/src/postgres/src/backend/access/transam/twophase.c

Source
/*-------------------------------------------------------------------------
 *
 * twophase.c
 *    Two-phase commit support functions.
 *
 * Portions Copyright (c) 1996-2025, 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 "storage/fd.h"
#include "storage/ipc.h"
#include "storage/md.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/builtins.h"
#include "utils/injection_point.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
 * proc number 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 */
  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 */
  FullTransactionId fxid;   /* The GXACT full xid */

  Oid     owner;      /* ID of user that executed the xact */
  ProcNumber  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 PrepareRedoRemoveFull(FullTransactionId fxid, bool giveWarning);
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, FullTransactionId fxid,
               const TwoPhaseCallback callbacks[]);
static void RemoveGXact(GlobalTransaction gxact);

static void XlogReadTwoPhaseData(XLogRecPtr lsn, char **buf, int *len);
static char *ProcessTwoPhaseBuffer(FullTransactionId fxid,
                   XLogRecPtr prepare_start_lsn,
                   bool fromdisk, bool setParent, bool setNextXid);
static void MarkAsPreparingGuts(GlobalTransaction gxact, FullTransactionId fxid,
                const char *gid, TimestampTz prepared_at, Oid owner,
                Oid databaseid);
static void RemoveTwoPhaseFile(FullTransactionId fxid, bool giveWarning);
static void RecreateTwoPhaseFile(FullTransactionId fxid, 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 = GetNumberFromPGProc(&PreparedXactProcs[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 = INVALID_PROC_NUMBER;
  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 = INVALID_PROC_NUMBER;
  LWLockRelease(TwoPhaseStateLock);

  MyLockedGxact = NULL;
}


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

  Assert(LWLockHeldByMeInMode(TwoPhaseStateLock, LW_EXCLUSIVE));

  Assert(gxact != NULL);
  proc = GetPGProcByNumber(gxact->pgprocno);

  /* Initialize the PGPROC entry */
  MemSet(proc, 0, sizeof(PGPROC));
  dlist_node_init(&proc->links);
  proc->waitStatus = PROC_WAIT_STATUS_OK;
  if (LocalTransactionIdIsValid(MyProc->vxid.lxid))
  {
    /* clone VXID, for TwoPhaseGetXidByVirtualXID() to find */
    proc->vxid.lxid = MyProc->vxid.lxid;
    proc->vxid.procNumber = MyProcNumber;
  }
  else
  {
    Assert(AmStartupProcess() || !IsPostmasterEnvironment);
    /* GetLockConflicts() uses this to specify a wait on the XID */
    proc->vxid.lxid = xid;
    proc->vxid.procNumber = INVALID_PROC_NUMBER;
  }
  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->isRegularBackend = 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->fxid = fxid;
  gxact->owner = owner;
  gxact->locking_backend = MyProcNumber;
  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 = GetPGProcByNumber(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(GetPGProcByNumber(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 = GetPGProcByNumber(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 != INVALID_PROC_NUMBER)
      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 = MyProcNumber;
    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 = 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 = GetPGProcByNumber(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(FullTransactionId fxid, bool lock_held)
{
  GlobalTransaction result = NULL;
  int     i;

  static FullTransactionId cached_fxid = {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 (FullTransactionIdEquals(fxid, cached_fxid))
    return cached_gxact;

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

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

    if (FullTransactionIdEquals(gxact->fxid, fxid))
    {
      result = gxact;
      break;
    }
  }

  if (!lock_held)
    LWLockRelease(TwoPhaseStateLock);

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

  cached_fxid = fxid;
  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
 * proc number 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 = GetPGProcByNumber(gxact->pgprocno);
    GET_VXID_FROM_PGPROC(proc_vxid, *proc);
    if (VirtualTransactionIdEquals(vxid, proc_vxid))
    {
      /*
       * Startup process sets proc->vxid.procNumber to
       * INVALID_PROC_NUMBER.
       */
      Assert(!gxact->inredo);

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

  LWLockRelease(TwoPhaseStateLock);

  return result;
}

/*
 * TwoPhaseGetDummyProcNumber
 *    Get the dummy proc number for prepared transaction
 *
 * Dummy proc numbers are similar to proc numbers of real backends.  They
 * start at MaxBackends, 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.
 */
ProcNumber
TwoPhaseGetDummyProcNumber(FullTransactionId fxid, bool lock_held)
{
  GlobalTransaction gxact = TwoPhaseGetGXact(fxid, lock_held);

  return gxact->pgprocno;
}

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

  return GetPGProcByNumber(gxact->pgprocno);
}

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

/*
 * Compute the FullTransactionId for the given TransactionId.
 *
 * This is safe if the xid has not yet reached COMMIT PREPARED or ROLLBACK
 * PREPARED.  After those commands, concurrent vac_truncate_clog() may make
 * the xid cease to qualify as allowable.  XXX Not all callers limit their
 * calls accordingly.
 */
static inline FullTransactionId
AdjustToFullTransactionId(TransactionId xid)
{
  Assert(TransactionIdIsValid(xid));
  return FullTransactionIdFromAllowableAt(ReadNextFullTransactionId(), xid);
}

static inline int
TwoPhaseFilePath(char *path, FullTransactionId fxid)
{
  return snprintf(path, MAXPGPATH, TWOPHASE_DIR "/%08X%08X",
          EpochFromFullTransactionId(fxid),
          XidFromFullTransactionId(fxid));
}

/*
 * 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 = GetPGProcByNumber(gxact->pgprocno);
  TransactionId xid = XidFromFullTransactionId(gxact->fxid);
  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_IN_COMMIT for; see notes
   * there.) Note that DELAY_CHKPT_IN_COMMIT is used to find transactions in
   * the critical commit section. We need to know about such transactions
   * for conflict detection in logical replication. See
   * GetOldestActiveTransactionId(true, false) and its use.
   *
   * 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
 * after discarding two-phase files from frozen epochs.
 */
static char *
ReadTwoPhaseFile(FullTransactionId fxid, 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, fxid);

  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/%08X: %s",
              LSN_FORMAT_ARGS(lsn), errormsg)));
    else
      ereport(ERROR,
          (errcode_for_file_access(),
           errmsg("could not read two-phase state from WAL at %X/%08X",
              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/%08X",
            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;
  FullTransactionId fxid;

  Assert(TransactionIdIsValid(xid));

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

  /* Read and validate file */
  fxid = AdjustToFullTransactionId(xid);
  buf = ReadTwoPhaseFile(fxid, 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;
  FullTransactionId fxid;
  TransactionId xid;
  bool    ondisk;
  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 = GetPGProcByNumber(gxact->pgprocno);
  fxid = gxact->fxid;
  xid = XidFromFullTransactionId(fxid);

  /*
   * 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(fxid, 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, fxid, twophase_postcommit_callbacks);
  else
    ProcessRecords(bufptr, fxid, twophase_postabort_callbacks);

  PredicateLockTwoPhaseFinish(fxid, isCommit);

  /*
   * Read this value while holding the two-phase lock, as the on-disk 2PC
   * file is physically removed after the lock is released.
   */
  ondisk = gxact->ondisk;

  /* 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 (ondisk)
    RemoveTwoPhaseFile(fxid, 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, FullTransactionId fxid,
         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] (fxid, record->info, bufptr, record->len);

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

/*
 * Remove the 2PC file.
 *
 * If giveWarning is false, do not complain about file-not-present;
 * this is an expected case during WAL replay.
 *
 * This routine is used at early stages at recovery where future and
 * past orphaned files are checked, hence the FullTransactionId to build
 * a complete file name fit for the removal.
 */
static void
RemoveTwoPhaseFile(FullTransactionId fxid, bool giveWarning)
{
  char    path[MAXPGPATH];

  TwoPhaseFilePath(path, fxid);
  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(FullTransactionId fxid, 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, fxid);

  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->fxid, 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) == 16 &&
      strspn(clde->d_name, "0123456789ABCDEF") == 16)
    {
      FullTransactionId fxid;
      char     *buf;

      fxid = FullTransactionIdFromU64(strtou64(clde->d_name, NULL, 16));

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

      PrepareRedoAdd(fxid, 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.  TransamVariables->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 TransamVariables->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 = TransamVariables->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);

    buf = ProcessTwoPhaseBuffer(gxact->fxid,
                  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.
     */
    xid = XidFromFullTransactionId(gxact->fxid);
    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.
 *
 * This updates pg_subtrans, so that any subtransactions will be correctly
 * seen as in-progress in snapshots taken during recovery.
 */
void
StandbyRecoverPreparedTransactions(void)
{
  int     i;

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

    Assert(gxact->inredo);

    buf = ProcessTwoPhaseBuffer(gxact->fxid,
                  gxact->prepare_start_lsn,
                  gxact->ondisk, true, 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++)
  {
    char     *buf;
    GlobalTransaction gxact = TwoPhaseState->prepXacts[i];
    FullTransactionId fxid = gxact->fxid;
    char     *bufptr;
    TwoPhaseFileHeader *hdr;
    TransactionId *subxids;
    const char *gid;

    /*
     * 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(gxact->fxid,
                  gxact->prepare_start_lsn,
                  gxact->ondisk, true, false);
    if (buf == NULL)
      continue;

    ereport(LOG,
        (errmsg("recovering prepared transaction %u of epoch %u from shared memory",
            XidFromFullTransactionId(gxact->fxid),
            EpochFromFullTransactionId(gxact->fxid))));

    hdr = (TwoPhaseFileHeader *) buf;
    Assert(TransactionIdEquals(hdr->xid,
                   XidFromFullTransactionId(gxact->fxid)));
    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, gxact->fxid, 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, fxid, 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(hdr->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 FullTransactionId, 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 TransamVariables->nextXid to the newest
 * value scanned.
 */
static char *
ProcessTwoPhaseBuffer(FullTransactionId fxid,
            XLogRecPtr prepare_start_lsn,
            bool fromdisk,
            bool setParent, bool setNextXid)
{
  FullTransactionId nextXid = TransamVariables->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(XidFromFullTransactionId(fxid)) ||
    TransactionIdDidAbort(XidFromFullTransactionId(fxid)))
  {
    if (fromdisk)
    {
      ereport(WARNING,
          (errmsg("removing stale two-phase state file for transaction %u of epoch %u",
              XidFromFullTransactionId(fxid),
              EpochFromFullTransactionId(fxid))));
      RemoveTwoPhaseFile(fxid, true);
    }
    else
    {
      ereport(WARNING,
          (errmsg("removing stale two-phase state from memory for transaction %u of epoch %u",
              XidFromFullTransactionId(fxid),
              EpochFromFullTransactionId(fxid))));
      PrepareRedoRemoveFull(fxid, true);
    }
    return NULL;
  }

  /* Reject XID if too new */
  if (FullTransactionIdFollowsOrEquals(fxid, nextXid))
  {
    if (fromdisk)
    {
      ereport(WARNING,
          (errmsg("removing future two-phase state file for transaction %u of epoch %u",
              XidFromFullTransactionId(fxid),
              EpochFromFullTransactionId(fxid))));
      RemoveTwoPhaseFile(fxid, true);
    }
    else
    {
      ereport(WARNING,
          (errmsg("removing future two-phase state from memory for transaction %u of epoch %u",
              XidFromFullTransactionId(fxid),
              EpochFromFullTransactionId(fxid))));
      PrepareRedoRemoveFull(fxid, true);
    }
    return NULL;
  }

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

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

  /*
   * 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, XidFromFullTransactionId(fxid)));

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

    if (setParent)
      SubTransSetParent(subxid, XidFromFullTransactionId(fxid));
  }

  return buf;
}


/*
 *  RecordTransactionCommitPrepared
 *
 * This is basically the same as RecordTransactionCommit (q.v. if you change
 * this function): in particular, we must set DELAY_CHKPT_IN_COMMIT 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;
  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);

  /* Load the injection point before entering the critical section */
  INJECTION_POINT_LOAD("commit-after-delay-checkpoint");

  START_CRIT_SECTION();

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

  INJECTION_POINT_CACHED("commit-after-delay-checkpoint", NULL);

  /*
   * Ensures the DELAY_CHKPT_IN_COMMIT flag write is globally visible before
   * commit time is written.
   */
  pg_write_barrier();

  /*
   * Note it is important to set committs value after marking ourselves as
   * in the commit critical section (DELAY_CHKPT_IN_COMMIT). This is because
   * we want to ensure all transactions that have acquired commit timestamp
   * are finished before we allow the logical replication client to advance
   * its xid which is used to hold back dead rows for conflict detection.
   * See comments atop worker.c.
   */
  committs = GetCurrentTimestamp();

  /*
   * 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_IN_COMMIT;

  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(FullTransactionId fxid, 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());

  if (!FullTransactionIdIsValid(fxid))
  {
    Assert(InRecovery);
    fxid = FullTransactionIdFromAllowableAt(TransamVariables->nextXid,
                        hdr->xid);
  }

  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];

    Assert(InRecovery);
    TwoPhaseFilePath(path, fxid);

    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/%08X, 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->fxid = fxid;
  gxact->owner = hdr->owner;
  gxact->locking_backend = INVALID_PROC_NUMBER;
  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 of epoch %u",
     XidFromFullTransactionId(gxact->fxid),
     EpochFromFullTransactionId(gxact->fxid));
}

/*
 * PrepareRedoRemoveFull
 *
 * 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.
 */
static void
PrepareRedoRemoveFull(FullTransactionId fxid, 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 (FullTransactionIdEquals(gxact->fxid, fxid))
    {
      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 of epoch %u ",
     XidFromFullTransactionId(fxid),
     EpochFromFullTransactionId(fxid));

  if (gxact->ondisk)
    RemoveTwoPhaseFile(fxid, giveWarning);

  RemoveGXact(gxact);
}

/*
 * Wrapper of PrepareRedoRemoveFull(), for TransactionIds.
 */
void
PrepareRedoRemove(TransactionId xid, bool giveWarning)
{
  FullTransactionId fxid =
    FullTransactionIdFromAllowableAt(TransamVariables->nextXid, xid);

  PrepareRedoRemoveFull(fxid, giveWarning);
}

/*
 * 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->fxid, 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;
}

/*
 * TwoPhaseTransactionGid
 *    Form the prepared transaction GID for two_phase transactions.
 *
 * Return the GID in the supplied buffer.
 */
void
TwoPhaseTransactionGid(Oid subid, TransactionId xid, char *gid_res, int szgid)
{
  Assert(OidIsValid(subid));

  if (!TransactionIdIsValid(xid))
    ereport(ERROR,
        (errcode(ERRCODE_PROTOCOL_VIOLATION),
         errmsg_internal("invalid two-phase transaction ID")));

  snprintf(gid_res, szgid, "pg_gid_%u_%u", subid, xid);
}

/*
 * IsTwoPhaseTransactionGidForSubid
 *    Check whether the given GID (as formed by TwoPhaseTransactionGid) is
 *    for the specified 'subid'.
 */
static bool
IsTwoPhaseTransactionGidForSubid(Oid subid, char *gid)
{
  int     ret;
  Oid     subid_from_gid;
  TransactionId xid_from_gid;
  char    gid_tmp[GIDSIZE];

  /* Extract the subid and xid from the given GID */
  ret = sscanf(gid, "pg_gid_%u_%u", &subid_from_gid, &xid_from_gid);

  /*
   * Check that the given GID has expected format, and at least the subid
   * matches.
   */
  if (ret != 2 || subid != subid_from_gid)
    return false;

  /*
   * Reconstruct a temporary GID based on the subid and xid extracted from
   * the given GID and check whether the temporary GID and the given GID
   * match.
   */
  TwoPhaseTransactionGid(subid, xid_from_gid, gid_tmp, sizeof(gid_tmp));

  return strcmp(gid, gid_tmp) == 0;
}

/*
 * LookupGXactBySubid
 *    Check if the prepared transaction done by apply worker exists.
 */
bool
LookupGXactBySubid(Oid subid)
{
  bool    found = false;

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

    /* Ignore not-yet-valid GIDs. */
    if (gxact->valid &&
      IsTwoPhaseTransactionGidForSubid(subid, gxact->gid))
    {
      found = true;
      break;
    }
  }
  LWLockRelease(TwoPhaseStateLock);

  return found;
}

/*
 * TwoPhaseGetOldestXidInCommit
 *    Return the oldest transaction ID from prepared transactions that are
 *    currently in the commit critical section.
 *
 * This function only considers transactions in the currently connected
 * database. If no matching transactions are found, it returns
 * InvalidTransactionId.
 */
TransactionId
TwoPhaseGetOldestXidInCommit(void)
{
  TransactionId oldestRunningXid = InvalidTransactionId;

  LWLockAcquire(TwoPhaseStateLock, LW_SHARED);

  for (int i = 0; i < TwoPhaseState->numPrepXacts; i++)
  {
    GlobalTransaction gxact = TwoPhaseState->prepXacts[i];
    PGPROC     *commitproc;
    TransactionId xid;

    if (!gxact->valid)
      continue;

    if (gxact->locking_backend == INVALID_PROC_NUMBER)
      continue;

    /*
     * Get the backend that is handling the transaction. It's safe to
     * access this backend while holding TwoPhaseStateLock, as the backend
     * can only be destroyed after either removing or unlocking the
     * current global transaction, both of which require an exclusive
     * TwoPhaseStateLock.
     */
    commitproc = GetPGProcByNumber(gxact->locking_backend);

    if (MyDatabaseId != commitproc->databaseId)
      continue;

    if ((commitproc->delayChkptFlags & DELAY_CHKPT_IN_COMMIT) == 0)
      continue;

    xid = XidFromFullTransactionId(gxact->fxid);

    if (!TransactionIdIsValid(oldestRunningXid) ||
      TransactionIdPrecedes(xid, oldestRunningXid))
      oldestRunningXid = xid;
  }

  LWLockRelease(TwoPhaseStateLock);

  return oldestRunningXid;
}

Coverage Report

Created: 2025-10-09 06:07