twophase.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * 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/xlogrecovery.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
     */
 
    /*
     * In the event of a crash while a checkpoint was running, it may be
     * possible that some two-phase data found its way to disk while its
     * corresponding record needs to be replayed in the follow-up recovery. As
     * the 2PC data was on disk, it has already been restored at the beginning
     * of recovery with restoreTwoPhaseData(), so skip this record to avoid
     * duplicates in TwoPhaseState.  If a consistent state has been reached,
     * the record is added to TwoPhaseState and it should have no
     * corresponding file in pg_twophase.
     */
    if (!XLogRecPtrIsInvalid(start_lsn))
    {
        char        path[MAXPGPATH];
 
        TwoPhaseFilePath(path, hdr->xid);
 
        if (access(path, F_OK) == 0)
        {
            ereport(reachedConsistency ? ERROR : WARNING,
                    (errmsg("could not recover two-phase state file for transaction %u",
                            hdr->xid),
                     errdetail("Two-phase state file has been found in WAL record %X/%X, but this transaction has already been restored from disk.",
                               LSN_FORMAT_ARGS(start_lsn))));
            return;
        }
 
        if (errno != ENOENT)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not access file \"%s\": %m", path)));
    }
 
    /* 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;
}