Improve comment about GetWALAvailability's WALAVAIL_REMOVED code.

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

/*-------------------------------------------------------------------------
 *
 * twophase.c
 *              Two-phase commit support functions.
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *              src/backend/access/transam/twophase.c
 *
 * NOTES
 *              Each global transaction is associated with a global transaction
 *              identifier (GID). The client assigns a GID to a postgres
 *              transaction with the PREPARE TRANSACTION command.
 *
 *              We keep all active global transactions in a shared memory array.
 *              When the PREPARE TRANSACTION command is issued, the GID is
 *              reserved for the transaction in the array. This is done before
 *              a WAL entry is made, because the reservation checks for duplicate
 *              GIDs and aborts the transaction if there already is a global
 *              transaction in prepared state with the same GID.
 *
 *              A global transaction (gxact) also has dummy PGPROC; this is what keeps
 *              the XID considered running by TransactionIdIsInProgress.  It is also
 *              convenient as a PGPROC to hook the gxact's locks to.
 *
 *              Information to recover prepared transactions in case of crash is
 *              now stored in WAL for the common case. In some cases there will be
 *              an extended period between preparing a GXACT and commit/abort, in
 *              which case we need to separately record prepared transaction data
 *              in permanent storage. This includes locking information, pending
 *              notifications etc. All that state information is written to the
 *              per-transaction state file in the pg_twophase directory.
 *              All prepared transactions will be written prior to shutdown.
 *
 *              Life track of state data is following:
 *
 *              * On PREPARE TRANSACTION backend writes state data only to the WAL and
 *                stores pointer to the start of the WAL record in
 *                gxact->prepare_start_lsn.
 *              * If COMMIT occurs before checkpoint then backend reads data from WAL
 *                using prepare_start_lsn.
 *              * On checkpoint state data copied to files in pg_twophase directory and
 *                fsynced
 *              * If COMMIT happens after checkpoint then backend reads state data from
 *                files
 *
 *              During replay and replication, TwoPhaseState also holds information
 *              about active prepared transactions that haven't been moved to disk yet.
 *
 *              Replay of twophase records happens by the following rules:
 *
 *              * At the beginning of recovery, pg_twophase is scanned once, filling
 *                TwoPhaseState with entries marked with gxact->inredo and
 *                gxact->ondisk.  Two-phase file data older than the XID horizon of
 *                the redo position are discarded.
 *              * On PREPARE redo, the transaction is added to TwoPhaseState->prepXacts.
 *                gxact->inredo is set to true for such entries.
 *              * On Checkpoint we iterate through TwoPhaseState->prepXacts entries
 *                that have gxact->inredo set and are behind the redo_horizon. We
 *                save them to disk and then switch gxact->ondisk to true.
 *              * On COMMIT/ABORT we delete the entry from TwoPhaseState->prepXacts.
 *                If gxact->ondisk is true, the corresponding entry from the disk
 *                is additionally deleted.
 *              * RecoverPreparedTransactions(), StandbyRecoverPreparedTransactions()
 *                and PrescanPreparedTransactions() have been modified to go through
 *                gxact->inredo entries that have not made it to disk.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

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

#include "access/commit_ts.h"
#include "access/htup_details.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogreader.h"
#include "access/xlogutils.h"
#include "catalog/pg_type.h"
#include "catalog/storage.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "replication/origin.h"
#include "replication/syncrep.h"
#include "replication/walsender.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/md.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/sinvaladt.h"
#include "storage/smgr.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/timestamp.h"

/*
 * Directory where Two-phase commit files reside within PGDATA
 */
#define TWOPHASE_DIR "pg_twophase"

/* GUC variable, can't be changed after startup */
int                     max_prepared_xacts = 0;

/*
 * This struct describes one global transaction that is in prepared state
 * or attempting to become prepared.
 *
 * The lifecycle of a global transaction is:
 *
 * 1. After checking that the requested GID is not in use, set up an entry in
 * the TwoPhaseState->prepXacts array with the correct GID and valid = false,
 * and mark it as locked by my backend.
 *
 * 2. After successfully completing prepare, set valid = true and enter the
 * referenced PGPROC into the global ProcArray.
 *
 * 3. To begin COMMIT PREPARED or ROLLBACK PREPARED, check that the entry is
 * valid and not locked, then mark the entry as locked by storing my current
 * backend ID into locking_backend.  This prevents concurrent attempts to
 * commit or rollback the same prepared xact.
 *
 * 4. On completion of COMMIT PREPARED or ROLLBACK PREPARED, remove the entry
 * from the ProcArray and the TwoPhaseState->prepXacts array and return it to
 * the freelist.
 *
 * Note that if the preparing transaction fails between steps 1 and 2, the
 * entry must be removed so that the GID and the GlobalTransaction struct
 * can be reused.  See AtAbort_Twophase().
 *
 * typedef struct GlobalTransactionData *GlobalTransaction appears in
 * twophase.h
 */

typedef struct GlobalTransactionData
{
        GlobalTransaction next;         /* list link for free list */
        int                     pgprocno;               /* ID of associated dummy PGPROC */
        BackendId       dummyBackendId; /* similar to backend id for backends */
        TimestampTz prepared_at;        /* time of preparation */

        /*
         * Note that we need to keep track of two LSNs for each GXACT. We keep
         * track of the start LSN because this is the address we must use to read
         * state data back from WAL when committing a prepared GXACT. We keep
         * track of the end LSN because that is the LSN we need to wait for prior
         * to commit.
         */
        XLogRecPtr      prepare_start_lsn;      /* XLOG offset of prepare record start */
        XLogRecPtr      prepare_end_lsn;        /* XLOG offset of prepare record end */
        TransactionId xid;                      /* The GXACT id */

        Oid                     owner;                  /* ID of user that executed the xact */
        BackendId       locking_backend;        /* backend currently working on the xact */
        bool            valid;                  /* true if PGPROC entry is in proc array */
        bool            ondisk;                 /* true if prepare state file is on disk */
        bool            inredo;                 /* true if entry was added via xlog_redo */
        char            gid[GIDSIZE];   /* The GID assigned to the prepared xact */
}                       GlobalTransactionData;

/*
 * Two Phase Commit shared state.  Access to this struct is protected
 * by TwoPhaseStateLock.
 */
typedef struct TwoPhaseStateData
{
        /* Head of linked list of free GlobalTransactionData structs */
        GlobalTransaction freeGXacts;

        /* Number of valid prepXacts entries. */
        int                     numPrepXacts;

        /* There are max_prepared_xacts items in this array */
        GlobalTransaction prepXacts[FLEXIBLE_ARRAY_MEMBER];
} TwoPhaseStateData;

static TwoPhaseStateData *TwoPhaseState;

/*
 * Global transaction entry currently locked by us, if any.  Note that any
 * access to the entry pointed to by this variable must be protected by
 * TwoPhaseStateLock, though obviously the pointer itself doesn't need to be
 * (since it's just local memory).
 */
static GlobalTransaction MyLockedGxact = NULL;

static bool twophaseExitRegistered = false;

static void RecordTransactionCommitPrepared(TransactionId xid,
                                                                                        int nchildren,
                                                                                        TransactionId *children,
                                                                                        int nrels,
                                                                                        RelFileLocator *rels,
                                                                                        int nstats,
                                                                                        xl_xact_stats_item *stats,
                                                                                        int ninvalmsgs,
                                                                                        SharedInvalidationMessage *invalmsgs,
                                                                                        bool initfileinval,
                                                                                        const char *gid);
static void RecordTransactionAbortPrepared(TransactionId xid,
                                                                                   int nchildren,
                                                                                   TransactionId *children,
                                                                                   int nrels,
                                                                                   RelFileLocator *rels,
                                                                                   int nstats,
                                                                                   xl_xact_stats_item *stats,
                                                                                   const char *gid);
static void ProcessRecords(char *bufptr, TransactionId xid,
                                                   const TwoPhaseCallback callbacks[]);
static void RemoveGXact(GlobalTransaction gxact);

static void XlogReadTwoPhaseData(XLogRecPtr lsn, char **buf, int *len);
static char *ProcessTwoPhaseBuffer(TransactionId xid,
                                                                   XLogRecPtr prepare_start_lsn,
                                                                   bool fromdisk, bool setParent, bool setNextXid);
static void MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid,
                                                                const char *gid, TimestampTz prepared_at, Oid owner,
                                                                Oid databaseid);
static void RemoveTwoPhaseFile(TransactionId xid, bool giveWarning);
static void RecreateTwoPhaseFile(TransactionId xid, void *content, int len);

/*
 * Initialization of shared memory
 */
Size
TwoPhaseShmemSize(void)
{
        Size            size;

        /* Need the fixed struct, the array of pointers, and the GTD structs */
        size = offsetof(TwoPhaseStateData, prepXacts);
        size = add_size(size, mul_size(max_prepared_xacts,
                                                                   sizeof(GlobalTransaction)));
        size = MAXALIGN(size);
        size = add_size(size, mul_size(max_prepared_xacts,
                                                                   sizeof(GlobalTransactionData)));

        return size;
}

void
TwoPhaseShmemInit(void)
{
        bool            found;

        TwoPhaseState = ShmemInitStruct("Prepared Transaction Table",
                                                                        TwoPhaseShmemSize(),
                                                                        &found);
        if (!IsUnderPostmaster)
        {
                GlobalTransaction gxacts;
                int                     i;

                Assert(!found);
                TwoPhaseState->freeGXacts = NULL;
                TwoPhaseState->numPrepXacts = 0;

                /*
                 * Initialize the linked list of free GlobalTransactionData structs
                 */
                gxacts = (GlobalTransaction)
                        ((char *) TwoPhaseState +
                         MAXALIGN(offsetof(TwoPhaseStateData, prepXacts) +
                                          sizeof(GlobalTransaction) * max_prepared_xacts));
                for (i = 0; i < max_prepared_xacts; i++)
                {
                        /* insert into linked list */
                        gxacts[i].next = TwoPhaseState->freeGXacts;
                        TwoPhaseState->freeGXacts = &gxacts[i];

                        /* associate it with a PGPROC assigned by InitProcGlobal */
                        gxacts[i].pgprocno = PreparedXactProcs[i].pgprocno;

                        /*
                         * Assign a unique ID for each dummy proc, so that the range of
                         * dummy backend IDs immediately follows the range of normal
                         * backend IDs. We don't dare to assign a real backend ID to dummy
                         * procs, because prepared transactions don't take part in cache
                         * invalidation like a real backend ID would imply, but having a
                         * unique ID for them is nevertheless handy. This arrangement
                         * allows you to allocate an array of size (MaxBackends +
                         * max_prepared_xacts + 1), and have a slot for every backend and
                         * prepared transaction. Currently multixact.c uses that
                         * technique.
                         */
                        gxacts[i].dummyBackendId = MaxBackends + 1 + i;
                }
        }
        else
                Assert(found);
}

/*
 * Exit hook to unlock the global transaction entry we're working on.
 */
static void
AtProcExit_Twophase(int code, Datum arg)
{
        /* same logic as abort */
        AtAbort_Twophase();
}

/*
 * Abort hook to unlock the global transaction entry we're working on.
 */
void
AtAbort_Twophase(void)
{
        if (MyLockedGxact == NULL)
                return;

        /*
         * What to do with the locked global transaction entry?  If we were in the
         * process of preparing the transaction, but haven't written the WAL
         * record and state file yet, the transaction must not be considered as
         * prepared.  Likewise, if we are in the process of finishing an
         * already-prepared transaction, and fail after having already written the
         * 2nd phase commit or rollback record to the WAL, the transaction should
         * not be considered as prepared anymore.  In those cases, just remove the
         * entry from shared memory.
         *
         * Otherwise, the entry must be left in place so that the transaction can
         * be finished later, so just unlock it.
         *
         * If we abort during prepare, after having written the WAL record, we
         * might not have transferred all locks and other state to the prepared
         * transaction yet.  Likewise, if we abort during commit or rollback,
         * after having written the WAL record, we might not have released all the
         * resources held by the transaction yet.  In those cases, the in-memory
         * state can be wrong, but it's too late to back out.
         */
        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        if (!MyLockedGxact->valid)
                RemoveGXact(MyLockedGxact);
        else
                MyLockedGxact->locking_backend = InvalidBackendId;
        LWLockRelease(TwoPhaseStateLock);

        MyLockedGxact = NULL;
}

/*
 * This is called after we have finished transferring state to the prepared
 * PGPROC entry.
 */
void
PostPrepare_Twophase(void)
{
        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        MyLockedGxact->locking_backend = InvalidBackendId;
        LWLockRelease(TwoPhaseStateLock);

        MyLockedGxact = NULL;
}


/*
 * MarkAsPreparing
 *              Reserve the GID for the given transaction.
 */
GlobalTransaction
MarkAsPreparing(TransactionId xid, const char *gid,
                                TimestampTz prepared_at, Oid owner, Oid databaseid)
{
        GlobalTransaction gxact;
        int                     i;

        if (strlen(gid) >= GIDSIZE)
                ereport(ERROR,
                                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                                 errmsg("transaction identifier \"%s\" is too long",
                                                gid)));

        /* fail immediately if feature is disabled */
        if (max_prepared_xacts == 0)
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("prepared transactions are disabled"),
                                 errhint("Set max_prepared_transactions to a nonzero value.")));

        /* on first call, register the exit hook */
        if (!twophaseExitRegistered)
        {
                before_shmem_exit(AtProcExit_Twophase, 0);
                twophaseExitRegistered = true;
        }

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

        /* Check for conflicting GID */
        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                gxact = TwoPhaseState->prepXacts[i];
                if (strcmp(gxact->gid, gid) == 0)
                {
                        ereport(ERROR,
                                        (errcode(ERRCODE_DUPLICATE_OBJECT),
                                         errmsg("transaction identifier \"%s\" is already in use",
                                                        gid)));
                }
        }

        /* Get a free gxact from the freelist */
        if (TwoPhaseState->freeGXacts == NULL)
                ereport(ERROR,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("maximum number of prepared transactions reached"),
                                 errhint("Increase max_prepared_transactions (currently %d).",
                                                 max_prepared_xacts)));
        gxact = TwoPhaseState->freeGXacts;
        TwoPhaseState->freeGXacts = gxact->next;

        MarkAsPreparingGuts(gxact, xid, gid, prepared_at, owner, databaseid);

        gxact->ondisk = false;

        /* And insert it into the active array */
        Assert(TwoPhaseState->numPrepXacts < max_prepared_xacts);
        TwoPhaseState->prepXacts[TwoPhaseState->numPrepXacts++] = gxact;

        LWLockRelease(TwoPhaseStateLock);

        return gxact;
}

/*
 * MarkAsPreparingGuts
 *
 * This uses a gxact struct and puts it into the active array.
 * NOTE: this is also used when reloading a gxact after a crash; so avoid
 * assuming that we can use very much backend context.
 *
 * Note: This function should be called with appropriate locks held.
 */
static void
MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
                                        TimestampTz prepared_at, Oid owner, Oid databaseid)
{
        PGPROC     *proc;
        int                     i;

        Assert(LWLockHeldByMeInMode(TwoPhaseStateLock, LW_EXCLUSIVE));

        Assert(gxact != NULL);
        proc = &ProcGlobal->allProcs[gxact->pgprocno];

        /* Initialize the PGPROC entry */
        MemSet(proc, 0, sizeof(PGPROC));
        proc->pgprocno = gxact->pgprocno;
        dlist_node_init(&proc->links);
        proc->waitStatus = PROC_WAIT_STATUS_OK;
        if (LocalTransactionIdIsValid(MyProc->lxid))
        {
                /* clone VXID, for TwoPhaseGetXidByVirtualXID() to find */
                proc->lxid = MyProc->lxid;
                proc->backendId = MyBackendId;
        }
        else
        {
                Assert(AmStartupProcess() || !IsPostmasterEnvironment);
                /* GetLockConflicts() uses this to specify a wait on the XID */
                proc->lxid = xid;
                proc->backendId = InvalidBackendId;
        }
        proc->xid = xid;
        Assert(proc->xmin == InvalidTransactionId);
        proc->delayChkptFlags = 0;
        proc->statusFlags = 0;
        proc->pid = 0;
        proc->databaseId = databaseid;
        proc->roleId = owner;
        proc->tempNamespaceId = InvalidOid;
        proc->isBackgroundWorker = false;
        proc->lwWaiting = LW_WS_NOT_WAITING;
        proc->lwWaitMode = 0;
        proc->waitLock = NULL;
        proc->waitProcLock = NULL;
        pg_atomic_init_u64(&proc->waitStart, 0);
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
                dlist_init(&proc->myProcLocks[i]);
        /* subxid data must be filled later by GXactLoadSubxactData */
        proc->subxidStatus.overflowed = false;
        proc->subxidStatus.count = 0;

        gxact->prepared_at = prepared_at;
        gxact->xid = xid;
        gxact->owner = owner;
        gxact->locking_backend = MyBackendId;
        gxact->valid = false;
        gxact->inredo = false;
        strcpy(gxact->gid, gid);

        /*
         * Remember that we have this GlobalTransaction entry locked for us. If we
         * abort after this, we must release it.
         */
        MyLockedGxact = gxact;
}

/*
 * GXactLoadSubxactData
 *
 * If the transaction being persisted had any subtransactions, this must
 * be called before MarkAsPrepared() to load information into the dummy
 * PGPROC.
 */
static void
GXactLoadSubxactData(GlobalTransaction gxact, int nsubxacts,
                                         TransactionId *children)
{
        PGPROC     *proc = &ProcGlobal->allProcs[gxact->pgprocno];

        /* We need no extra lock since the GXACT isn't valid yet */
        if (nsubxacts > PGPROC_MAX_CACHED_SUBXIDS)
        {
                proc->subxidStatus.overflowed = true;
                nsubxacts = PGPROC_MAX_CACHED_SUBXIDS;
        }
        if (nsubxacts > 0)
        {
                memcpy(proc->subxids.xids, children,
                           nsubxacts * sizeof(TransactionId));
                proc->subxidStatus.count = nsubxacts;
        }
}

/*
 * MarkAsPrepared
 *              Mark the GXACT as fully valid, and enter it into the global ProcArray.
 *
 * lock_held indicates whether caller already holds TwoPhaseStateLock.
 */
static void
MarkAsPrepared(GlobalTransaction gxact, bool lock_held)
{
        /* Lock here may be overkill, but I'm not convinced of that ... */
        if (!lock_held)
                LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        Assert(!gxact->valid);
        gxact->valid = true;
        if (!lock_held)
                LWLockRelease(TwoPhaseStateLock);

        /*
         * Put it into the global ProcArray so TransactionIdIsInProgress considers
         * the XID as still running.
         */
        ProcArrayAdd(&ProcGlobal->allProcs[gxact->pgprocno]);
}

/*
 * LockGXact
 *              Locate the prepared transaction and mark it busy for COMMIT or PREPARE.
 */
static GlobalTransaction
LockGXact(const char *gid, Oid user)
{
        int                     i;

        /* on first call, register the exit hook */
        if (!twophaseExitRegistered)
        {
                before_shmem_exit(AtProcExit_Twophase, 0);
                twophaseExitRegistered = true;
        }

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];
                PGPROC     *proc = &ProcGlobal->allProcs[gxact->pgprocno];

                /* Ignore not-yet-valid GIDs */
                if (!gxact->valid)
                        continue;
                if (strcmp(gxact->gid, gid) != 0)
                        continue;

                /* Found it, but has someone else got it locked? */
                if (gxact->locking_backend != InvalidBackendId)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                         errmsg("prepared transaction with identifier \"%s\" is busy",
                                                        gid)));

                if (user != gxact->owner && !superuser_arg(user))
                        ereport(ERROR,
                                        (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                                         errmsg("permission denied to finish prepared transaction"),
                                         errhint("Must be superuser or the user that prepared the transaction.")));

                /*
                 * Note: it probably would be possible to allow committing from
                 * another database; but at the moment NOTIFY is known not to work and
                 * there may be some other issues as well.  Hence disallow until
                 * someone gets motivated to make it work.
                 */
                if (MyDatabaseId != proc->databaseId)
                        ereport(ERROR,
                                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                         errmsg("prepared transaction belongs to another database"),
                                         errhint("Connect to the database where the transaction was prepared to finish it.")));

                /* OK for me to lock it */
                gxact->locking_backend = MyBackendId;
                MyLockedGxact = gxact;

                LWLockRelease(TwoPhaseStateLock);

                return gxact;
        }

        LWLockRelease(TwoPhaseStateLock);

        ereport(ERROR,
                        (errcode(ERRCODE_UNDEFINED_OBJECT),
                         errmsg("prepared transaction with identifier \"%s\" does not exist",
                                        gid)));

        /* NOTREACHED */
        return NULL;
}

/*
 * RemoveGXact
 *              Remove the prepared transaction from the shared memory array.
 *
 * NB: caller should have already removed it from ProcArray
 */
static void
RemoveGXact(GlobalTransaction gxact)
{
        int                     i;

        Assert(LWLockHeldByMeInMode(TwoPhaseStateLock, LW_EXCLUSIVE));

        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                if (gxact == TwoPhaseState->prepXacts[i])
                {
                        /* remove from the active array */
                        TwoPhaseState->numPrepXacts--;
                        TwoPhaseState->prepXacts[i] = TwoPhaseState->prepXacts[TwoPhaseState->numPrepXacts];

                        /* and put it back in the freelist */
                        gxact->next = TwoPhaseState->freeGXacts;
                        TwoPhaseState->freeGXacts = gxact;

                        return;
                }
        }

        elog(ERROR, "failed to find %p in GlobalTransaction array", gxact);
}

/*
 * Returns an array of all prepared transactions for the user-level
 * function pg_prepared_xact.
 *
 * The returned array and all its elements are copies of internal data
 * structures, to minimize the time we need to hold the TwoPhaseStateLock.
 *
 * WARNING -- we return even those transactions that are not fully prepared
 * yet.  The caller should filter them out if he doesn't want them.
 *
 * The returned array is palloc'd.
 */
static int
GetPreparedTransactionList(GlobalTransaction *gxacts)
{
        GlobalTransaction array;
        int                     num;
        int                     i;

        LWLockAcquire(TwoPhaseStateLock, LW_SHARED);

        if (TwoPhaseState->numPrepXacts == 0)
        {
                LWLockRelease(TwoPhaseStateLock);

                *gxacts = NULL;
                return 0;
        }

        num = TwoPhaseState->numPrepXacts;
        array = (GlobalTransaction) palloc(sizeof(GlobalTransactionData) * num);
        *gxacts = array;
        for (i = 0; i < num; i++)
                memcpy(array + i, TwoPhaseState->prepXacts[i],
                           sizeof(GlobalTransactionData));

        LWLockRelease(TwoPhaseStateLock);

        return num;
}


/* Working status for pg_prepared_xact */
typedef struct
{
        GlobalTransaction array;
        int                     ngxacts;
        int                     currIdx;
} Working_State;

/*
 * pg_prepared_xact
 *              Produce a view with one row per prepared transaction.
 *
 * This function is here so we don't have to export the
 * GlobalTransactionData struct definition.
 */
Datum
pg_prepared_xact(PG_FUNCTION_ARGS)
{
        FuncCallContext *funcctx;
        Working_State *status;

        if (SRF_IS_FIRSTCALL())
        {
                TupleDesc       tupdesc;
                MemoryContext oldcontext;

                /* create a function context for cross-call persistence */
                funcctx = SRF_FIRSTCALL_INIT();

                /*
                 * Switch to memory context appropriate for multiple function calls
                 */
                oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

                /* build tupdesc for result tuples */
                /* this had better match pg_prepared_xacts view in system_views.sql */
                tupdesc = CreateTemplateTupleDesc(5);
                TupleDescInitEntry(tupdesc, (AttrNumber) 1, "transaction",
                                                   XIDOID, -1, 0);
                TupleDescInitEntry(tupdesc, (AttrNumber) 2, "gid",
                                                   TEXTOID, -1, 0);
                TupleDescInitEntry(tupdesc, (AttrNumber) 3, "prepared",
                                                   TIMESTAMPTZOID, -1, 0);
                TupleDescInitEntry(tupdesc, (AttrNumber) 4, "ownerid",
                                                   OIDOID, -1, 0);
                TupleDescInitEntry(tupdesc, (AttrNumber) 5, "dbid",
                                                   OIDOID, -1, 0);

                funcctx->tuple_desc = BlessTupleDesc(tupdesc);

                /*
                 * Collect all the 2PC status information that we will format and send
                 * out as a result set.
                 */
                status = (Working_State *) palloc(sizeof(Working_State));
                funcctx->user_fctx = (void *) status;

                status->ngxacts = GetPreparedTransactionList(&status->array);
                status->currIdx = 0;

                MemoryContextSwitchTo(oldcontext);
        }

        funcctx = SRF_PERCALL_SETUP();
        status = (Working_State *) funcctx->user_fctx;

        while (status->array != NULL && status->currIdx < status->ngxacts)
        {
                GlobalTransaction gxact = &status->array[status->currIdx++];
                PGPROC     *proc = &ProcGlobal->allProcs[gxact->pgprocno];
                Datum           values[5] = {0};
                bool            nulls[5] = {0};
                HeapTuple       tuple;
                Datum           result;

                if (!gxact->valid)
                        continue;

                /*
                 * Form tuple with appropriate data.
                 */

                values[0] = TransactionIdGetDatum(proc->xid);
                values[1] = CStringGetTextDatum(gxact->gid);
                values[2] = TimestampTzGetDatum(gxact->prepared_at);
                values[3] = ObjectIdGetDatum(gxact->owner);
                values[4] = ObjectIdGetDatum(proc->databaseId);

                tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
                result = HeapTupleGetDatum(tuple);
                SRF_RETURN_NEXT(funcctx, result);
        }

        SRF_RETURN_DONE(funcctx);
}

/*
 * TwoPhaseGetGXact
 *              Get the GlobalTransaction struct for a prepared transaction
 *              specified by XID
 *
 * If lock_held is set to true, TwoPhaseStateLock will not be taken, so the
 * caller had better hold it.
 */
static GlobalTransaction
TwoPhaseGetGXact(TransactionId xid, bool lock_held)
{
        GlobalTransaction result = NULL;
        int                     i;

        static TransactionId cached_xid = InvalidTransactionId;
        static GlobalTransaction cached_gxact = NULL;

        Assert(!lock_held || LWLockHeldByMe(TwoPhaseStateLock));

        /*
         * During a recovery, COMMIT PREPARED, or ABORT PREPARED, we'll be called
         * repeatedly for the same XID.  We can save work with a simple cache.
         */
        if (xid == cached_xid)
                return cached_gxact;

        if (!lock_held)
                LWLockAcquire(TwoPhaseStateLock, LW_SHARED);

        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];

                if (gxact->xid == xid)
                {
                        result = gxact;
                        break;
                }
        }

        if (!lock_held)
                LWLockRelease(TwoPhaseStateLock);

        if (result == NULL)                     /* should not happen */
                elog(ERROR, "failed to find GlobalTransaction for xid %u", xid);

        cached_xid = xid;
        cached_gxact = result;

        return result;
}

/*
 * TwoPhaseGetXidByVirtualXID
 *              Lookup VXID among xacts prepared since last startup.
 *
 * (This won't find recovered xacts.)  If more than one matches, return any
 * and set "have_more" to true.  To witness multiple matches, a single
 * BackendId must consume 2^32 LXIDs, with no intervening database restart.
 */
TransactionId
TwoPhaseGetXidByVirtualXID(VirtualTransactionId vxid,
                                                   bool *have_more)
{
        int                     i;
        TransactionId result = InvalidTransactionId;

        Assert(VirtualTransactionIdIsValid(vxid));
        LWLockAcquire(TwoPhaseStateLock, LW_SHARED);

        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];
                PGPROC     *proc;
                VirtualTransactionId proc_vxid;

                if (!gxact->valid)
                        continue;
                proc = &ProcGlobal->allProcs[gxact->pgprocno];
                GET_VXID_FROM_PGPROC(proc_vxid, *proc);
                if (VirtualTransactionIdEquals(vxid, proc_vxid))
                {
                        /* Startup process sets proc->backendId to InvalidBackendId. */
                        Assert(!gxact->inredo);

                        if (result != InvalidTransactionId)
                        {
                                *have_more = true;
                                break;
                        }
                        result = gxact->xid;
                }
        }

        LWLockRelease(TwoPhaseStateLock);

        return result;
}

/*
 * TwoPhaseGetDummyBackendId
 *              Get the dummy backend ID for prepared transaction specified by XID
 *
 * Dummy backend IDs are similar to real backend IDs of real backends.
 * They start at MaxBackends + 1, and are unique across all currently active
 * real backends and prepared transactions.  If lock_held is set to true,
 * TwoPhaseStateLock will not be taken, so the caller had better hold it.
 */
BackendId
TwoPhaseGetDummyBackendId(TransactionId xid, bool lock_held)
{
        GlobalTransaction gxact = TwoPhaseGetGXact(xid, lock_held);

        return gxact->dummyBackendId;
}

/*
 * TwoPhaseGetDummyProc
 *              Get the PGPROC that represents a prepared transaction specified by XID
 *
 * If lock_held is set to true, TwoPhaseStateLock will not be taken, so the
 * caller had better hold it.
 */
PGPROC *
TwoPhaseGetDummyProc(TransactionId xid, bool lock_held)
{
        GlobalTransaction gxact = TwoPhaseGetGXact(xid, lock_held);

        return &ProcGlobal->allProcs[gxact->pgprocno];
}

/************************************************************************/
/* State file support                                                                                                   */
/************************************************************************/

#define TwoPhaseFilePath(path, xid) \
        snprintf(path, MAXPGPATH, TWOPHASE_DIR "/%08X", xid)

/*
 * 2PC state file format:
 *
 *      1. TwoPhaseFileHeader
 *      2. TransactionId[] (subtransactions)
 *      3. RelFileLocator[] (files to be deleted at commit)
 *      4. RelFileLocator[] (files to be deleted at abort)
 *      5. SharedInvalidationMessage[] (inval messages to be sent at commit)
 *      6. TwoPhaseRecordOnDisk
 *      7. ...
 *      8. TwoPhaseRecordOnDisk (end sentinel, rmid == TWOPHASE_RM_END_ID)
 *      9. checksum (CRC-32C)
 *
 * Each segment except the final checksum is MAXALIGN'd.
 */

/*
 * Header for a 2PC state file
 */
#define TWOPHASE_MAGIC  0x57F94534      /* format identifier */

typedef xl_xact_prepare TwoPhaseFileHeader;

/*
 * Header for each record in a state file
 *
 * NOTE: len counts only the rmgr data, not the TwoPhaseRecordOnDisk header.
 * The rmgr data will be stored starting on a MAXALIGN boundary.
 */
typedef struct TwoPhaseRecordOnDisk
{
        uint32          len;                    /* length of rmgr data */
        TwoPhaseRmgrId rmid;            /* resource manager for this record */
        uint16          info;                   /* flag bits for use by rmgr */
} TwoPhaseRecordOnDisk;

/*
 * During prepare, the state file is assembled in memory before writing it
 * to WAL and the actual state file.  We use a chain of StateFileChunk blocks
 * for that.
 */
typedef struct StateFileChunk
{
        char       *data;
        uint32          len;
        struct StateFileChunk *next;
} StateFileChunk;

static struct xllist
{
        StateFileChunk *head;           /* first data block in the chain */
        StateFileChunk *tail;           /* last block in chain */
        uint32          num_chunks;
        uint32          bytes_free;             /* free bytes left in tail block */
        uint32          total_len;              /* total data bytes in chain */
}                       records;


/*
 * Append a block of data to records data structure.
 *
 * NB: each block is padded to a MAXALIGN multiple.  This must be
 * accounted for when the file is later read!
 *
 * The data is copied, so the caller is free to modify it afterwards.
 */
static void
save_state_data(const void *data, uint32 len)
{
        uint32          padlen = MAXALIGN(len);

        if (padlen > records.bytes_free)
        {
                records.tail->next = palloc0(sizeof(StateFileChunk));
                records.tail = records.tail->next;
                records.tail->len = 0;
                records.tail->next = NULL;
                records.num_chunks++;

                records.bytes_free = Max(padlen, 512);
                records.tail->data = palloc(records.bytes_free);
        }

        memcpy(((char *) records.tail->data) + records.tail->len, data, len);
        records.tail->len += padlen;
        records.bytes_free -= padlen;
        records.total_len += padlen;
}

/*
 * Start preparing a state file.
 *
 * Initializes data structure and inserts the 2PC file header record.
 */
void
StartPrepare(GlobalTransaction gxact)
{
        PGPROC     *proc = &ProcGlobal->allProcs[gxact->pgprocno];
        TransactionId xid = gxact->xid;
        TwoPhaseFileHeader hdr;
        TransactionId *children;
        RelFileLocator *commitrels;
        RelFileLocator *abortrels;
        xl_xact_stats_item *abortstats = NULL;
        xl_xact_stats_item *commitstats = NULL;
        SharedInvalidationMessage *invalmsgs;

        /* Initialize linked list */
        records.head = palloc0(sizeof(StateFileChunk));
        records.head->len = 0;
        records.head->next = NULL;

        records.bytes_free = Max(sizeof(TwoPhaseFileHeader), 512);
        records.head->data = palloc(records.bytes_free);

        records.tail = records.head;
        records.num_chunks = 1;

        records.total_len = 0;

        /* Create header */
        hdr.magic = TWOPHASE_MAGIC;
        hdr.total_len = 0;                      /* EndPrepare will fill this in */
        hdr.xid = xid;
        hdr.database = proc->databaseId;
        hdr.prepared_at = gxact->prepared_at;
        hdr.owner = gxact->owner;
        hdr.nsubxacts = xactGetCommittedChildren(&children);
        hdr.ncommitrels = smgrGetPendingDeletes(true, &commitrels);
        hdr.nabortrels = smgrGetPendingDeletes(false, &abortrels);
        hdr.ncommitstats =
                pgstat_get_transactional_drops(true, &commitstats);
        hdr.nabortstats =
                pgstat_get_transactional_drops(false, &abortstats);
        hdr.ninvalmsgs = xactGetCommittedInvalidationMessages(&invalmsgs,
                                                                                                                  &hdr.initfileinval);
        hdr.gidlen = strlen(gxact->gid) + 1;    /* Include '\0' */
        /* EndPrepare will fill the origin data, if necessary */
        hdr.origin_lsn = InvalidXLogRecPtr;
        hdr.origin_timestamp = 0;

        save_state_data(&hdr, sizeof(TwoPhaseFileHeader));
        save_state_data(gxact->gid, hdr.gidlen);

        /*
         * Add the additional info about subxacts, deletable files and cache
         * invalidation messages.
         */
        if (hdr.nsubxacts > 0)
        {
                save_state_data(children, hdr.nsubxacts * sizeof(TransactionId));
                /* While we have the child-xact data, stuff it in the gxact too */
                GXactLoadSubxactData(gxact, hdr.nsubxacts, children);
        }
        if (hdr.ncommitrels > 0)
        {
                save_state_data(commitrels, hdr.ncommitrels * sizeof(RelFileLocator));
                pfree(commitrels);
        }
        if (hdr.nabortrels > 0)
        {
                save_state_data(abortrels, hdr.nabortrels * sizeof(RelFileLocator));
                pfree(abortrels);
        }
        if (hdr.ncommitstats > 0)
        {
                save_state_data(commitstats,
                                                hdr.ncommitstats * sizeof(xl_xact_stats_item));
                pfree(commitstats);
        }
        if (hdr.nabortstats > 0)
        {
                save_state_data(abortstats,
                                                hdr.nabortstats * sizeof(xl_xact_stats_item));
                pfree(abortstats);
        }
        if (hdr.ninvalmsgs > 0)
        {
                save_state_data(invalmsgs,
                                                hdr.ninvalmsgs * sizeof(SharedInvalidationMessage));
                pfree(invalmsgs);
        }
}

/*
 * Finish preparing state data and writing it to WAL.
 */
void
EndPrepare(GlobalTransaction gxact)
{
        TwoPhaseFileHeader *hdr;
        StateFileChunk *record;
        bool            replorigin;

        /* Add the end sentinel to the list of 2PC records */
        RegisterTwoPhaseRecord(TWOPHASE_RM_END_ID, 0,
                                                   NULL, 0);

        /* Go back and fill in total_len in the file header record */
        hdr = (TwoPhaseFileHeader *) records.head->data;
        Assert(hdr->magic == TWOPHASE_MAGIC);
        hdr->total_len = records.total_len + sizeof(pg_crc32c);

        replorigin = (replorigin_session_origin != InvalidRepOriginId &&
                                  replorigin_session_origin != DoNotReplicateId);

        if (replorigin)
        {
                hdr->origin_lsn = replorigin_session_origin_lsn;
                hdr->origin_timestamp = replorigin_session_origin_timestamp;
        }

        /*
         * If the data size exceeds MaxAllocSize, we won't be able to read it in
         * ReadTwoPhaseFile. Check for that now, rather than fail in the case
         * where we write data to file and then re-read at commit time.
         */
        if (hdr->total_len > MaxAllocSize)
                ereport(ERROR,
                                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                                 errmsg("two-phase state file maximum length exceeded")));

        /*
         * Now writing 2PC state data to WAL. We let the WAL's CRC protection
         * cover us, so no need to calculate a separate CRC.
         *
         * We have to set DELAY_CHKPT_START here, too; otherwise a checkpoint
         * starting immediately after the WAL record is inserted could complete
         * without fsync'ing our state file.  (This is essentially the same kind
         * of race condition as the COMMIT-to-clog-write case that
         * RecordTransactionCommit uses DELAY_CHKPT_START for; see notes there.)
         *
         * We save the PREPARE record's location in the gxact for later use by
         * CheckPointTwoPhase.
         */
        XLogEnsureRecordSpace(0, records.num_chunks);

        START_CRIT_SECTION();

        Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
        MyProc->delayChkptFlags |= DELAY_CHKPT_START;

        XLogBeginInsert();
        for (record = records.head; record != NULL; record = record->next)
                XLogRegisterData(record->data, record->len);

        XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);

        gxact->prepare_end_lsn = XLogInsert(RM_XACT_ID, XLOG_XACT_PREPARE);

        if (replorigin)
        {
                /* Move LSNs forward for this replication origin */
                replorigin_session_advance(replorigin_session_origin_lsn,
                                                                   gxact->prepare_end_lsn);
        }

        XLogFlush(gxact->prepare_end_lsn);

        /* If we crash now, we have prepared: WAL replay will fix things */

        /* Store record's start location to read that later on Commit */
        gxact->prepare_start_lsn = ProcLastRecPtr;

        /*
         * Mark the prepared transaction as valid.  As soon as xact.c marks MyProc
         * as not running our XID (which it will do immediately after this
         * function returns), others can commit/rollback the xact.
         *
         * NB: a side effect of this is to make a dummy ProcArray entry for the
         * prepared XID.  This must happen before we clear the XID from MyProc /
         * ProcGlobal->xids[], else there is a window where the XID is not running
         * according to TransactionIdIsInProgress, and onlookers would be entitled
         * to assume the xact crashed.  Instead we have a window where the same
         * XID appears twice in ProcArray, which is OK.
         */
        MarkAsPrepared(gxact, false);

        /*
         * Now we can mark ourselves as out of the commit critical section: a
         * checkpoint starting after this will certainly see the gxact as a
         * candidate for fsyncing.
         */
        MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;

        /*
         * Remember that we have this GlobalTransaction entry locked for us.  If
         * we crash after this point, it's too late to abort, but we must unlock
         * it so that the prepared transaction can be committed or rolled back.
         */
        MyLockedGxact = gxact;

        END_CRIT_SECTION();

        /*
         * Wait for synchronous replication, if required.
         *
         * Note that at this stage we have marked the prepare, but still show as
         * running in the procarray (twice!) and continue to hold locks.
         */
        SyncRepWaitForLSN(gxact->prepare_end_lsn, false);

        records.tail = records.head = NULL;
        records.num_chunks = 0;
}

/*
 * Register a 2PC record to be written to state file.
 */
void
RegisterTwoPhaseRecord(TwoPhaseRmgrId rmid, uint16 info,
                                           const void *data, uint32 len)
{
        TwoPhaseRecordOnDisk record;

        record.rmid = rmid;
        record.info = info;
        record.len = len;
        save_state_data(&record, sizeof(TwoPhaseRecordOnDisk));
        if (len > 0)
                save_state_data(data, len);
}


/*
 * Read and validate the state file for xid.
 *
 * If it looks OK (has a valid magic number and CRC), return the palloc'd
 * contents of the file, issuing an error when finding corrupted data.  If
 * missing_ok is true, which indicates that missing files can be safely
 * ignored, then return NULL.  This state can be reached when doing recovery.
 */
static char *
ReadTwoPhaseFile(TransactionId xid, bool missing_ok)
{
        char            path[MAXPGPATH];
        char       *buf;
        TwoPhaseFileHeader *hdr;
        int                     fd;
        struct stat stat;
        uint32          crc_offset;
        pg_crc32c       calc_crc,
                                file_crc;
        int                     r;

        TwoPhaseFilePath(path, xid);

        fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
        if (fd < 0)
        {
                if (missing_ok && errno == ENOENT)
                        return NULL;

                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));
        }

        /*
         * Check file length.  We can determine a lower bound pretty easily. We
         * set an upper bound to avoid palloc() failure on a corrupt file, though
         * we can't guarantee that we won't get an out of memory error anyway,
         * even on a valid file.
         */
        if (fstat(fd, &stat))
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not stat file \"%s\": %m", path)));

        if (stat.st_size < (MAXALIGN(sizeof(TwoPhaseFileHeader)) +
                                                MAXALIGN(sizeof(TwoPhaseRecordOnDisk)) +
                                                sizeof(pg_crc32c)) ||
                stat.st_size > MaxAllocSize)
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg_plural("incorrect size of file \"%s\": %lld byte",
                                                           "incorrect size of file \"%s\": %lld bytes",
                                                           (long long int) stat.st_size, path,
                                                           (long long int) stat.st_size)));

        crc_offset = stat.st_size - sizeof(pg_crc32c);
        if (crc_offset != MAXALIGN(crc_offset))
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("incorrect alignment of CRC offset for file \"%s\"",
                                                path)));

        /*
         * OK, slurp in the file.
         */
        buf = (char *) palloc(stat.st_size);

        pgstat_report_wait_start(WAIT_EVENT_TWOPHASE_FILE_READ);
        r = read(fd, buf, stat.st_size);
        if (r != stat.st_size)
        {
                if (r < 0)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m", path)));
                else
                        ereport(ERROR,
                                        (errmsg("could not read file \"%s\": read %d of %lld",
                                                        path, r, (long long int) stat.st_size)));
        }

        pgstat_report_wait_end();

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

        hdr = (TwoPhaseFileHeader *) buf;
        if (hdr->magic != TWOPHASE_MAGIC)
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("invalid magic number stored in file \"%s\"",
                                                path)));

        if (hdr->total_len != stat.st_size)
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("invalid size stored in file \"%s\"",
                                                path)));

        INIT_CRC32C(calc_crc);
        COMP_CRC32C(calc_crc, buf, crc_offset);
        FIN_CRC32C(calc_crc);

        file_crc = *((pg_crc32c *) (buf + crc_offset));

        if (!EQ_CRC32C(calc_crc, file_crc))
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("calculated CRC checksum does not match value stored in file \"%s\"",
                                                path)));

        return buf;
}


/*
 * Reads 2PC data from xlog. During checkpoint this data will be moved to
 * twophase files and ReadTwoPhaseFile should be used instead.
 *
 * Note clearly that this function can access WAL during normal operation,
 * similarly to the way WALSender or Logical Decoding would do.
 */
static void
XlogReadTwoPhaseData(XLogRecPtr lsn, char **buf, int *len)
{
        XLogRecord *record;
        XLogReaderState *xlogreader;
        char       *errormsg;

        xlogreader = XLogReaderAllocate(wal_segment_size, NULL,
                                                                        XL_ROUTINE(.page_read = &read_local_xlog_page,
                                                                                           .segment_open = &wal_segment_open,
                                                                                           .segment_close = &wal_segment_close),
                                                                        NULL);
        if (!xlogreader)
                ereport(ERROR,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("out of memory"),
                                 errdetail("Failed while allocating a WAL reading processor.")));

        XLogBeginRead(xlogreader, lsn);
        record = XLogReadRecord(xlogreader, &errormsg);

        if (record == NULL)
        {
                if (errormsg)
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not read two-phase state from WAL at %X/%X: %s",
                                                        LSN_FORMAT_ARGS(lsn), errormsg)));
                else
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not read two-phase state from WAL at %X/%X",
                                                        LSN_FORMAT_ARGS(lsn))));
        }

        if (XLogRecGetRmid(xlogreader) != RM_XACT_ID ||
                (XLogRecGetInfo(xlogreader) & XLOG_XACT_OPMASK) != XLOG_XACT_PREPARE)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("expected two-phase state data is not present in WAL at %X/%X",
                                                LSN_FORMAT_ARGS(lsn))));

        if (len != NULL)
                *len = XLogRecGetDataLen(xlogreader);

        *buf = palloc(sizeof(char) * XLogRecGetDataLen(xlogreader));
        memcpy(*buf, XLogRecGetData(xlogreader), sizeof(char) * XLogRecGetDataLen(xlogreader));

        XLogReaderFree(xlogreader);
}


/*
 * Confirms an xid is prepared, during recovery
 */
bool
StandbyTransactionIdIsPrepared(TransactionId xid)
{
        char       *buf;
        TwoPhaseFileHeader *hdr;
        bool            result;

        Assert(TransactionIdIsValid(xid));

        if (max_prepared_xacts <= 0)
                return false;                   /* nothing to do */

        /* Read and validate file */
        buf = ReadTwoPhaseFile(xid, true);
        if (buf == NULL)
                return false;

        /* Check header also */
        hdr = (TwoPhaseFileHeader *) buf;
        result = TransactionIdEquals(hdr->xid, xid);
        pfree(buf);

        return result;
}

/*
 * FinishPreparedTransaction: execute COMMIT PREPARED or ROLLBACK PREPARED
 */
void
FinishPreparedTransaction(const char *gid, bool isCommit)
{
        GlobalTransaction gxact;
        PGPROC     *proc;
        TransactionId xid;
        char       *buf;
        char       *bufptr;
        TwoPhaseFileHeader *hdr;
        TransactionId latestXid;
        TransactionId *children;
        RelFileLocator *commitrels;
        RelFileLocator *abortrels;
        RelFileLocator *delrels;
        int                     ndelrels;
        xl_xact_stats_item *commitstats;
        xl_xact_stats_item *abortstats;
        SharedInvalidationMessage *invalmsgs;

        /*
         * Validate the GID, and lock the GXACT to ensure that two backends do not
         * try to commit the same GID at once.
         */
        gxact = LockGXact(gid, GetUserId());
        proc = &ProcGlobal->allProcs[gxact->pgprocno];
        xid = gxact->xid;

        /*
         * Read and validate 2PC state data. State data will typically be stored
         * in WAL files if the LSN is after the last checkpoint record, or moved
         * to disk if for some reason they have lived for a long time.
         */
        if (gxact->ondisk)
                buf = ReadTwoPhaseFile(xid, false);
        else
                XlogReadTwoPhaseData(gxact->prepare_start_lsn, &buf, NULL);


        /*
         * Disassemble the header area
         */
        hdr = (TwoPhaseFileHeader *) buf;
        Assert(TransactionIdEquals(hdr->xid, xid));
        bufptr = buf + MAXALIGN(sizeof(TwoPhaseFileHeader));
        bufptr += MAXALIGN(hdr->gidlen);
        children = (TransactionId *) bufptr;
        bufptr += MAXALIGN(hdr->nsubxacts * sizeof(TransactionId));
        commitrels = (RelFileLocator *) bufptr;
        bufptr += MAXALIGN(hdr->ncommitrels * sizeof(RelFileLocator));
        abortrels = (RelFileLocator *) bufptr;
        bufptr += MAXALIGN(hdr->nabortrels * sizeof(RelFileLocator));
        commitstats = (xl_xact_stats_item *) bufptr;
        bufptr += MAXALIGN(hdr->ncommitstats * sizeof(xl_xact_stats_item));
        abortstats = (xl_xact_stats_item *) bufptr;
        bufptr += MAXALIGN(hdr->nabortstats * sizeof(xl_xact_stats_item));
        invalmsgs = (SharedInvalidationMessage *) bufptr;
        bufptr += MAXALIGN(hdr->ninvalmsgs * sizeof(SharedInvalidationMessage));

        /* compute latestXid among all children */
        latestXid = TransactionIdLatest(xid, hdr->nsubxacts, children);

        /* Prevent cancel/die interrupt while cleaning up */
        HOLD_INTERRUPTS();

        /*
         * The order of operations here is critical: make the XLOG entry for
         * commit or abort, then mark the transaction committed or aborted in
         * pg_xact, then remove its PGPROC from the global ProcArray (which means
         * TransactionIdIsInProgress will stop saying the prepared xact is in
         * progress), then run the post-commit or post-abort callbacks. The
         * callbacks will release the locks the transaction held.
         */
        if (isCommit)
                RecordTransactionCommitPrepared(xid,
                                                                                hdr->nsubxacts, children,
                                                                                hdr->ncommitrels, commitrels,
                                                                                hdr->ncommitstats,
                                                                                commitstats,
                                                                                hdr->ninvalmsgs, invalmsgs,
                                                                                hdr->initfileinval, gid);
        else
                RecordTransactionAbortPrepared(xid,
                                                                           hdr->nsubxacts, children,
                                                                           hdr->nabortrels, abortrels,
                                                                           hdr->nabortstats,
                                                                           abortstats,
                                                                           gid);

        ProcArrayRemove(proc, latestXid);

        /*
         * In case we fail while running the callbacks, mark the gxact invalid so
         * no one else will try to commit/rollback, and so it will be recycled if
         * we fail after this point.  It is still locked by our backend so it
         * won't go away yet.
         *
         * (We assume it's safe to do this without taking TwoPhaseStateLock.)
         */
        gxact->valid = false;

        /*
         * We have to remove any files that were supposed to be dropped. For
         * consistency with the regular xact.c code paths, must do this before
         * releasing locks, so do it before running the callbacks.
         *
         * NB: this code knows that we couldn't be dropping any temp rels ...
         */
        if (isCommit)
        {
                delrels = commitrels;
                ndelrels = hdr->ncommitrels;
        }
        else
        {
                delrels = abortrels;
                ndelrels = hdr->nabortrels;
        }

        /* Make sure files supposed to be dropped are dropped */
        DropRelationFiles(delrels, ndelrels, false);

        if (isCommit)
                pgstat_execute_transactional_drops(hdr->ncommitstats, commitstats, false);
        else
                pgstat_execute_transactional_drops(hdr->nabortstats, abortstats, false);

        /*
         * Handle cache invalidation messages.
         *
         * Relcache init file invalidation requires processing both before and
         * after we send the SI messages, only when committing.  See
         * AtEOXact_Inval().
         */
        if (isCommit)
        {
                if (hdr->initfileinval)
                        RelationCacheInitFilePreInvalidate();
                SendSharedInvalidMessages(invalmsgs, hdr->ninvalmsgs);
                if (hdr->initfileinval)
                        RelationCacheInitFilePostInvalidate();
        }

        /*
         * Acquire the two-phase lock.  We want to work on the two-phase callbacks
         * while holding it to avoid potential conflicts with other transactions
         * attempting to use the same GID, so the lock is released once the shared
         * memory state is cleared.
         */
        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

        /* And now do the callbacks */
        if (isCommit)
                ProcessRecords(bufptr, xid, twophase_postcommit_callbacks);
        else
                ProcessRecords(bufptr, xid, twophase_postabort_callbacks);

        PredicateLockTwoPhaseFinish(xid, isCommit);

        /* Clear shared memory state */
        RemoveGXact(gxact);

        /*
         * Release the lock as all callbacks are called and shared memory cleanup
         * is done.
         */
        LWLockRelease(TwoPhaseStateLock);

        /* Count the prepared xact as committed or aborted */
        AtEOXact_PgStat(isCommit, false);

        /*
         * And now we can clean up any files we may have left.
         */
        if (gxact->ondisk)
                RemoveTwoPhaseFile(xid, true);

        MyLockedGxact = NULL;

        RESUME_INTERRUPTS();

        pfree(buf);
}

/*
 * Scan 2PC state data in memory and call the indicated callbacks for each 2PC record.
 */
static void
ProcessRecords(char *bufptr, TransactionId xid,
                           const TwoPhaseCallback callbacks[])
{
        for (;;)
        {
                TwoPhaseRecordOnDisk *record = (TwoPhaseRecordOnDisk *) bufptr;

                Assert(record->rmid <= TWOPHASE_RM_MAX_ID);
                if (record->rmid == TWOPHASE_RM_END_ID)
                        break;

                bufptr += MAXALIGN(sizeof(TwoPhaseRecordOnDisk));

                if (callbacks[record->rmid] != NULL)
                        callbacks[record->rmid] (xid, record->info,
                                                                         (void *) bufptr, record->len);

                bufptr += MAXALIGN(record->len);
        }
}

/*
 * Remove the 2PC file for the specified XID.
 *
 * If giveWarning is false, do not complain about file-not-present;
 * this is an expected case during WAL replay.
 */
static void
RemoveTwoPhaseFile(TransactionId xid, bool giveWarning)
{
        char            path[MAXPGPATH];

        TwoPhaseFilePath(path, xid);
        if (unlink(path))
                if (errno != ENOENT || giveWarning)
                        ereport(WARNING,
                                        (errcode_for_file_access(),
                                         errmsg("could not remove file \"%s\": %m", path)));
}

/*
 * Recreates a state file. This is used in WAL replay and during
 * checkpoint creation.
 *
 * Note: content and len don't include CRC.
 */
static void
RecreateTwoPhaseFile(TransactionId xid, void *content, int len)
{
        char            path[MAXPGPATH];
        pg_crc32c       statefile_crc;
        int                     fd;

        /* Recompute CRC */
        INIT_CRC32C(statefile_crc);
        COMP_CRC32C(statefile_crc, content, len);
        FIN_CRC32C(statefile_crc);

        TwoPhaseFilePath(path, xid);

        fd = OpenTransientFile(path,
                                                   O_CREAT | O_TRUNC | O_WRONLY | PG_BINARY);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not recreate file \"%s\": %m", path)));

        /* Write content and CRC */
        errno = 0;
        pgstat_report_wait_start(WAIT_EVENT_TWOPHASE_FILE_WRITE);
        if (write(fd, content, len) != len)
        {
                /* if write didn't set errno, assume problem is no disk space */
                if (errno == 0)
                        errno = ENOSPC;
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write file \"%s\": %m", path)));
        }
        if (write(fd, &statefile_crc, sizeof(pg_crc32c)) != sizeof(pg_crc32c))
        {
                /* if write didn't set errno, assume problem is no disk space */
                if (errno == 0)
                        errno = ENOSPC;
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write file \"%s\": %m", path)));
        }
        pgstat_report_wait_end();

        /*
         * We must fsync the file because the end-of-replay checkpoint will not do
         * so, there being no GXACT in shared memory yet to tell it to.
         */
        pgstat_report_wait_start(WAIT_EVENT_TWOPHASE_FILE_SYNC);
        if (pg_fsync(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m", path)));
        pgstat_report_wait_end();

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

/*
 * CheckPointTwoPhase -- handle 2PC component of checkpointing.
 *
 * We must fsync the state file of any GXACT that is valid or has been
 * generated during redo and has a PREPARE LSN <= the checkpoint's redo
 * horizon.  (If the gxact isn't valid yet, has not been generated in
 * redo, or has a later LSN, this checkpoint is not responsible for
 * fsyncing it.)
 *
 * This is deliberately run as late as possible in the checkpoint sequence,
 * because GXACTs ordinarily have short lifespans, and so it is quite
 * possible that GXACTs that were valid at checkpoint start will no longer
 * exist if we wait a little bit. With typical checkpoint settings this
 * will be about 3 minutes for an online checkpoint, so as a result we
 * expect that there will be no GXACTs that need to be copied to disk.
 *
 * If a GXACT remains valid across multiple checkpoints, it will already
 * be on disk so we don't bother to repeat that write.
 */
void
CheckPointTwoPhase(XLogRecPtr redo_horizon)
{
        int                     i;
        int                     serialized_xacts = 0;

        if (max_prepared_xacts <= 0)
                return;                                 /* nothing to do */

        TRACE_POSTGRESQL_TWOPHASE_CHECKPOINT_START();

        /*
         * We are expecting there to be zero GXACTs that need to be copied to
         * disk, so we perform all I/O while holding TwoPhaseStateLock for
         * simplicity. This prevents any new xacts from preparing while this
         * occurs, which shouldn't be a problem since the presence of long-lived
         * prepared xacts indicates the transaction manager isn't active.
         *
         * It's also possible to move I/O out of the lock, but on every error we
         * should check whether somebody committed our transaction in different
         * backend. Let's leave this optimization for future, if somebody will
         * spot that this place cause bottleneck.
         *
         * Note that it isn't possible for there to be a GXACT with a
         * prepare_end_lsn set prior to the last checkpoint yet is marked invalid,
         * because of the efforts with delayChkptFlags.
         */
        LWLockAcquire(TwoPhaseStateLock, LW_SHARED);
        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                /*
                 * Note that we are using gxact not PGPROC so this works in recovery
                 * also
                 */
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];

                if ((gxact->valid || gxact->inredo) &&
                        !gxact->ondisk &&
                        gxact->prepare_end_lsn <= redo_horizon)
                {
                        char       *buf;
                        int                     len;

                        XlogReadTwoPhaseData(gxact->prepare_start_lsn, &buf, &len);
                        RecreateTwoPhaseFile(gxact->xid, buf, len);
                        gxact->ondisk = true;
                        gxact->prepare_start_lsn = InvalidXLogRecPtr;
                        gxact->prepare_end_lsn = InvalidXLogRecPtr;
                        pfree(buf);
                        serialized_xacts++;
                }
        }
        LWLockRelease(TwoPhaseStateLock);

        /*
         * Flush unconditionally the parent directory to make any information
         * durable on disk.  Two-phase files could have been removed and those
         * removals need to be made persistent as well as any files newly created
         * previously since the last checkpoint.
         */
        fsync_fname(TWOPHASE_DIR, true);

        TRACE_POSTGRESQL_TWOPHASE_CHECKPOINT_DONE();

        if (log_checkpoints && serialized_xacts > 0)
                ereport(LOG,
                                (errmsg_plural("%u two-phase state file was written "
                                                           "for a long-running prepared transaction",
                                                           "%u two-phase state files were written "
                                                           "for long-running prepared transactions",
                                                           serialized_xacts,
                                                           serialized_xacts)));
}

/*
 * restoreTwoPhaseData
 *
 * Scan pg_twophase and fill TwoPhaseState depending on the on-disk data.
 * This is called once at the beginning of recovery, saving any extra
 * lookups in the future.  Two-phase files that are newer than the
 * minimum XID horizon are discarded on the way.
 */
void
restoreTwoPhaseData(void)
{
        DIR                *cldir;
        struct dirent *clde;

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        cldir = AllocateDir(TWOPHASE_DIR);
        while ((clde = ReadDir(cldir, TWOPHASE_DIR)) != NULL)
        {
                if (strlen(clde->d_name) == 8 &&
                        strspn(clde->d_name, "0123456789ABCDEF") == 8)
                {
                        TransactionId xid;
                        char       *buf;

                        xid = (TransactionId) strtoul(clde->d_name, NULL, 16);

                        buf = ProcessTwoPhaseBuffer(xid, InvalidXLogRecPtr,
                                                                                true, false, false);
                        if (buf == NULL)
                                continue;

                        PrepareRedoAdd(buf, InvalidXLogRecPtr,
                                                   InvalidXLogRecPtr, InvalidRepOriginId);
                }
        }
        LWLockRelease(TwoPhaseStateLock);
        FreeDir(cldir);
}

/*
 * PrescanPreparedTransactions
 *
 * Scan the shared memory entries of TwoPhaseState and determine the range
 * of valid XIDs present.  This is run during database startup, after we
 * have completed reading WAL.  ShmemVariableCache->nextXid has been set to
 * one more than the highest XID for which evidence exists in WAL.
 *
 * We throw away any prepared xacts with main XID beyond nextXid --- if any
 * are present, it suggests that the DBA has done a PITR recovery to an
 * earlier point in time without cleaning out pg_twophase.  We dare not
 * try to recover such prepared xacts since they likely depend on database
 * state that doesn't exist now.
 *
 * However, we will advance nextXid beyond any subxact XIDs belonging to
 * valid prepared xacts.  We need to do this since subxact commit doesn't
 * write a WAL entry, and so there might be no evidence in WAL of those
 * subxact XIDs.
 *
 * On corrupted two-phase files, fail immediately.  Keeping around broken
 * entries and let replay continue causes harm on the system, and a new
 * backup should be rolled in.
 *
 * Our other responsibility is to determine and return the oldest valid XID
 * among the prepared xacts (if none, return ShmemVariableCache->nextXid).
 * This is needed to synchronize pg_subtrans startup properly.
 *
 * If xids_p and nxids_p are not NULL, pointer to a palloc'd array of all
 * top-level xids is stored in *xids_p. The number of entries in the array
 * is returned in *nxids_p.
 */
TransactionId
PrescanPreparedTransactions(TransactionId **xids_p, int *nxids_p)
{
        FullTransactionId nextXid = ShmemVariableCache->nextXid;
        TransactionId origNextXid = XidFromFullTransactionId(nextXid);
        TransactionId result = origNextXid;
        TransactionId *xids = NULL;
        int                     nxids = 0;
        int                     allocsize = 0;
        int                     i;

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                TransactionId xid;
                char       *buf;
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];

                Assert(gxact->inredo);

                xid = gxact->xid;

                buf = ProcessTwoPhaseBuffer(xid,
                                                                        gxact->prepare_start_lsn,
                                                                        gxact->ondisk, false, true);

                if (buf == NULL)
                        continue;

                /*
                 * OK, we think this file is valid.  Incorporate xid into the
                 * running-minimum result.
                 */
                if (TransactionIdPrecedes(xid, result))
                        result = xid;

                if (xids_p)
                {
                        if (nxids == allocsize)
                        {
                                if (nxids == 0)
                                {
                                        allocsize = 10;
                                        xids = palloc(allocsize * sizeof(TransactionId));
                                }
                                else
                                {
                                        allocsize = allocsize * 2;
                                        xids = repalloc(xids, allocsize * sizeof(TransactionId));
                                }
                        }
                        xids[nxids++] = xid;
                }

                pfree(buf);
        }
        LWLockRelease(TwoPhaseStateLock);

        if (xids_p)
        {
                *xids_p = xids;
                *nxids_p = nxids;
        }

        return result;
}

/*
 * StandbyRecoverPreparedTransactions
 *
 * Scan the shared memory entries of TwoPhaseState and setup all the required
 * information to allow standby queries to treat prepared transactions as still
 * active.
 *
 * This is never called at the end of recovery - we use
 * RecoverPreparedTransactions() at that point.
 *
 * The lack of calls to SubTransSetParent() calls here is by design;
 * those calls are made by RecoverPreparedTransactions() at the end of recovery
 * for those xacts that need this.
 */
void
StandbyRecoverPreparedTransactions(void)
{
        int                     i;

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                TransactionId xid;
                char       *buf;
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];

                Assert(gxact->inredo);

                xid = gxact->xid;

                buf = ProcessTwoPhaseBuffer(xid,
                                                                        gxact->prepare_start_lsn,
                                                                        gxact->ondisk, false, false);
                if (buf != NULL)
                        pfree(buf);
        }
        LWLockRelease(TwoPhaseStateLock);
}

/*
 * RecoverPreparedTransactions
 *
 * Scan the shared memory entries of TwoPhaseState and reload the state for
 * each prepared transaction (reacquire locks, etc).
 *
 * This is run at the end of recovery, but before we allow backends to write
 * WAL.
 *
 * At the end of recovery the way we take snapshots will change. We now need
 * to mark all running transactions with their full SubTransSetParent() info
 * to allow normal snapshots to work correctly if snapshots overflow.
 * We do this here because by definition prepared transactions are the only
 * type of write transaction still running, so this is necessary and
 * complete.
 */
void
RecoverPreparedTransactions(void)
{
        int                     i;

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                TransactionId xid;
                char       *buf;
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];
                char       *bufptr;
                TwoPhaseFileHeader *hdr;
                TransactionId *subxids;
                const char *gid;

                xid = gxact->xid;

                /*
                 * Reconstruct subtrans state for the transaction --- needed because
                 * pg_subtrans is not preserved over a restart.  Note that we are
                 * linking all the subtransactions directly to the top-level XID;
                 * there may originally have been a more complex hierarchy, but
                 * there's no need to restore that exactly. It's possible that
                 * SubTransSetParent has been set before, if the prepared transaction
                 * generated xid assignment records.
                 */
                buf = ProcessTwoPhaseBuffer(xid,
                                                                        gxact->prepare_start_lsn,
                                                                        gxact->ondisk, true, false);
                if (buf == NULL)
                        continue;

                ereport(LOG,
                                (errmsg("recovering prepared transaction %u from shared memory", xid)));

                hdr = (TwoPhaseFileHeader *) buf;
                Assert(TransactionIdEquals(hdr->xid, xid));
                bufptr = buf + MAXALIGN(sizeof(TwoPhaseFileHeader));
                gid = (const char *) bufptr;
                bufptr += MAXALIGN(hdr->gidlen);
                subxids = (TransactionId *) bufptr;
                bufptr += MAXALIGN(hdr->nsubxacts * sizeof(TransactionId));
                bufptr += MAXALIGN(hdr->ncommitrels * sizeof(RelFileLocator));
                bufptr += MAXALIGN(hdr->nabortrels * sizeof(RelFileLocator));
                bufptr += MAXALIGN(hdr->ncommitstats * sizeof(xl_xact_stats_item));
                bufptr += MAXALIGN(hdr->nabortstats * sizeof(xl_xact_stats_item));
                bufptr += MAXALIGN(hdr->ninvalmsgs * sizeof(SharedInvalidationMessage));

                /*
                 * Recreate its GXACT and dummy PGPROC. But, check whether it was
                 * added in redo and already has a shmem entry for it.
                 */
                MarkAsPreparingGuts(gxact, xid, gid,
                                                        hdr->prepared_at,
                                                        hdr->owner, hdr->database);

                /* recovered, so reset the flag for entries generated by redo */
                gxact->inredo = false;

                GXactLoadSubxactData(gxact, hdr->nsubxacts, subxids);
                MarkAsPrepared(gxact, true);

                LWLockRelease(TwoPhaseStateLock);

                /*
                 * Recover other state (notably locks) using resource managers.
                 */
                ProcessRecords(bufptr, xid, twophase_recover_callbacks);

                /*
                 * Release locks held by the standby process after we process each
                 * prepared transaction. As a result, we don't need too many
                 * additional locks at any one time.
                 */
                if (InHotStandby)
                        StandbyReleaseLockTree(xid, hdr->nsubxacts, subxids);

                /*
                 * We're done with recovering this transaction. Clear MyLockedGxact,
                 * like we do in PrepareTransaction() during normal operation.
                 */
                PostPrepare_Twophase();

                pfree(buf);

                LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
        }

        LWLockRelease(TwoPhaseStateLock);
}

/*
 * ProcessTwoPhaseBuffer
 *
 * Given a transaction id, read it either from disk or read it directly
 * via shmem xlog record pointer using the provided "prepare_start_lsn".
 *
 * If setParent is true, set up subtransaction parent linkages.
 *
 * If setNextXid is true, set ShmemVariableCache->nextXid to the newest
 * value scanned.
 */
static char *
ProcessTwoPhaseBuffer(TransactionId xid,
                                          XLogRecPtr prepare_start_lsn,
                                          bool fromdisk,
                                          bool setParent, bool setNextXid)
{
        FullTransactionId nextXid = ShmemVariableCache->nextXid;
        TransactionId origNextXid = XidFromFullTransactionId(nextXid);
        TransactionId *subxids;
        char       *buf;
        TwoPhaseFileHeader *hdr;
        int                     i;

        Assert(LWLockHeldByMeInMode(TwoPhaseStateLock, LW_EXCLUSIVE));

        if (!fromdisk)
                Assert(prepare_start_lsn != InvalidXLogRecPtr);

        /* Already processed? */
        if (TransactionIdDidCommit(xid) || TransactionIdDidAbort(xid))
        {
                if (fromdisk)
                {
                        ereport(WARNING,
                                        (errmsg("removing stale two-phase state file for transaction %u",
                                                        xid)));
                        RemoveTwoPhaseFile(xid, true);
                }
                else
                {
                        ereport(WARNING,
                                        (errmsg("removing stale two-phase state from memory for transaction %u",
                                                        xid)));
                        PrepareRedoRemove(xid, true);
                }
                return NULL;
        }

        /* Reject XID if too new */
        if (TransactionIdFollowsOrEquals(xid, origNextXid))
        {
                if (fromdisk)
                {
                        ereport(WARNING,
                                        (errmsg("removing future two-phase state file for transaction %u",
                                                        xid)));
                        RemoveTwoPhaseFile(xid, true);
                }
                else
                {
                        ereport(WARNING,
                                        (errmsg("removing future two-phase state from memory for transaction %u",
                                                        xid)));
                        PrepareRedoRemove(xid, true);
                }
                return NULL;
        }

        if (fromdisk)
        {
                /* Read and validate file */
                buf = ReadTwoPhaseFile(xid, false);
        }
        else
        {
                /* Read xlog data */
                XlogReadTwoPhaseData(prepare_start_lsn, &buf, NULL);
        }

        /* Deconstruct header */
        hdr = (TwoPhaseFileHeader *) buf;
        if (!TransactionIdEquals(hdr->xid, xid))
        {
                if (fromdisk)
                        ereport(ERROR,
                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                         errmsg("corrupted two-phase state file for transaction %u",
                                                        xid)));
                else
                        ereport(ERROR,
                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                         errmsg("corrupted two-phase state in memory for transaction %u",
                                                        xid)));
        }

        /*
         * Examine subtransaction XIDs ... they should all follow main XID, and
         * they may force us to advance nextXid.
         */
        subxids = (TransactionId *) (buf +
                                                                 MAXALIGN(sizeof(TwoPhaseFileHeader)) +
                                                                 MAXALIGN(hdr->gidlen));
        for (i = 0; i < hdr->nsubxacts; i++)
        {
                TransactionId subxid = subxids[i];

                Assert(TransactionIdFollows(subxid, xid));

                /* update nextXid if needed */
                if (setNextXid)
                        AdvanceNextFullTransactionIdPastXid(subxid);

                if (setParent)
                        SubTransSetParent(subxid, xid);
        }

        return buf;
}


/*
 *      RecordTransactionCommitPrepared
 *
 * This is basically the same as RecordTransactionCommit (q.v. if you change
 * this function): in particular, we must set DELAY_CHKPT_START to avoid a
 * race condition.
 *
 * We know the transaction made at least one XLOG entry (its PREPARE),
 * so it is never possible to optimize out the commit record.
 */
static void
RecordTransactionCommitPrepared(TransactionId xid,
                                                                int nchildren,
                                                                TransactionId *children,
                                                                int nrels,
                                                                RelFileLocator *rels,
                                                                int nstats,
                                                                xl_xact_stats_item *stats,
                                                                int ninvalmsgs,
                                                                SharedInvalidationMessage *invalmsgs,
                                                                bool initfileinval,
                                                                const char *gid)
{
        XLogRecPtr      recptr;
        TimestampTz committs = GetCurrentTimestamp();
        bool            replorigin;

        /*
         * Are we using the replication origins feature?  Or, in other words, are
         * we replaying remote actions?
         */
        replorigin = (replorigin_session_origin != InvalidRepOriginId &&
                                  replorigin_session_origin != DoNotReplicateId);

        START_CRIT_SECTION();

        /* See notes in RecordTransactionCommit */
        Assert((MyProc->delayChkptFlags & DELAY_CHKPT_START) == 0);
        MyProc->delayChkptFlags |= DELAY_CHKPT_START;

        /*
         * Emit the XLOG commit record. Note that we mark 2PC commits as
         * potentially having AccessExclusiveLocks since we don't know whether or
         * not they do.
         */
        recptr = XactLogCommitRecord(committs,
                                                                 nchildren, children, nrels, rels,
                                                                 nstats, stats,
                                                                 ninvalmsgs, invalmsgs,
                                                                 initfileinval,
                                                                 MyXactFlags | XACT_FLAGS_ACQUIREDACCESSEXCLUSIVELOCK,
                                                                 xid, gid);


        if (replorigin)
                /* Move LSNs forward for this replication origin */
                replorigin_session_advance(replorigin_session_origin_lsn,
                                                                   XactLastRecEnd);

        /*
         * Record commit timestamp.  The value comes from plain commit timestamp
         * if replorigin is not enabled, or replorigin already set a value for us
         * in replorigin_session_origin_timestamp otherwise.
         *
         * We don't need to WAL-log anything here, as the commit record written
         * above already contains the data.
         */
        if (!replorigin || replorigin_session_origin_timestamp == 0)
                replorigin_session_origin_timestamp = committs;

        TransactionTreeSetCommitTsData(xid, nchildren, children,
                                                                   replorigin_session_origin_timestamp,
                                                                   replorigin_session_origin);

        /*
         * We don't currently try to sleep before flush here ... nor is there any
         * support for async commit of a prepared xact (the very idea is probably
         * a contradiction)
         */

        /* Flush XLOG to disk */
        XLogFlush(recptr);

        /* Mark the transaction committed in pg_xact */
        TransactionIdCommitTree(xid, nchildren, children);

        /* Checkpoint can proceed now */
        MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;

        END_CRIT_SECTION();

        /*
         * Wait for synchronous replication, if required.
         *
         * Note that at this stage we have marked clog, but still show as running
         * in the procarray and continue to hold locks.
         */
        SyncRepWaitForLSN(recptr, true);
}

/*
 *      RecordTransactionAbortPrepared
 *
 * This is basically the same as RecordTransactionAbort.
 *
 * We know the transaction made at least one XLOG entry (its PREPARE),
 * so it is never possible to optimize out the abort record.
 */
static void
RecordTransactionAbortPrepared(TransactionId xid,
                                                           int nchildren,
                                                           TransactionId *children,
                                                           int nrels,
                                                           RelFileLocator *rels,
                                                           int nstats,
                                                           xl_xact_stats_item *stats,
                                                           const char *gid)
{
        XLogRecPtr      recptr;
        bool            replorigin;

        /*
         * Are we using the replication origins feature?  Or, in other words, are
         * we replaying remote actions?
         */
        replorigin = (replorigin_session_origin != InvalidRepOriginId &&
                                  replorigin_session_origin != DoNotReplicateId);

        /*
         * Catch the scenario where we aborted partway through
         * RecordTransactionCommitPrepared ...
         */
        if (TransactionIdDidCommit(xid))
                elog(PANIC, "cannot abort transaction %u, it was already committed",
                         xid);

        START_CRIT_SECTION();

        /*
         * Emit the XLOG commit record. Note that we mark 2PC aborts as
         * potentially having AccessExclusiveLocks since we don't know whether or
         * not they do.
         */
        recptr = XactLogAbortRecord(GetCurrentTimestamp(),
                                                                nchildren, children,
                                                                nrels, rels,
                                                                nstats, stats,
                                                                MyXactFlags | XACT_FLAGS_ACQUIREDACCESSEXCLUSIVELOCK,
                                                                xid, gid);

        if (replorigin)
                /* Move LSNs forward for this replication origin */
                replorigin_session_advance(replorigin_session_origin_lsn,
                                                                   XactLastRecEnd);

        /* Always flush, since we're about to remove the 2PC state file */
        XLogFlush(recptr);

        /*
         * Mark the transaction aborted in clog.  This is not absolutely necessary
         * but we may as well do it while we are here.
         */
        TransactionIdAbortTree(xid, nchildren, children);

        END_CRIT_SECTION();

        /*
         * Wait for synchronous replication, if required.
         *
         * Note that at this stage we have marked clog, but still show as running
         * in the procarray and continue to hold locks.
         */
        SyncRepWaitForLSN(recptr, false);
}

/*
 * PrepareRedoAdd
 *
 * Store pointers to the start/end of the WAL record along with the xid in
 * a gxact entry in shared memory TwoPhaseState structure.  If caller
 * specifies InvalidXLogRecPtr as WAL location to fetch the two-phase
 * data, the entry is marked as located on disk.
 */
void
PrepareRedoAdd(char *buf, XLogRecPtr start_lsn,
                           XLogRecPtr end_lsn, RepOriginId origin_id)
{
        TwoPhaseFileHeader *hdr = (TwoPhaseFileHeader *) buf;
        char       *bufptr;
        const char *gid;
        GlobalTransaction gxact;

        Assert(LWLockHeldByMeInMode(TwoPhaseStateLock, LW_EXCLUSIVE));
        Assert(RecoveryInProgress());

        bufptr = buf + MAXALIGN(sizeof(TwoPhaseFileHeader));
        gid = (const char *) bufptr;

        /*
         * Reserve the GID for the given transaction in the redo code path.
         *
         * This creates a gxact struct and puts it into the active array.
         *
         * In redo, this struct is mainly used to track PREPARE/COMMIT entries in
         * shared memory. Hence, we only fill up the bare minimum contents here.
         * The gxact also gets marked with gxact->inredo set to true to indicate
         * that it got added in the redo phase
         */

        /* Get a free gxact from the freelist */
        if (TwoPhaseState->freeGXacts == NULL)
                ereport(ERROR,
                                (errcode(ERRCODE_OUT_OF_MEMORY),
                                 errmsg("maximum number of prepared transactions reached"),
                                 errhint("Increase max_prepared_transactions (currently %d).",
                                                 max_prepared_xacts)));
        gxact = TwoPhaseState->freeGXacts;
        TwoPhaseState->freeGXacts = gxact->next;

        gxact->prepared_at = hdr->prepared_at;
        gxact->prepare_start_lsn = start_lsn;
        gxact->prepare_end_lsn = end_lsn;
        gxact->xid = hdr->xid;
        gxact->owner = hdr->owner;
        gxact->locking_backend = InvalidBackendId;
        gxact->valid = false;
        gxact->ondisk = XLogRecPtrIsInvalid(start_lsn);
        gxact->inredo = true;           /* yes, added in redo */
        strcpy(gxact->gid, gid);

        /* And insert it into the active array */
        Assert(TwoPhaseState->numPrepXacts < max_prepared_xacts);
        TwoPhaseState->prepXacts[TwoPhaseState->numPrepXacts++] = gxact;

        if (origin_id != InvalidRepOriginId)
        {
                /* recover apply progress */
                replorigin_advance(origin_id, hdr->origin_lsn, end_lsn,
                                                   false /* backward */ , false /* WAL */ );
        }

        elog(DEBUG2, "added 2PC data in shared memory for transaction %u", gxact->xid);
}

/*
 * PrepareRedoRemove
 *
 * Remove the corresponding gxact entry from TwoPhaseState. Also remove
 * the 2PC file if a prepared transaction was saved via an earlier checkpoint.
 *
 * Caller must hold TwoPhaseStateLock in exclusive mode, because TwoPhaseState
 * is updated.
 */
void
PrepareRedoRemove(TransactionId xid, bool giveWarning)
{
        GlobalTransaction gxact = NULL;
        int                     i;
        bool            found = false;

        Assert(LWLockHeldByMeInMode(TwoPhaseStateLock, LW_EXCLUSIVE));
        Assert(RecoveryInProgress());

        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                gxact = TwoPhaseState->prepXacts[i];

                if (gxact->xid == xid)
                {
                        Assert(gxact->inredo);
                        found = true;
                        break;
                }
        }

        /*
         * Just leave if there is nothing, this is expected during WAL replay.
         */
        if (!found)
                return;

        /*
         * And now we can clean up any files we may have left.
         */
        elog(DEBUG2, "removing 2PC data for transaction %u", xid);
        if (gxact->ondisk)
                RemoveTwoPhaseFile(xid, giveWarning);
        RemoveGXact(gxact);
}

/*
 * LookupGXact
 *              Check if the prepared transaction with the given GID, lsn and timestamp
 *              exists.
 *
 * Note that we always compare with the LSN where prepare ends because that is
 * what is stored as origin_lsn in the 2PC file.
 *
 * This function is primarily used to check if the prepared transaction
 * received from the upstream (remote node) already exists. Checking only GID
 * is not sufficient because a different prepared xact with the same GID can
 * exist on the same node. So, we are ensuring to match origin_lsn and
 * origin_timestamp of prepared xact to avoid the possibility of a match of
 * prepared xact from two different nodes.
 */
bool
LookupGXact(const char *gid, XLogRecPtr prepare_end_lsn,
                        TimestampTz origin_prepare_timestamp)
{
        int                     i;
        bool            found = false;

        LWLockAcquire(TwoPhaseStateLock, LW_SHARED);
        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                GlobalTransaction gxact = TwoPhaseState->prepXacts[i];

                /* Ignore not-yet-valid GIDs. */
                if (gxact->valid && strcmp(gxact->gid, gid) == 0)
                {
                        char       *buf;
                        TwoPhaseFileHeader *hdr;

                        /*
                         * We are not expecting collisions of GXACTs (same gid) between
                         * publisher and subscribers, so we perform all I/O while holding
                         * TwoPhaseStateLock for simplicity.
                         *
                         * To move the I/O out of the lock, we need to ensure that no
                         * other backend commits the prepared xact in the meantime. We can
                         * do this optimization if we encounter many collisions in GID
                         * between publisher and subscriber.
                         */
                        if (gxact->ondisk)
                                buf = ReadTwoPhaseFile(gxact->xid, false);
                        else
                        {
                                Assert(gxact->prepare_start_lsn);
                                XlogReadTwoPhaseData(gxact->prepare_start_lsn, &buf, NULL);
                        }

                        hdr = (TwoPhaseFileHeader *) buf;

                        if (hdr->origin_lsn == prepare_end_lsn &&
                                hdr->origin_timestamp == origin_prepare_timestamp)
                        {
                                found = true;
                                pfree(buf);
                                break;
                        }

                        pfree(buf);
                }
        }
        LWLockRelease(TwoPhaseStateLock);
        return found;
}