoint.nextOid)));
        ereport(DEBUG1,
                        (errmsg("next MultiXactId: %u; next MultiXactOffset: %u",
                                        checkPoint.nextMulti, checkPoint.nextMultiOffset)));
        if (!TransactionIdIsNormal(checkPoint.nextXid))
                ereport(PANIC,
                                (errmsg("invalid next transaction ID")));

        ShmemVariableCache->nextXid = checkPoint.nextXid;
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);

        /*
         * We must replay WAL entries using the same TimeLineID they were created
         * under, so temporarily adopt the TLI indicated by the checkpoint (see
         * also xlog_redo()).
         */
        ThisTimeLineID = checkPoint.ThisTimeLineID;

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

        if (XLByteLT(RecPtr, checkPoint.redo))
                ereport(PANIC,
                                (errmsg("invalid redo in checkpoint record")));

        /*
         * Check whether we need to force recovery from WAL.  If it appears to
         * have been a clean shutdown and we did not have a recovery.conf file,
         * then assume no recovery needed.
         */
        if (XLByteLT(checkPoint.redo, RecPtr))
        {
                if (wasShutdown)
                        ereport(PANIC,
                                        (errmsg("invalid redo record in shutdown checkpoint")));
                InRecovery = true;
        }
        else if (ControlFile->state != DB_SHUTDOWNED)
                InRecovery = true;
        else if (InArchiveRecovery)
        {
                /* force recovery due to presence of recovery.conf */
                InRecovery = true;
        }

        /* REDO */
        if (InRecovery)
        {
                int                     rmid;

                /*
                 * 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.
                 */
                if (InArchiveRecovery)
                {
                        ereport(LOG,
                                        (errmsg("automatic recovery in progress")));
                        ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
                }
                else
                {
                        ereport(LOG,
                                        (errmsg("database system was not properly shut down; "
                                                        "automatic recovery in progress")));
                        ControlFile->state = DB_IN_CRASH_RECOVERY;
                }
                ControlFile->prevCheckPoint = ControlFile->checkPoint;
                ControlFile->checkPoint = checkPointLoc;
                ControlFile->checkPointCopy = checkPoint;
                if (minRecoveryLoc.xlogid != 0 || minRecoveryLoc.xrecoff != 0)
                        ControlFile->minRecoveryPoint = minRecoveryLoc;
                ControlFile->time = (pg_time_t) time(NULL);
                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);
                        if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) != 0)
                                ereport(FATAL,
                                                (errcode_for_file_access(),
                                                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                                BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
                }

                /* Initialize resource managers */
                for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
                {
                        if (RmgrTable[rmid].rm_startup != NULL)
                                RmgrTable[rmid].rm_startup();
                }

                /*
                 * Find the first record that logically follows the checkpoint --- it
                 * might physically precede it, though.
                 */
                if (XLByteLT(checkPoint.redo, RecPtr))
                {
                        /* back up to find the record */
                        record = ReadRecord(&(checkPoint.redo), PANIC);
                }
                else
                {
                        /* just have to read next record after CheckPoint */
                        record = ReadRecord(NULL, LOG);
                }

                if (record != NULL)
                {
                        bool            recoveryContinue = true;
                        bool            recoveryApply = true;
                        ErrorContextCallback errcontext;

                        InRedo = true;
                        ereport(LOG,
                                        (errmsg("redo starts at %X/%X",
                                                        ReadRecPtr.xlogid, ReadRecPtr.xrecoff)));

                        /*
                         * main redo apply loop
                         */
                        do
                        {
#ifdef WAL_DEBUG
                                if (XLOG_DEBUG)
                                {
                                        StringInfoData buf;

                                        initStringInfo(&buf);
                                        appendStringInfo(&buf, "REDO @ %X/%X; LSN %X/%X: ",
                                                                         ReadRecPtr.xlogid, ReadRecPtr.xrecoff,
                                                                         EndRecPtr.xlogid, EndRecPtr.xrecoff);
                                        xlog_outrec(&buf, record);
                                        appendStringInfo(&buf, " - ");
                                        RmgrTable[record->xl_rmid].rm_desc(&buf,
                                                                                                           record->xl_info,
                                                                                                         XLogRecGetData(record));
                                        elog(LOG, "%s", buf.data);
                                        pfree(buf.data);
                                }
#endif

                                /*
                                 * Have we reached our recovery target?
                                 */
                                if (recoveryStopsHere(record, &recoveryApply))
                                {
                                        reachedStopPoint = true;        /* see below */
                                        recoveryContinue = false;
                                        if (!recoveryApply)
                                                break;
                                }

                                /* Setup error traceback support for ereport() */
                                errcontext.callback = rm_redo_error_callback;
                                errcontext.arg = (void *) record;
                                errcontext.previous = error_context_stack;
                                error_context_stack = &errcontext;

                                /* nextXid must be beyond record's xid */
                                if (TransactionIdFollowsOrEquals(record->xl_xid,
                                                                                                 ShmemVariableCache->nextXid))
                                {
                                        ShmemVariableCache->nextXid = record->xl_xid;
                                        TransactionIdAdvance(ShmemVariableCache->nextXid);
                                }

                                if (record->xl_info & XLR_BKP_BLOCK_MASK)
                                        RestoreBkpBlocks(record, EndRecPtr);

                                RmgrTable[record->xl_rmid].rm_redo(EndRecPtr, record);

                                /* Pop the error context stack */
                                error_context_stack = errcontext.previous;

                                LastRec = ReadRecPtr;

                                record = ReadRecord(NULL, LOG);
                        } while (record != NULL && recoveryContinue);

                        /*
                         * end of main redo apply loop
                         */

                        ereport(LOG,
                                        (errmsg("redo done at %X/%X",
                                                        ReadRecPtr.xlogid, ReadRecPtr.xrecoff)));
                        if (recoveryLastXTime)
                                ereport(LOG,
                                         (errmsg("last completed transaction was at log time %s",
                                                         timestamptz_to_str(recoveryLastXTime))));
                        InRedo = false;
                }
                else
                {
                        /* there are no WAL records following the checkpoint */
                        ereport(LOG,
                                        (errmsg("redo is not required")));
                }
        }

        /*
         * Re-fetch the last valid or last applied record, so we can identify the
         * exact endpoint of what we consider the valid portion of WAL.
         */
        record = ReadRecord(&LastRec, PANIC);
        EndOfLog = EndRecPtr;
        XLByteToPrevSeg(EndOfLog, endLogId, endLogSeg);

        /*
         * Complain if we did not roll forward far enough to render the backup
         * dump consistent.
         */
        if (XLByteLT(EndOfLog, ControlFile->minRecoveryPoint))
        {
                if (reachedStopPoint)   /* stopped because of stop request */
                        ereport(FATAL,
                                        (errmsg("requested recovery stop point is before end time of backup dump")));
                else    /* ran off end of WAL */
                        ereport(FATAL,
                                        (errmsg("WAL ends before end time of backup dump")));
        }

        /*
         * Consider whether we need to assign a new timeline ID.
         *
         * If we are doing an 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.
         */
        if (InArchiveRecovery)
        {
                ThisTimeLineID = findNewestTimeLine(recoveryTargetTLI) + 1;
                ereport(LOG,
                                (errmsg("selected new timeline ID: %u", ThisTimeLineID)));
                writeTimeLineHistory(ThisTimeLineID, recoveryTargetTLI,
                                                         curFileTLI, endLogId, endLogSeg);
        }

        /* Save the selected TimeLineID in shared memory, too */
        XLogCtl->ThisTimeLineID = ThisTimeLineID;

        /*
         * We are now done reading the old WAL.  Turn off archive fetching if it
         * was active, and 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 readBuf;
         * we will use that below.)
         */
        if (InArchiveRecovery)
                exitArchiveRecovery(curFileTLI, endLogId, endLogSeg);

        /*
         * Prepare to write WAL starting at EndOfLog position, and init xlog
         * buffer cache using the block containing the last record from the
         * previous incarnation.
         */
        openLogId = endLogId;
        openLogSeg = endLogSeg;
        openLogFile = XLogFileOpen(openLogId, openLogSeg);
        openLogOff = 0;
        Insert = &XLogCtl->Insert;
        Insert->PrevRecord = LastRec;
        XLogCtl->xlblocks[0].xlogid = openLogId;
        XLogCtl->xlblocks[0].xrecoff =
                ((EndOfLog.xrecoff - 1) / XLOG_BLCKSZ + 1) * XLOG_BLCKSZ;

        /*
         * Tricky point here: readBuf 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.
         */
        Assert(readOff == (XLogCtl->xlblocks[0].xrecoff - XLOG_BLCKSZ) % XLogSegSize);
        memcpy((char *) Insert->currpage, readBuf, XLOG_BLCKSZ);
        Insert->currpos = (char *) Insert->currpage +
                (EndOfLog.xrecoff + XLOG_BLCKSZ - XLogCtl->xlblocks[0].xrecoff);

        LogwrtResult.Write = LogwrtResult.Flush = EndOfLog;

        XLogCtl->Write.LogwrtResult = LogwrtResult;
        Insert->LogwrtResult = LogwrtResult;
        XLogCtl->LogwrtResult = LogwrtResult;

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

        freespace = INSERT_FREESPACE(Insert);
        if (freespace > 0)
        {
                /* Make sure rest of page is zero */
                MemSet(Insert->currpos, 0, freespace);
                XLogCtl->Write.curridx = 0;
        }
        else
        {
                /*
                 * Whenever Write.LogwrtResult points to exactly the end of a page,
                 * Write.curridx must point to the *next* page (see XLogWrite()).
                 *
                 * Note: it might seem we should do AdvanceXLInsertBuffer() here, but
                 * this is sufficient.  The first actual attempt to insert a log
                 * record will advance the insert state.
                 */
                XLogCtl->Write.curridx = NextBufIdx(0);
        }

        /* Pre-scan prepared transactions to find out the range of XIDs present */
        oldestActiveXID = PrescanPreparedTransactions();

        if (InRecovery)
        {
                int                     rmid;

                /*
                 * Allow resource managers to do any required cleanup.
                 */
                for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
                {
                        if (RmgrTable[rmid].rm_cleanup != NULL)
                                RmgrTable[rmid].rm_cleanup();
                }

                /*
                 * Check to see if the XLOG sequence contained any unresolved
                 * references to uninitialized pages.
                 */
                XLogCheckInvalidPages();

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

                /*
                 * 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.
                 */
                CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
        }

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

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

        ControlFile->state = DB_IN_PRODUCTION;
        ControlFile->time = (pg_time_t) time(NULL);
        UpdateControlFile();

        /* start the archive_timeout timer running */
        XLogCtl->Write.lastSegSwitchTime = ControlFile->time;

        /* initialize shared-memory copy of latest checkpoint XID/epoch */
        XLogCtl->ckptXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
        XLogCtl->ckptXid = ControlFile->checkPointCopy.nextXid;

        /* also initialize latestCompletedXid, to nextXid - 1 */
        ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
        TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);

        /* Start up the commit log and related stuff, too */
        StartupCLOG();
        StartupSUBTRANS(oldestActiveXID);
        StartupMultiXact();

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

        /* Shut down readFile facility, free space */
        if (readFile >= 0)
        {
                close(readFile);
                readFile = -1;
        }
        if (readBuf)
        {
                free(readBuf);
                readBuf = NULL;
        }
        if (readRecordBuf)
        {
                free(readRecordBuf);
                readRecordBuf = NULL;
                readRecordBufSize = 0;
        }
}

/*
 * Subroutine to try to fetch and validate a prior checkpoint record.
 *
 * whichChkpt identifies the checkpoint (merely for reporting purposes).
 * 1 for "primary", 2 for "secondary", 0 for "other" (backup_label)
 */
static XLogRecord *
ReadCheckpointRecord(XLogRecPtr RecPtr, int whichChkpt)
{
        XLogRecord *record;

        if (!XRecOffIsValid(RecPtr.xrecoff))
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                (errmsg("invalid primary checkpoint link in control file")));
                                break;
                        case 2:
                                ereport(LOG,
                                                (errmsg("invalid secondary checkpoint link in control file")));
                                break;
                        default:
                                ereport(LOG,
                                   (errmsg("invalid checkpoint link in backup_label file")));
                                break;
                }
                return NULL;
        }

        record = ReadRecord(&RecPtr, LOG);

        if (record == NULL)
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                                (errmsg("invalid primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                                (errmsg("invalid secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                                (errmsg("invalid checkpoint record")));
                                break;
                }
                return NULL;
        }
        if (record->xl_rmid != RM_XLOG_ID)
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                                (errmsg("invalid resource manager ID in primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                                (errmsg("invalid resource manager ID in secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                (errmsg("invalid resource manager ID in checkpoint record")));
                                break;
                }
                return NULL;
        }
        if (record->xl_info != XLOG_CHECKPOINT_SHUTDOWN &&
                record->xl_info != XLOG_CHECKPOINT_ONLINE)
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                   (errmsg("invalid xl_info in primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                 (errmsg("invalid xl_info in secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                                (errmsg("invalid xl_info in checkpoint record")));
                                break;
                }
                return NULL;
        }
        if (record->xl_len != sizeof(CheckPoint) ||
                record->xl_tot_len != SizeOfXLogRecord + sizeof(CheckPoint))
        {
                switch (whichChkpt)
                {
                        case 1:
                                ereport(LOG,
                                        (errmsg("invalid length of primary checkpoint record")));
                                break;
                        case 2:
                                ereport(LOG,
                                  (errmsg("invalid length of secondary checkpoint record")));
                                break;
                        default:
                                ereport(LOG,
                                                (errmsg("invalid length of checkpoint record")));
                                break;
                }
                return NULL;
        }
        return record;
}

/*
 * This must be called during startup of a backend process, except that
 * it need not be called in a standalone backend (which does StartupXLOG
 * instead).  We need to initialize the local copies of ThisTimeLineID and
 * RedoRecPtr.
 *
 * Note: before Postgres 8.0, we went to some effort to keep the postmaster
 * process's copies of ThisTimeLineID and RedoRecPtr valid too.  This was
 * unnecessary however, since the postmaster itself never touches XLOG anyway.
 */
void
InitXLOGAccess(void)
{
        /* ThisTimeLineID doesn't change so we need no lock to copy it */
        ThisTimeLineID = XLogCtl->ThisTimeLineID;
        /* Use GetRedoRecPtr to copy the RedoRecPtr safely */
        (void) GetRedoRecPtr();
}

/*
 * Once spawned, a backend may update its local RedoRecPtr from
 * XLogCtl->Insert.RedoRecPtr; it must hold the insert lock or info_lck
 * to do so.  This is done in XLogInsert() or GetRedoRecPtr().
 */
XLogRecPtr
GetRedoRecPtr(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        Assert(XLByteLE(RedoRecPtr, xlogctl->Insert.RedoRecPtr));
        RedoRecPtr = xlogctl->Insert.RedoRecPtr;
        SpinLockRelease(&xlogctl->info_lck);

        return RedoRecPtr;
}

/*
 * 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 acquire WALInsertLock which can be quite
 * heavily contended, and an approximation is enough for the current
 * usage of this function.
 */
XLogRecPtr
GetInsertRecPtr(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogRecPtr      recptr;

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

        return recptr;
}

/*
 * Get the time of the last xlog segment switch
 */
pg_time_t
GetLastSegSwitchTime(void)
{
        pg_time_t       result;

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

        return result;
}

/*
 * GetNextXidAndEpoch - get the current nextXid value and associated epoch
 *
 * This is exported for use by code that would like to have 64-bit XIDs.
 * We don't really support such things, but all XIDs within the system
 * can be presumed "close to" the result, and thus the epoch associated
 * with them can be determined.
 */
void
GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch)
{
        uint32          ckptXidEpoch;
        TransactionId ckptXid;
        TransactionId nextXid;

        /* Must read checkpoint info first, else have race condition */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                ckptXidEpoch = xlogctl->ckptXidEpoch;
                ckptXid = xlogctl->ckptXid;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /* Now fetch current nextXid */
        nextXid = ReadNewTransactionId();

        /*
         * nextXid is certainly logically later than ckptXid.  So if it's
         * numerically less, it must have wrapped into the next epoch.
         */
        if (nextXid < ckptXid)
                ckptXidEpoch++;

        *xid = nextXid;
        *epoch = ckptXidEpoch;
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownXLOG(int code, Datum arg)
{
        ereport(LOG,
                        (errmsg("shutting down")));

        CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
        ShutdownCLOG();
        ShutdownSUBTRANS();
        ShutdownMultiXact();

        ereport(LOG,
                        (errmsg("database system is shut down")));
}

/*
 * Log start of a checkpoint.
 */
static void
LogCheckpointStart(int flags)
{
        elog(LOG, "checkpoint starting:%s%s%s%s%s%s",
                 (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
                 (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
                 (flags & CHECKPOINT_FORCE) ? " force" : "",
                 (flags & CHECKPOINT_WAIT) ? " wait" : "",
                 (flags & CHECKPOINT_CAUSE_XLOG) ? " xlog" : "",
                 (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "");
}

/*
 * Log end of a checkpoint.
 */
static void
LogCheckpointEnd(void)
{
        long            write_secs,
                                sync_secs,
                                total_secs;
        int                     write_usecs,
                                sync_usecs,
                                total_usecs;

        CheckpointStats.ckpt_end_t = GetCurrentTimestamp();

        TimestampDifference(CheckpointStats.ckpt_start_t,
                                                CheckpointStats.ckpt_end_t,
                                                &total_secs, &total_usecs);

        TimestampDifference(CheckpointStats.ckpt_write_t,
                                                CheckpointStats.ckpt_sync_t,
                                                &write_secs, &write_usecs);

        TimestampDifference(CheckpointStats.ckpt_sync_t,
                                                CheckpointStats.ckpt_sync_end_t,
                                                &sync_secs, &sync_usecs);

        elog(LOG, "checkpoint complete: wrote %d buffers (%.1f%%); "
                 "%d transaction log file(s) added, %d removed, %d recycled; "
                 "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s",
                 CheckpointStats.ckpt_bufs_written,
                 (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
                 CheckpointStats.ckpt_segs_added,
                 CheckpointStats.ckpt_segs_removed,
                 CheckpointStats.ckpt_segs_recycled,
                 write_secs, write_usecs / 1000,
                 sync_secs, sync_usecs / 1000,
                 total_secs, total_usecs / 1000);
}

/*
 * 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_IMMEDIATE: finish the checkpoint ASAP,
 *              ignoring checkpoint_completion_target parameter.
 *      CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occured
 *              since the last one (implied by CHECKPOINT_IS_SHUTDOWN).
 *
 * 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.
 */
void
CreateCheckPoint(int flags)
{
        bool            shutdown = (flags & CHECKPOINT_IS_SHUTDOWN) != 0;
        CheckPoint      checkPoint;
        XLogRecPtr      recptr;
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        XLogRecData rdata;
        uint32          freespace;
        uint32          _logId;
        uint32          _logSeg;
        TransactionId *inCommitXids;
        int                     nInCommit;

        /*
         * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
         * (This is just pro forma, since in the present system structure there is
         * only one process that is allowed to issue checkpoints at any given
         * time.)
         */
        LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

        /*
         * 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();

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

        if (shutdown)
        {
                ControlFile->state = DB_SHUTDOWNING;
                ControlFile->time = (pg_time_t) time(NULL);
                UpdateControlFile();
        }

        /*
         * Let smgr prepare for checkpoint; this has to happen 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.
         */
        smgrpreckpt();

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

        /*
         * We must hold WALInsertLock while examining insert state to determine
         * the checkpoint REDO pointer.
         */
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);

        /*
         * If this isn't a shutdown or forced checkpoint, and we have not inserted
         * any XLOG records since the start of the last checkpoint, skip the
         * checkpoint.  The idea here is to avoid inserting duplicate checkpoints
         * when the system is idle. That wastes log space, and more importantly it
         * exposes us to possible loss of both current and previous checkpoint
         * records if the machine crashes just as we're writing the update.
         * (Perhaps it'd make even more sense to checkpoint only when the previous
         * checkpoint record is in a different xlog page?)
         *
         * We have to make two tests to determine that nothing has happened since
         * the start of the last checkpoint: current insertion point must match
         * the end of the last checkpoint record, and its redo pointer must point
         * to itself.
         */
        if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_FORCE)) == 0)
        {
                XLogRecPtr      curInsert;

                INSERT_RECPTR(curInsert, Insert, Insert->curridx);
                if (curInsert.xlogid == ControlFile->checkPoint.xlogid &&
                        curInsert.xrecoff == ControlFile->checkPoint.xrecoff +
                        MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
                        ControlFile->checkPoint.xlogid ==
                        ControlFile->checkPointCopy.redo.xlogid &&
                        ControlFile->checkPoint.xrecoff ==
                        ControlFile->checkPointCopy.redo.xrecoff)
                {
                        LWLockRelease(WALInsertLock);
                        LWLockRelease(CheckpointLock);
                        END_CRIT_SECTION();
                        return;
                }
        }

        /*
         * 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(Insert);
        if (freespace < SizeOfXLogRecord)
        {
                (void) AdvanceXLInsertBuffer(false);
                /* OK to ignore update return flag, since we will do flush anyway */
                freespace = INSERT_FREESPACE(Insert);
        }
        INSERT_RECPTR(checkPoint.redo, Insert, Insert->curridx);

        /*
         * Here we update the shared RedoRecPtr for future XLogInsert calls; this
         * must be done while holding the insert lock AND the info_lck.
         *
         * 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.
         */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                RedoRecPtr = xlogctl->Insert.RedoRecPtr = checkPoint.redo;
                SpinLockRelease(&xlogctl->info_lck);
        }

        /*
         * Now we can release WAL insert lock, allowing other xacts to proceed
         * while we are flushing disk buffers.
         */
        LWLockRelease(WALInsertLock);

        /*
         * 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);

        /*
         * Before flushing data, we must wait for any transactions that are
         * currently in their 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_clog.  So
         * we wait till he's out of his commit critical section before proceeding.
         * See notes in RecordTransactionCommit().
         *
         * Because we've already released WALInsertLock, 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 locks 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 inCommit 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.
         */
        nInCommit = GetTransactionsInCommit(&inCommitXids);
        if (nInCommit > 0)
        {
                do
                {
                        pg_usleep(10000L);      /* wait for 10 msec */
                } while (HaveTransactionsInCommit(inCommitXids, nInCommit));
        }
        pfree(inCommitXids);

        /*
         * Get the other info we need for the checkpoint record.
         */
        LWLockAcquire(XidGenLock, LW_SHARED);
        checkPoint.nextXid = ShmemVariableCache->nextXid;
        LWLockRelease(XidGenLock);

        /* Increase XID epoch if we've wrapped around since last checkpoint */
        checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
        if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
                checkPoint.nextXidEpoch++;

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

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

        /*
         * 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();

        CheckPointGuts(checkPoint.redo, flags);

        START_CRIT_SECTION();

        /*
         * Now insert the checkpoint record into XLOG.
         */
        rdata.data = (char *) (&checkPoint);
        rdata.len = sizeof(checkPoint);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;

        recptr = XLogInsert(RM_XLOG_ID,
                                                shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
                                                XLOG_CHECKPOINT_ONLINE,
                                                &rdata);

        XLogFlush(recptr);

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

        /*
         * Select point at which we can truncate the log, which we base on the
         * prior checkpoint's earliest info.
         */
        XLByteToSeg(ControlFile->checkPointCopy.redo, _logId, _logSeg);

        /*
         * Update the control file.
         */
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        if (shutdown)
                ControlFile->state = DB_SHUTDOWNED;
        ControlFile->prevCheckPoint = ControlFile->checkPoint;
        ControlFile->checkPoint = ProcLastRecPtr;
        ControlFile->checkPointCopy = checkPoint;
        ControlFile->time = (pg_time_t) time(NULL);
        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        /* Update shared-memory copy of checkpoint XID/epoch */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
                xlogctl->ckptXid = 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).
         */
        smgrpostckpt();

        /*
         * Delete old log files (those no longer needed even for previous
         * checkpoint).
         */
        if (_logId || _logSeg)
        {
                PrevLogSeg(_logId, _logSeg);
                RemoveOldXlogFiles(_logId, _logSeg, recptr);
        }

        /*
         * 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);

        /*
         * 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 (!InRecovery)
                TruncateSUBTRANS(GetOldestXmin(true, false));

        /* All real work is done, but log before releasing lock. */
        if (log_checkpoints)
                LogCheckpointEnd();

        LWLockRelease(CheckpointLock);
}

/*
 * 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)
{
        CheckPointCLOG();
        CheckPointSUBTRANS();
        CheckPointMultiXact();
        CheckPointBuffers(flags);       /* performs all required fsyncs */
        /* We deliberately delay 2PC checkpointing as long as possible */
        CheckPointTwoPhase(checkPointRedo);
}

/*
 * Set a recovery restart point if appropriate
 *
 * 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.  This function is called each time
 * a checkpoint record is read from XLOG; it must determine whether a
 * restartpoint is needed or not.
 */
static void
RecoveryRestartPoint(const CheckPoint *checkPoint)
{
        int                     elapsed_secs;
        int                     rmid;

        /*
         * Do nothing if the elapsed time since the last restartpoint is less than
         * half of checkpoint_timeout.  (We use a value less than
         * checkpoint_timeout so that variations in the timing of checkpoints on
         * the master, or speed of transmission of WAL segments to a slave, won't
         * make the slave skip a restartpoint once it's synced with the master.)
         * Checking true elapsed time keeps us from doing restartpoints too often
         * while rapidly scanning large amounts of WAL.
         */
        elapsed_secs = (pg_time_t) time(NULL) - ControlFile->time;
        if (elapsed_secs < CheckPointTimeout / 2)
                return;

        /*
         * Is it safe to checkpoint?  We must ask each of the resource managers
         * whether they have any partial state information that might prevent a
         * correct restart from this point.  If so, we skip this opportunity, but
         * return at the next checkpoint record for another try.
         */
        for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
        {
                if (RmgrTable[rmid].rm_safe_restartpoint != NULL)
                        if (!(RmgrTable[rmid].rm_safe_restartpoint()))
                        {
                                elog(DEBUG2, "RM %d not safe to record restart point at %X/%X",
                                         rmid,
                                         checkPoint->redo.xlogid,
                                         checkPoint->redo.xrecoff);
                                return;
                        }
        }

        /*
         * OK, force data out to disk
         */
        CheckPointGuts(checkPoint->redo, CHECKPOINT_IMMEDIATE);

        /*
         * Update pg_control so that any subsequent crash will restart from this
         * checkpoint.  Note: ReadRecPtr gives the XLOG address of the checkpoint
         * record itself.
         */
        ControlFile->prevCheckPoint = ControlFile->checkPoint;
        ControlFile->checkPoint = ReadRecPtr;
        ControlFile->checkPointCopy = *checkPoint;
        ControlFile->time = (pg_time_t) time(NULL);
        UpdateControlFile();

        ereport((recoveryLogRestartpoints ? LOG : DEBUG2),
                        (errmsg("recovery restart point at %X/%X",
                                        checkPoint->redo.xlogid, checkPoint->redo.xrecoff)));
        if (recoveryLastXTime)
                ereport((recoveryLogRestartpoints ? LOG : DEBUG2),
                                (errmsg("last completed transaction was at log time %s",
                                                timestamptz_to_str(recoveryLastXTime))));
}

/*
 * Write a NEXTOID log record
 */
void
XLogPutNextOid(Oid nextOid)
{
        XLogRecData rdata;

        rdata.data = (char *) (&nextOid);
        rdata.len = sizeof(Oid);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;
        (void) XLogInsert(RM_XLOG_ID, XLOG_NEXTOID, &rdata);

        /*
         * 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(void)
{
        XLogRecPtr      RecPtr;
        XLogRecData rdata;

        /* XLOG SWITCH, alone among xlog record types, has no data */
        rdata.buffer = InvalidBuffer;
        rdata.data = NULL;
        rdata.len = 0;
        rdata.next = NULL;

        RecPtr = XLogInsert(RM_XLOG_ID, XLOG_SWITCH, &rdata);

        return RecPtr;
}

/*
 * XLOG resource manager's routines
 */
void
xlog_redo(XLogRecPtr lsn, XLogRecord *record)
{
        uint8           info = record->xl_info & ~XLR_INFO_MASK;

        if (info == XLOG_NEXTOID)
        {
                Oid                     nextOid;

                memcpy(&nextOid, XLogRecGetData(record), sizeof(Oid));
                if (ShmemVariableCache->nextOid < nextOid)
                {
                        ShmemVariableCache->nextOid = nextOid;
                        ShmemVariableCache->oidCount = 0;
                }
        }
        else if (info == XLOG_CHECKPOINT_SHUTDOWN)
        {
                CheckPoint      checkPoint;

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

                /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
                ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
                ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

                /*
                 * TLI may change in a shutdown checkpoint, but it shouldn't decrease
                 */
                if (checkPoint.ThisTimeLineID != ThisTimeLineID)
                {
                        if (checkPoint.ThisTimeLineID < ThisTimeLineID ||
                                !list_member_int(expectedTLIs,
                                                                 (int) checkPoint.ThisTimeLineID))
                                ereport(PANIC,
                                                (errmsg("unexpected timeline ID %u (after %u) in checkpoint record",
                                                                checkPoint.ThisTimeLineID, ThisTimeLineID)));
                        /* Following WAL records should be run with new TLI */
                        ThisTimeLineID = checkPoint.ThisTimeLineID;
                }

                RecoveryRestartPoint(&checkPoint);
        }
        else if (info == XLOG_CHECKPOINT_ONLINE)
        {
                CheckPoint      checkPoint;

                memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                /* In an ONLINE checkpoint, treat the counters like NEXTOID */
                if (TransactionIdPrecedes(ShmemVariableCache->nextXid,
                                                                  checkPoint.nextXid))
                        ShmemVariableCache->nextXid = checkPoint.nextXid;
                if (ShmemVariableCache->nextOid < checkPoint.nextOid)
                {
                        ShmemVariableCache->nextOid = checkPoint.nextOid;
                        ShmemVariableCache->oidCount = 0;
                }
                MultiXactAdvanceNextMXact(checkPoint.nextMulti,
                                                                  checkPoint.nextMultiOffset);

                /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
                ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
                ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

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

                RecoveryRestartPoint(&checkPoint);
        }
        else if (info == XLOG_NOOP)
        {
                /* nothing to do here */
        }
        else if (info == XLOG_SWITCH)
        {
                /* nothing to do here */
        }
}

void
xlog_desc(StringInfo buf, uint8 xl_info, char *rec)
{
        uint8           info = xl_info & ~XLR_INFO_MASK;

        if (info == XLOG_CHECKPOINT_SHUTDOWN ||
                info == XLOG_CHECKPOINT_ONLINE)
        {
                CheckPoint *checkpoint = (CheckPoint *) rec;

                appendStringInfo(buf, "checkpoint: redo %X/%X; "
                                                 "tli %u; xid %u/%u; oid %u; multi %u; offset %u; %s",
                                                 checkpoint->redo.xlogid, checkpoint->redo.xrecoff,
                                                 checkpoint->ThisTimeLineID,
                                                 checkpoint->nextXidEpoch, checkpoint->nextXid,
                                                 checkpoint->nextOid,
                                                 checkpoint->nextMulti,
                                                 checkpoint->nextMultiOffset,
                                 (info == XLOG_CHECKPOINT_SHUTDOWN) ? "shutdown" : "online");
        }
        else if (info == XLOG_NOOP)
        {
                appendStringInfo(buf, "xlog no-op");
        }
        else if (info == XLOG_NEXTOID)
        {
                Oid                     nextOid;

                memcpy(&nextOid, rec, sizeof(Oid));
                appendStringInfo(buf, "nextOid: %u", nextOid);
        }
        else if (info == XLOG_SWITCH)
        {
                appendStringInfo(buf, "xlog switch");
        }
        else
                appendStringInfo(buf, "UNKNOWN");
}

#ifdef WAL_DEBUG

static void
xlog_outrec(StringInfo buf, XLogRecord *record)
{
        int                     i;

        appendStringInfo(buf, "prev %X/%X; xid %u",
                                         record->xl_prev.xlogid, record->xl_prev.xrecoff,
                                         record->xl_xid);

        for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
        {
                if (record->xl_info & XLR_SET_BKP_BLOCK(i))
                        appendStringInfo(buf, "; bkpb%d", i + 1);
        }

        appendStringInfo(buf, ": %s", RmgrTable[record->xl_rmid].rm_name);
}
#endif   /* WAL_DEBUG */


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

        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;
#endif
#ifdef OPEN_DATASYNC_FLAG
                case SYNC_METHOD_OPEN_DSYNC:
                        return OPEN_DATASYNC_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
 */
bool
assign_xlog_sync_method(int new_sync_method, bool doit, GucSource source)
{
        if (!doit)
                return true;

        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)
                {
                        if (pg_fsync(openLogFile) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not fsync log file %u, segment %u: %m",
                                                                openLogId, openLogSeg)));
                        if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
                                XLogFileClose();
                }
        }

        return true;
}


/*
 * Issue appropriate kind of fsync (if any) on the current XLOG output file
 */
static void
issue_xlog_fsync(void)
{
        switch (sync_method)
        {
                case SYNC_METHOD_FSYNC:
                        if (pg_fsync_no_writethrough(openLogFile) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not fsync log file %u, segment %u: %m",
                                                                openLogId, openLogSeg)));
                        break;
#ifdef HAVE_FSYNC_WRITETHROUGH
                case SYNC_METHOD_FSYNC_WRITETHROUGH:
                        if (pg_fsync_writethrough(openLogFile) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                                 errmsg("could not fsync write-through log file %u, segment %u: %m",
                                                                openLogId, openLogSeg)));
                        break;
#endif
#ifdef HAVE_FDATASYNC
                case SYNC_METHOD_FDATASYNC:
                        if (pg_fdatasync(openLogFile) != 0)
                                ereport(PANIC,
                                                (errcode_for_file_access(),
                                        errmsg("could not fdatasync log file %u, segment %u: %m",
                                                   openLogId, openLogSeg)));
                        break;
#endif
                case SYNC_METHOD_OPEN:
                case SYNC_METHOD_OPEN_DSYNC:
                        /* write synced it already */
                        break;
                default:
                        elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
                        break;
        }
}


/*
 * pg_start_backup: set up for taking an on-line backup dump
 *
 * Essentially what this does is to create a backup label file in $PGDATA,
 * where it will be archived as part of the backup dump.  The label file
 * contains the user-supplied label string (typically this would be used
 * to tell where the backup dump will be stored) and the starting time and
 * starting WAL location for the dump.
 */
Datum
pg_start_backup(PG_FUNCTION_ARGS)
{
        text       *backupid = PG_GETARG_TEXT_P(0);
        char       *backupidstr;
        XLogRecPtr      checkpointloc;
        XLogRecPtr      startpoint;
        pg_time_t       stamp_time;
        char            strfbuf[128];
        char            xlogfilename[MAXFNAMELEN];
        uint32          _logId;
        uint32          _logSeg;
        struct stat stat_buf;
        FILE       *fp;

        if (!superuser())
                ereport(ERROR,
                                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                                 errmsg("must be superuser to run a backup")));

        if (!XLogArchivingActive())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("WAL archiving is not active"),
                                 errhint("archive_mode must be enabled at server start.")));

        if (!XLogArchiveCommandSet())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("WAL archiving is not active"),
                                 errhint("archive_command must be defined before "
                                                 "online backups can be made safely.")));

        backupidstr = text_to_cstring(backupid);

        /*
         * 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.)
         *
         * We must hold WALInsertLock to change the value of forcePageWrites, to
         * ensure adequate interlocking against XLogInsert().
         */
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
        if (XLogCtl->Insert.forcePageWrites)
        {
                LWLockRelease(WALInsertLock);
                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.forcePageWrites = true;
        LWLockRelease(WALInsertLock);

        /* Ensure we release forcePageWrites if fail below */
        PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) 0);
        {
                /*
                 * 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.
                 *
                 * We don't use CHECKPOINT_IMMEDIATE, hence this can take awhile.
                 */
                RequestCheckpoint(CHECKPOINT_FORCE | CHECKPOINT_WAIT);

                /*
                 * 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_EXCLUSIVE);
                checkpointloc = ControlFile->checkPoint;
                startpoint = ControlFile->checkPointCopy.redo;
                LWLockRelease(ControlFileLock);

                XLByteToSeg(startpoint, _logId, _logSeg);
                XLogFileName(xlogfilename, ThisTimeLineID, _logId, _logSeg);

                /* 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));

                /*
                 * Check for existing backup label --- implies a backup is already
                 * running.  (XXX given that we checked forcePageWrites 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)));

                /*
                 * Okay, write the 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)));
                fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
                                startpoint.xlogid, startpoint.xrecoff, xlogfilename);
                fprintf(fp, "CHECKPOINT LOCATION: %X/%X\n",
                                checkpointloc.xlogid, checkpointloc.xrecoff);
                fprintf(fp, "START TIME: %s\n", strfbuf);
                fprintf(fp, "LABEL: %s\n", backupidstr);
                if (fflush(fp) || ferror(fp) || FreeFile(fp))
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not write file \"%s\": %m",
                                                        BACKUP_LABEL_FILE)));
        }
        PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) 0);

        /*
         * We're done.  As a convenience, return the starting WAL location.
         */
        snprintf(xlogfilename, sizeof(xlogfilename), "%X/%X",
                         startpoint.xlogid, startpoint.xrecoff);
        PG_RETURN_TEXT_P(cstring_to_text(xlogfilename));
}

/* Error cleanup callback for pg_start_backup */
static void
pg_start_backup_callback(int code, Datum arg)
{
        /* Turn off forcePageWrites on failure */
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
        XLogCtl->Insert.forcePageWrites = false;
        LWLockRelease(WALInsertLock);
}

/*
 * pg_stop_backup: finish taking an on-line backup dump
 *
 * We remove the backup label file created by pg_start_backup, and instead
 * create a backup history file in pg_xlog (whence it will immediately be
 * archived).  The backup history file contains the same info found in
 * the label file, plus the backup-end time and WAL location.
 * Note: different from CancelBackup which just cancels online backup mode.
 */
Datum
pg_stop_backup(PG_FUNCTION_ARGS)
{
        XLogRecPtr      startpoint;
        XLogRecPtr      stoppoint;
        pg_time_t       stamp_time;
        char            strfbuf[128];
        char            histfilepath[MAXPGPATH];
        char            startxlogfilename[MAXFNAMELEN];
        char            stopxlogfilename[MAXFNAMELEN];
        char            lastxlogfilename[MAXFNAMELEN];
        char            histfilename[MAXFNAMELEN];
        uint32          _logId;
        uint32          _logSeg;
        FILE       *lfp;
        FILE       *fp;
        char            ch;
        int                     ich;
        int                     seconds_before_warning;
        int                     waits = 0;

        if (!superuser())
                ereport(ERROR,
                                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                                 (errmsg("must be superuser to run a backup"))));

        if (!XLogArchivingActive())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("WAL archiving is not active"),
                                 errhint("archive_mode must be enabled at server start.")));

        /*
         * OK to clear forcePageWrites
         */
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
        XLogCtl->Insert.forcePageWrites = false;
        LWLockRelease(WALInsertLock);

        /*
         * 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. We'll report
         * the end address of the XLOG SWITCH record as the backup stopping point.
         */
        stoppoint = RequestXLogSwitch();

        XLByteToSeg(stoppoint, _logId, _logSeg);
        XLogFileName(stopxlogfilename, ThisTimeLineID, _logId, _logSeg);

        /* 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));

        /*
         * Open the existing label file
         */
        lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
        if (!lfp)
        {
                if (errno != ENOENT)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m",
                                                        BACKUP_LABEL_FILE)));
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("a backup is not in progress")));
        }

        /*
         * 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 (fscanf(lfp, "START WAL LOCATION: %X/%X (file %24s)%c",
                           &startpoint.xlogid, &startpoint.xrecoff, startxlogfilename,
                           &ch) != 4 || ch != '\n')
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));

        /*
         * Write the backup history file
         */
        XLByteToSeg(startpoint, _logId, _logSeg);
        BackupHistoryFilePath(histfilepath, ThisTimeLineID, _logId, _logSeg,
                                                  startpoint.xrecoff % XLogSegSize);
        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",
                        startpoint.xlogid, startpoint.xrecoff, startxlogfilename);
        fprintf(fp, "STOP WAL LOCATION: %X/%X (file %s)\n",
                        stoppoint.xlogid, stoppoint.xrecoff, stopxlogfilename);
        /* transfer remaining lines from label to history file */
        while ((ich = fgetc(lfp)) != EOF)
                fputc(ich, fp);
        fprintf(fp, "STOP TIME: %s\n", strfbuf);
        if (fflush(fp) || ferror(fp) || FreeFile(fp))
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write file \"%s\": %m",
                                                histfilepath)));

        /*
         * Close and remove the backup label file
         */
        if (ferror(lfp) || FreeFile(lfp))
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not read file \"%s\": %m",
                                                BACKUP_LABEL_FILE)));
        if (unlink(BACKUP_LABEL_FILE) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not remove file \"%s\": %m",
                                                BACKUP_LABEL_FILE)));

        /*
         * 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();

        /*
         * Wait until both the last WAL file filled during backup and the history
         * file have been archived.  We 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, you can set statement_timeout.
         */
        XLByteToPrevSeg(stoppoint, _logId, _logSeg);
        XLogFileName(lastxlogfilename, ThisTimeLineID, _logId, _logSeg);

        XLByteToSeg(startpoint, _logId, _logSeg);
        BackupHistoryFileName(histfilename, ThisTimeLineID, _logId, _logSeg,
                                                  startpoint.xrecoff % XLogSegSize);

        seconds_before_warning = 60;
        waits = 0;

        while (XLogArchiveIsBusy(lastxlogfilename) ||
                   XLogArchiveIsBusy(histfilename))
        {
                CHECK_FOR_INTERRUPTS();

                pg_usleep(1000000L);

                if (++waits >= seconds_before_warning)
                {
                        seconds_before_warning *= 2;     /* This wraps in >10 years... */
                        ereport(WARNING,
                                        (errmsg("pg_stop_backup still waiting for archive to complete (%d seconds elapsed)",
                                                        waits)));
                }
        }

        /*
         * We're done.  As a convenience, return the ending WAL location.
         */
        snprintf(stopxlogfilename, sizeof(stopxlogfilename), "%X/%X",
                         stoppoint.xlogid, stoppoint.xrecoff);
        PG_RETURN_TEXT_P(cstring_to_text(stopxlogfilename));
}

/*
 * pg_switch_xlog: switch to next xlog file
 */
Datum
pg_switch_xlog(PG_FUNCTION_ARGS)
{
        XLogRecPtr      switchpoint;
        char            location[MAXFNAMELEN];

        if (!superuser())
                ereport(ERROR,
                                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                         (errmsg("must be superuser to switch transaction log files"))));

        switchpoint = RequestXLogSwitch();

        /*
         * As a convenience, return the WAL location of the switch record
         */
        snprintf(location, sizeof(location), "%X/%X",
                         switchpoint.xlogid, switchpoint.xrecoff);
        PG_RETURN_TEXT_P(cstring_to_text(location));
}

/*
 * Report the current WAL write location (same format as pg_start_backup etc)
 *
 * This is useful for determining how much of WAL is visible to an external
 * archiving process.  Note that the data before this point is written out
 * to the kernel, but is not necessarily synced to disk.
 */
Datum
pg_current_xlog_location(PG_FUNCTION_ARGS)
{
        char            location[MAXFNAMELEN];

        /* Make sure we have an up-to-date local LogwrtResult */
        {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;

                SpinLockAcquire(&xlogctl->info_lck);
                LogwrtResult = xlogctl->LogwrtResult;
                SpinLockRelease(&xlogctl->info_lck);
        }

        snprintf(location, sizeof(location), "%X/%X",
                         LogwrtResult.Write.xlogid, LogwrtResult.Write.xrecoff);
        PG_RETURN_TEXT_P(cstring_to_text(location));
}

/*
 * Report the current WAL insert location (same format as pg_start_backup etc)
 *
 * This function is mostly for debugging purposes.
 */
Datum
pg_current_xlog_insert_location(PG_FUNCTION_ARGS)
{
        XLogCtlInsert *Insert = &XLogCtl->Insert;
        XLogRecPtr      current_recptr;
        char            location[MAXFNAMELEN];

        /*
         * Get the current end-of-WAL position ... shared lock is sufficient
         */
        LWLockAcquire(WALInsertLock, LW_SHARED);
        INSERT_RECPTR(current_recptr, Insert, Insert->curridx);
        LWLockRelease(WALInsertLock);

        snprintf(location, sizeof(location), "%X/%X",
                         current_recptr.xlogid, current_recptr.xrecoff);
        PG_RETURN_TEXT_P(cstring_to_text(location));
}

/*
 * Compute an xlog file name and decimal byte offset given a WAL location,
 * such as is returned by pg_stop_backup() or pg_xlog_switch().
 *
 * Note that a location exactly at a segment boundary is taken to be in
 * the previous segment.  This is usually the right thing, since the
 * expected usage is to determine which xlog file(s) are ready to archive.
 */
Datum
pg_xlogfile_name_offset(PG_FUNCTION_ARGS)
{
        text       *location = PG_GETARG_TEXT_P(0);
        char       *locationstr;
        unsigned int uxlogid;
        unsigned int uxrecoff;
        uint32          xlogid;
        uint32          xlogseg;
        uint32          xrecoff;
        XLogRecPtr      locationpoint;
        char            xlogfilename[MAXFNAMELEN];
        Datum           values[2];
        bool            isnull[2];
        TupleDesc       resultTupleDesc;
        HeapTuple       resultHeapTuple;
        Datum           result;

        /*
         * Read input and parse
         */
        locationstr = text_to_cstring(location);

        if (sscanf(locationstr, "%X/%X", &uxlogid, &uxrecoff) != 2)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("could not parse transaction log location \"%s\"",
                                                locationstr)));

        locationpoint.xlogid = uxlogid;
        locationpoint.xrecoff = uxrecoff;

        /*
         * Construct a tuple descriptor for the result row.  This must match this
         * function's pg_proc entry!
         */
        resultTupleDesc = CreateTemplateTupleDesc(2, false);
        TupleDescInitEntry(resultTupleDesc, (AttrNumber) 1, "file_name",
                                           TEXTOID, -1, 0);
        TupleDescInitEntry(resultTupleDesc, (AttrNumber) 2, "file_offset",
                                           INT4OID, -1, 0);

        resultTupleDesc = BlessTupleDesc(resultTupleDesc);

        /*
         * xlogfilename
         */
        XLByteToPrevSeg(locationpoint, xlogid, xlogseg);
        XLogFileName(xlogfilename, ThisTimeLineID, xlogid, xlogseg);

        values[0] = CStringGetTextDatum(xlogfilename);
        isnull[0] = false;

        /*
         * offset
         */
        xrecoff = locationpoint.xrecoff - xlogseg * XLogSegSize;

        values[1] = UInt32GetDatum(xrecoff);
        isnull[1] = false;

        /*
         * Tuple jam: Having first prepared your Datums, then squash together
         */
        resultHeapTuple = heap_form_tuple(resultTupleDesc, values, isnull);

        result = HeapTupleGetDatum(resultHeapTuple);

        PG_RETURN_DATUM(result);
}

/*
 * Compute an xlog file name given a WAL location,
 * such as is returned by pg_stop_backup() or pg_xlog_switch().
 */
Datum
pg_xlogfile_name(PG_FUNCTION_ARGS)
{
        text       *location = PG_GETARG_TEXT_P(0);
        char       *locationstr;
        unsigned int uxlogid;
        unsigned int uxrecoff;
        uint32          xlogid;
        uint32          xlogseg;
        XLogRecPtr      locationpoint;
        char            xlogfilename[MAXFNAMELEN];

        locationstr = text_to_cstring(location);

        if (sscanf(locationstr, "%X/%X", &uxlogid, &uxrecoff) != 2)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("could not parse transaction log location \"%s\"",
                                                locationstr)));

        locationpoint.xlogid = uxlogid;
        locationpoint.xrecoff = uxrecoff;

        XLByteToPrevSeg(locationpoint, xlogid, xlogseg);
        XLogFileName(xlogfilename, ThisTimeLineID, xlogid, xlogseg);

        PG_RETURN_TEXT_P(cstring_to_text(xlogfilename));
}

/*
 * read_backup_label: check to see if a backup_label file is present
 *
 * If we see a backup_label during recovery, we assume that we are recovering
 * from a backup dump file, and we therefore roll forward from the checkpoint
 * identified by the label file, NOT what pg_control says.      This avoids the
 * problem that pg_control might have been archived one or more checkpoints
 * later than the start of the dump, and so if we rely on it as the start
 * point, we will fail to restore a consistent database state.
 *
 * We also attempt to retrieve the corresponding backup history file.
 * If successful, set *minRecoveryLoc to constrain valid PITR stopping
 * points.
 *
 * Returns TRUE if a backup_label was found (and fills the checkpoint
 * location into *checkPointLoc); returns FALSE if not.
 */
static bool
read_backup_label(XLogRecPtr *checkPointLoc, XLogRecPtr *minRecoveryLoc)
{
        XLogRecPtr      startpoint;
        XLogRecPtr      stoppoint;
        char            histfilename[MAXFNAMELEN];
        char            histfilepath[MAXPGPATH];
        char            startxlogfilename[MAXFNAMELEN];
        char            stopxlogfilename[MAXFNAMELEN];
        TimeLineID      tli;
        uint32          _logId;
        uint32          _logSeg;
        FILE       *lfp;
        FILE       *fp;
        char            ch;

        /* Default is to not constrain recovery stop point */
        minRecoveryLoc->xlogid = 0;
        minRecoveryLoc->xrecoff = 0;

        /*
         * See if label file is present
         */
        lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
        if (!lfp)
        {
                if (errno != ENOENT)
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m",
                                                        BACKUP_LABEL_FILE)));
                return false;                   /* it's not there, all is fine */
        }

        /*
         * Read and parse the START WAL LOCATION and CHECKPOINT lines (this code
         * is pretty crude, but we are not expecting any variability in the file
         * format).
         */
        if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %08X%16s)%c",
                           &startpoint.xlogid, &startpoint.xrecoff, &tli,
                           startxlogfilename, &ch) != 5 || ch != '\n')
                ereport(FATAL,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
                           &checkPointLoc->xlogid, &checkPointLoc->xrecoff,
                           &ch) != 3 || ch != '\n')
                ereport(FATAL,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
        if (ferror(lfp) || FreeFile(lfp))
                ereport(FATAL,
                                (errcode_for_file_access(),
                                 errmsg("could not read file \"%s\": %m",
                                                BACKUP_LABEL_FILE)));

        /*
         * Try to retrieve the backup history file (no error if we can't)
         */
        XLByteToSeg(startpoint, _logId, _logSeg);
        BackupHistoryFileName(histfilename, tli, _logId, _logSeg,
                                                  startpoint.xrecoff % XLogSegSize);

        if (InArchiveRecovery)
                RestoreArchivedFile(histfilepath, histfilename, "RECOVERYHISTORY", 0);
        else
                BackupHistoryFilePath(histfilepath, tli, _logId, _logSeg,
                                                          startpoint.xrecoff % XLogSegSize);

        fp = AllocateFile(histfilepath, "r");
        if (fp)
        {
                /*
                 * Parse history file to identify stop point.
                 */
                if (fscanf(fp, "START WAL LOCATION: %X/%X (file %24s)%c",
                                   &startpoint.xlogid, &startpoint.xrecoff, startxlogfilename,
                                   &ch) != 4 || ch != '\n')
                        ereport(FATAL,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("invalid data in file \"%s\"", histfilename)));
                if (fscanf(fp, "STOP WAL LOCATION: %X/%X (file %24s)%c",
                                   &stoppoint.xlogid, &stoppoint.xrecoff, stopxlogfilename,
                                   &ch) != 4 || ch != '\n')
                        ereport(FATAL,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("invalid data in file \"%s\"", histfilename)));
                *minRecoveryLoc = stoppoint;
                if (ferror(fp) || FreeFile(fp))
                        ereport(FATAL,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m",
                                                        histfilepath)));
        }

        return true;
}

/*
 * Error context callback for errors occurring during rm_redo().
 */
static void
rm_redo_error_callback(void *arg)
{
        XLogRecord *record = (XLogRecord *) arg;
        StringInfoData buf;

        initStringInfo(&buf);
        RmgrTable[record->xl_rmid].rm_desc(&buf,
                                                                           record->xl_info,
                                                                           XLogRecGetData(record));

        /* don't bother emitting empty description */
        if (buf.len > 0)
                errcontext("xlog redo %s", buf.data);

        pfree(buf.data);
}

/*
 * 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" file to cancel backup mode
 *
 * If the "backup_label" file exists, it will be renamed to "backup_label.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 file is not there, return */
        if (stat(BACKUP_LABEL_FILE, &stat_buf) < 0)
                return;

        /* remove leftover file from previously cancelled backup if it exists */
        unlink(BACKUP_LABEL_OLD);

        if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) == 0)
        {
                ereport(LOG,
                                (errmsg("online backup mode cancelled"),
                                 errdetail("\"%s\" was renamed to \"%s\".",
                                                BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
        }
        else
        {
                ereport(WARNING,
                                (errcode_for_file_access(),
                                 errmsg("online backup mode was not cancelled"),
                                 errdetail("Could not rename \"%s\" to \"%s\": %m.",
                                                BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
        }
}