Query in SQL function still not schema-safe; add a couple

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

/*-------------------------------------------------------------------------
 *
 * twophase.c
 *              Two-phase commit support functions.
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *              $PostgreSQL$
 *
 * 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 a dummy PGPROC that is entered
 *              into the ProcArray array; this is what keeps the XID considered
 *              running by TransactionIdIsInProgress.  It is also convenient as a
 *              PGPROC to hook the gxact's locks to.
 *
 *              In order to survive crashes and shutdowns, all prepared
 *              transactions must be stored 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.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

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

#include "access/htup.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.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 "storage/fd.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "utils/builtins.h"
#include "utils/memutils.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 first component of the struct is a dummy PGPROC that is inserted
 * into the global ProcArray so that the transaction appears to still be
 * running and holding locks.  It must be first because we cast pointers
 * to PGPROC and pointers to GlobalTransactionData back and forth.
 *
 * 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 XID and GID,
 * with locking_xid = my own XID and valid = false.
 *
 * 2. After successfully completing prepare, set valid = true and enter the
 * contained PGPROC into the global ProcArray.
 *
 * 3. To begin COMMIT PREPARED or ROLLBACK PREPARED, check that the entry
 * is valid and its locking_xid is no longer active, then store my current
 * XID into locking_xid.  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 will remain in prepXacts until recycled.  We can detect recyclable
 * entries by checking for valid = false and locking_xid no longer active.
 *
 * typedef struct GlobalTransactionData *GlobalTransaction appears in
 * twophase.h
 */
#define GIDSIZE 200

typedef struct GlobalTransactionData
{
        PGPROC          proc;                   /* dummy proc */
        TimestampTz prepared_at;        /* time of preparation */
        XLogRecPtr      prepare_lsn;    /* XLOG offset of prepare record */
        Oid                     owner;                  /* ID of user that executed the xact */
        TransactionId locking_xid;      /* top-level XID of backend working on xact */
        bool            valid;                  /* TRUE if fully prepared */
        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, but C wants a
         * fixed-size array.
         */
        GlobalTransaction prepXacts[1];         /* VARIABLE LENGTH ARRAY */
} TwoPhaseStateData;                    /* VARIABLE LENGTH STRUCT */

static TwoPhaseStateData *TwoPhaseState;


static void RecordTransactionCommitPrepared(TransactionId xid,
                                                                int nchildren,
                                                                TransactionId *children,
                                                                int nrels,
                                                                RelFileNode *rels);
static void RecordTransactionAbortPrepared(TransactionId xid,
                                                           int nchildren,
                                                           TransactionId *children,
                                                           int nrels,
                                                           RelFileNode *rels);
static void ProcessRecords(char *bufptr, TransactionId xid,
                           const TwoPhaseCallback callbacks[]);


/*
 * 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++)
                {
                        gxacts[i].proc.links.next = (SHM_QUEUE *) TwoPhaseState->freeGXacts;
                        TwoPhaseState->freeGXacts = &gxacts[i];
                }
        }
        else
                Assert(found);
}


/*
 * MarkAsPreparing
 *              Reserve the GID for the given transaction.
 *
 * Internally, this creates 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.
 */
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.")));

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

        /*
         * First, find and recycle any gxacts that failed during prepare. We do
         * this partly to ensure we don't mistakenly say their GIDs are still
         * reserved, and partly so we don't fail on out-of-slots unnecessarily.
         */
        for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
        {
                gxact = TwoPhaseState->prepXacts[i];
                if (!gxact->valid && !TransactionIdIsActive(gxact->locking_xid))
                {
                        /* It's dead Jim ... remove from the active array */
                        TwoPhaseState->numPrepXacts--;
                        TwoPhaseState->prepXacts[i] = TwoPhaseState->prepXacts[TwoPhaseState->numPrepXacts];
                        /* and put it back in the freelist */
                        gxact->proc.links.next = (SHM_QUEUE *) TwoPhaseState->freeGXacts;
                        TwoPhaseState->freeGXacts = gxact;
                        /* Back up index count too, so we don't miss scanning one */
                        i--;
                }
        }

        /* 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 = (GlobalTransaction) gxact->proc.links.next;

        /* Initialize it */
        MemSet(&gxact->proc, 0, sizeof(PGPROC));
        SHMQueueElemInit(&(gxact->proc.links));
        gxact->proc.waitStatus = STATUS_OK;
        /* We set up the gxact's VXID as InvalidBackendId/XID */
        gxact->proc.lxid = (LocalTransactionId) xid;
        gxact->proc.xid = xid;
        gxact->proc.xmin = InvalidTransactionId;
        gxact->proc.pid = 0;
        gxact->proc.backendId = InvalidBackendId;
        gxact->proc.databaseId = databaseid;
        gxact->proc.roleId = owner;
        gxact->proc.inCommit = false;
        gxact->proc.vacuumFlags = 0;
        gxact->proc.lwWaiting = false;
        gxact->proc.lwExclusive = false;
        gxact->proc.lwWaitLink = NULL;
        gxact->proc.waitLock = NULL;
        gxact->proc.waitProcLock = NULL;
        for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
                SHMQueueInit(&(gxact->proc.myProcLocks[i]));
        /* subxid data must be filled later by GXactLoadSubxactData */
        gxact->proc.subxids.overflowed = false;
        gxact->proc.subxids.nxids = 0;

        gxact->prepared_at = prepared_at;
        /* initialize LSN to 0 (start of WAL) */
        gxact->prepare_lsn.xlogid = 0;
        gxact->prepare_lsn.xrecoff = 0;
        gxact->owner = owner;
        gxact->locking_xid = xid;
        gxact->valid = false;
        strcpy(gxact->gid, gid);

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

        LWLockRelease(TwoPhaseStateLock);

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

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

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

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

        LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);

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

                /* 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 (TransactionIdIsValid(gxact->locking_xid))
                {
                        if (TransactionIdIsActive(gxact->locking_xid))
                                ereport(ERROR,
                                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                errmsg("prepared transaction with identifier \"%s\" is busy",
                                           gid)));
                        gxact->locking_xid = InvalidTransactionId;
                }

                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 != gxact->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_xid = GetTopTransactionId();

                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;

        LWLockAcquire(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->proc.links.next = (SHM_QUEUE *) TwoPhaseState->freeGXacts;
                        TwoPhaseState->freeGXacts = gxact;

                        LWLockRelease(TwoPhaseStateLock);

                        return;
                }
        }

        LWLockRelease(TwoPhaseStateLock);

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

/*
 * TransactionIdIsPrepared
 *              True iff transaction associated with the identifier is prepared
 *              for two-phase commit
 *
 * Note: only gxacts marked "valid" are considered; but notice we do not
 * check the locking status.
 *
 * This is not currently exported, because it is only needed internally.
 */
static bool
TransactionIdIsPrepared(TransactionId xid)
{
        bool            result = false;
        int                     i;

        LWLockAcquire(TwoPhaseStateLock, LW_SHARED);

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

                if (gxact->valid && gxact->proc.xid == xid)
                {
                        result = true;
                        break;
                }
        }

        LWLockRelease(TwoPhaseStateLock);

        return result;
}

/*
 * 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, false);
                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++];
                Datum           values[5];
                bool            nulls[5];
                HeapTuple       tuple;
                Datum           result;

                if (!gxact->valid)
                        continue;

                /*
                 * Form tuple with appropriate data.
                 */
                MemSet(values, 0, sizeof(values));
                MemSet(nulls, 0, sizeof(nulls));

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

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

        SRF_RETURN_DONE(funcctx);
}

/*
 * TwoPhaseGetDummyProc
 *              Get the PGPROC that represents a prepared transaction specified by XID
 */
PGPROC *
TwoPhaseGetDummyProc(TransactionId xid)
{
        PGPROC     *result = NULL;
        int                     i;

        static TransactionId cached_xid = InvalidTransactionId;
        static PGPROC *cached_proc = NULL;

        /*
         * 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_proc;

        LWLockAcquire(TwoPhaseStateLock, LW_SHARED);

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

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

        LWLockRelease(TwoPhaseStateLock);

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

        cached_xid = xid;
        cached_proc = result;

        return result;
}

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

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

/*
 * 2PC state file format:
 *
 *      1. TwoPhaseFileHeader
 *      2. TransactionId[] (subtransactions)
 *      3. RelFileNode[] (files to be deleted at commit)
 *      4. RelFileNode[] (files to be deleted at abort)
 *      5. TwoPhaseRecordOnDisk
 *      6. ...
 *      7. TwoPhaseRecordOnDisk (end sentinel, rmid == TWOPHASE_RM_END_ID)
 *      8. CRC32
 *
 * Each segment except the final CRC32 is MAXALIGN'd.
 */

/*
 * Header for a 2PC state file
 */
#define TWOPHASE_MAGIC  0x57F94531              /* format identifier */

typedef struct TwoPhaseFileHeader
{
        uint32          magic;                  /* format identifier */
        uint32          total_len;              /* actual file length */
        TransactionId xid;                      /* original transaction XID */
        Oid                     database;               /* OID of database it was in */
        TimestampTz prepared_at;        /* time of preparation */
        Oid                     owner;                  /* user running the transaction */
        int32           nsubxacts;              /* number of following subxact XIDs */
        int32           ncommitrels;    /* number of delete-on-commit rels */
        int32           nabortrels;             /* number of delete-on-abort rels */
        char            gid[GIDSIZE];   /* GID for transaction */
} 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 XLogRecData blocks
 * so that we will be able to pass the state file contents directly to
 * XLogInsert.
 */
static struct xllist
{
        XLogRecData *head;                      /* first data block in the chain */
        XLogRecData *tail;                      /* last block in chain */
        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(XLogRecData));
                records.tail = records.tail->next;
                records.tail->buffer = InvalidBuffer;
                records.tail->len = 0;
                records.tail->next = NULL;

                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)
{
        TransactionId xid = gxact->proc.xid;
        TwoPhaseFileHeader hdr;
        TransactionId *children;
        RelFileNode *commitrels;
        RelFileNode *abortrels;

        /* Initialize linked list */
        records.head = palloc0(sizeof(XLogRecData));
        records.head->buffer = InvalidBuffer;
        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.total_len = 0;

        /* Create header */
        hdr.magic = TWOPHASE_MAGIC;
        hdr.total_len = 0;                      /* EndPrepare will fill this in */
        hdr.xid = xid;
        hdr.database = gxact->proc.databaseId;
        hdr.prepared_at = gxact->prepared_at;
        hdr.owner = gxact->owner;
        hdr.nsubxacts = xactGetCommittedChildren(&children);
        hdr.ncommitrels = smgrGetPendingDeletes(true, &commitrels, NULL);
        hdr.nabortrels = smgrGetPendingDeletes(false, &abortrels, NULL);
        StrNCpy(hdr.gid, gxact->gid, GIDSIZE);

        save_state_data(&hdr, sizeof(TwoPhaseFileHeader));

        /* Add the additional info about subxacts and deletable files */
        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(RelFileNode));
                pfree(commitrels);
        }
        if (hdr.nabortrels > 0)
        {
                save_state_data(abortrels, hdr.nabortrels * sizeof(RelFileNode));
                pfree(abortrels);
        }
}

/*
 * Finish preparing state file.
 *
 * Calculates CRC and writes state file to WAL and in pg_twophase directory.
 */
void
EndPrepare(GlobalTransaction gxact)
{
        TransactionId xid = gxact->proc.xid;
        TwoPhaseFileHeader *hdr;
        char            path[MAXPGPATH];
        XLogRecData *record;
        pg_crc32        statefile_crc;
        pg_crc32        bogus_crc;
        int                     fd;

        /* 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_crc32);

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

        /*
         * Create the 2PC state file.
         *
         * Note: because we use BasicOpenFile(), we are responsible for ensuring
         * the FD gets closed in any error exit path.  Once we get into the
         * critical section, though, it doesn't matter since any failure causes
         * PANIC anyway.
         */
        TwoPhaseFilePath(path, xid);

        fd = BasicOpenFile(path,
                                           O_CREAT | O_EXCL | O_WRONLY | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not create two-phase state file \"%s\": %m",
                                                path)));

        /* Write data to file, and calculate CRC as we pass over it */
        INIT_CRC32(statefile_crc);

        for (record = records.head; record != NULL; record = record->next)
        {
                COMP_CRC32(statefile_crc, record->data, record->len);
                if ((write(fd, record->data, record->len)) != record->len)
                {
                        close(fd);
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not write two-phase state file: %m")));
                }
        }

        FIN_CRC32(statefile_crc);

        /*
         * Write a deliberately bogus CRC to the state file; this is just paranoia
         * to catch the case where four more bytes will run us out of disk space.
         */
        bogus_crc = ~statefile_crc;

        if ((write(fd, &bogus_crc, sizeof(pg_crc32))) != sizeof(pg_crc32))
        {
                close(fd);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write two-phase state file: %m")));
        }

        /* Back up to prepare for rewriting the CRC */
        if (lseek(fd, -((off_t) sizeof(pg_crc32)), SEEK_CUR) < 0)
        {
                close(fd);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not seek in two-phase state file: %m")));
        }

        /*
         * The state file isn't valid yet, because we haven't written the correct
         * CRC yet.  Before we do that, insert entry in WAL and flush it to disk.
         *
         * Between the time we have written the WAL entry and the time we write
         * out the correct state file CRC, we have an inconsistency: the xact is
         * prepared according to WAL but not according to our on-disk state. We
         * use a critical section to force a PANIC if we are unable to complete
         * the write --- then, WAL replay should repair the inconsistency.      The
         * odds of a PANIC actually occurring should be very tiny given that we
         * were able to write the bogus CRC above.
         *
         * We have to set inCommit 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 inCommit for; see notes there.)
         *
         * We save the PREPARE record's location in the gxact for later use by
         * CheckPointTwoPhase.
         */
        START_CRIT_SECTION();

        MyProc->inCommit = true;

        gxact->prepare_lsn = XLogInsert(RM_XACT_ID, XLOG_XACT_PREPARE,
                                                                        records.head);
        XLogFlush(gxact->prepare_lsn);

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

        /* write correct CRC and close file */
        if ((write(fd, &statefile_crc, sizeof(pg_crc32))) != sizeof(pg_crc32))
        {
                close(fd);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write two-phase state file: %m")));
        }

        if (close(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close two-phase state file: %m")));

        /*
         * 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,
         * 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);

        /*
         * 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->inCommit = false;

        END_CRIT_SECTION();

        records.tail = records.head = NULL;
}

/*
 * 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.  Otherwise return NULL.
 */
static char *
ReadTwoPhaseFile(TransactionId xid)
{
        char            path[MAXPGPATH];
        char       *buf;
        TwoPhaseFileHeader *hdr;
        int                     fd;
        struct stat stat;
        uint32          crc_offset;
        pg_crc32        calc_crc,
                                file_crc;

        TwoPhaseFilePath(path, xid);

        fd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
        if (fd < 0)
        {
                ereport(WARNING,
                                (errcode_for_file_access(),
                                 errmsg("could not open two-phase state file \"%s\": %m",
                                                path)));
                return NULL;
        }

        /*
         * 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))
        {
                close(fd);
                ereport(WARNING,
                                (errcode_for_file_access(),
                                 errmsg("could not stat two-phase state file \"%s\": %m",
                                                path)));
                return NULL;
        }

        if (stat.st_size < (MAXALIGN(sizeof(TwoPhaseFileHeader)) +
                                                MAXALIGN(sizeof(TwoPhaseRecordOnDisk)) +
                                                sizeof(pg_crc32)) ||
                stat.st_size > MaxAllocSize)
        {
                close(fd);
                return NULL;
        }

        crc_offset = stat.st_size - sizeof(pg_crc32);
        if (crc_offset != MAXALIGN(crc_offset))
        {
                close(fd);
                return NULL;
        }

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

        if (read(fd, buf, stat.st_size) != stat.st_size)
        {
                close(fd);
                ereport(WARNING,
                                (errcode_for_file_access(),
                                 errmsg("could not read two-phase state file \"%s\": %m",
                                                path)));
                pfree(buf);
                return NULL;
        }

        close(fd);

        hdr = (TwoPhaseFileHeader *) buf;
        if (hdr->magic != TWOPHASE_MAGIC || hdr->total_len != stat.st_size)
        {
                pfree(buf);
                return NULL;
        }

        INIT_CRC32(calc_crc);
        COMP_CRC32(calc_crc, buf, crc_offset);
        FIN_CRC32(calc_crc);

        file_crc = *((pg_crc32 *) (buf + crc_offset));

        if (!EQ_CRC32(calc_crc, file_crc))
        {
                pfree(buf);
                return NULL;
        }

        return buf;
}


/*
 * FinishPreparedTransaction: execute COMMIT PREPARED or ROLLBACK PREPARED
 */
void
FinishPreparedTransaction(const char *gid, bool isCommit)
{
        GlobalTransaction gxact;
        TransactionId xid;
        char       *buf;
        char       *bufptr;
        TwoPhaseFileHeader *hdr;
        TransactionId latestXid;
        TransactionId *children;
        RelFileNode *commitrels;
        RelFileNode *abortrels;
        RelFileNode *delrels;
        int                     ndelrels;
        int                     i;

        /*
         * 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());
        xid = gxact->proc.xid;

        /*
         * Read and validate the state file
         */
        buf = ReadTwoPhaseFile(xid);
        if (buf == NULL)
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("two-phase state file for transaction %u is corrupt",
                                                xid)));

        /*
         * Disassemble the header area
         */
        hdr = (TwoPhaseFileHeader *) buf;
        Assert(TransactionIdEquals(hdr->xid, xid));
        bufptr = buf + MAXALIGN(sizeof(TwoPhaseFileHeader));
        children = (TransactionId *) bufptr;
        bufptr += MAXALIGN(hdr->nsubxacts * sizeof(TransactionId));
        commitrels = (RelFileNode *) bufptr;
        bufptr += MAXALIGN(hdr->ncommitrels * sizeof(RelFileNode));
        abortrels = (RelFileNode *) bufptr;
        bufptr += MAXALIGN(hdr->nabortrels * sizeof(RelFileNode));

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

        /*
         * The order of operations here is critical: make the XLOG entry for
         * commit or abort, then mark the transaction committed or aborted in
         * pg_clog, 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);
        else
                RecordTransactionAbortPrepared(xid,
                                                                           hdr->nsubxacts, children,
                                                                           hdr->nabortrels, abortrels);

        ProcArrayRemove(&gxact->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 can be recycled
         * properly later.      It is still locked by our XID 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;
        }
        for (i = 0; i < ndelrels; i++)
        {
                SMgrRelation srel = smgropen(delrels[i]);
                ForkNumber      fork;

                for (fork = 0; fork <= MAX_FORKNUM; fork++)
                {
                        if (smgrexists(srel, fork))
                                smgrdounlink(srel, fork, false, false);
                }
                smgrclose(srel);
        }

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

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

        /*
         * And now we can clean up our mess.
         */
        RemoveTwoPhaseFile(xid, true);

        RemoveGXact(gxact);

        pfree(buf);
}

/*
 * Scan a 2PC state file (already read into memory by ReadTwoPhaseFile)
 * 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.
 */
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 two-phase state file \"%s\": %m",
                                                  path)));
}

/*
 * Recreates a state file. This is used in WAL replay.
 *
 * Note: content and len don't include CRC.
 */
void
RecreateTwoPhaseFile(TransactionId xid, void *content, int len)
{
        char            path[MAXPGPATH];
        pg_crc32        statefile_crc;
        int                     fd;

        /* Recompute CRC */
        INIT_CRC32(statefile_crc);
        COMP_CRC32(statefile_crc, content, len);
        FIN_CRC32(statefile_crc);

        TwoPhaseFilePath(path, xid);

        fd = BasicOpenFile(path,
                                           O_CREAT | O_TRUNC | O_WRONLY | PG_BINARY,
                                           S_IRUSR | S_IWUSR);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not recreate two-phase state file \"%s\": %m",
                                                path)));

        /* Write content and CRC */
        if (write(fd, content, len) != len)
        {
                close(fd);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write two-phase state file: %m")));
        }
        if (write(fd, &statefile_crc, sizeof(pg_crc32)) != sizeof(pg_crc32))
        {
                close(fd);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write two-phase state file: %m")));
        }

        /*
         * 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.
         */
        if (pg_fsync(fd) != 0)
        {
                close(fd);
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync two-phase state file: %m")));
        }

        if (close(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close two-phase state file: %m")));
}

/*
 * CheckPointTwoPhase -- handle 2PC component of checkpointing.
 *
 * We must fsync the state file of any GXACT that is valid and has a PREPARE
 * LSN <= the checkpoint's redo horizon.  (If the gxact isn't valid yet 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.
 *
 * If a GXACT remains valid across multiple checkpoints, it'll be fsynced
 * each time.  This is considered unusual enough that we don't bother to
 * expend any extra code to avoid the redundant fsyncs.  (They should be
 * reasonably cheap anyway, since they won't cause I/O.)
 */
void
CheckPointTwoPhase(XLogRecPtr redo_horizon)
{
        TransactionId *xids;
        int                     nxids;
        char            path[MAXPGPATH];
        int                     i;

        /*
         * We don't want to hold the TwoPhaseStateLock while doing I/O, so we grab
         * it just long enough to make a list of the XIDs that require fsyncing,
         * and then do the I/O afterwards.
         *
         * This approach creates a race condition: someone else could delete a
         * GXACT between the time we release TwoPhaseStateLock and the time we try
         * to open its state file.      We handle this by special-casing ENOENT
         * failures: if we see that, we verify that the GXACT is no longer valid,
         * and if so ignore the failure.
         */
        if (max_prepared_xacts <= 0)
                return;                                 /* nothing to do */

        TRACE_POSTGRESQL_TWOPHASE_CHECKPOINT_START();

        xids = (TransactionId *) palloc(max_prepared_xacts * sizeof(TransactionId));
        nxids = 0;

        LWLockAcquire(TwoPhaseStateLock, LW_SHARED);

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

                if (gxact->valid &&
                        XLByteLE(gxact->prepare_lsn, redo_horizon))
                        xids[nxids++] = gxact->proc.xid;
        }

        LWLockRelease(TwoPhaseStateLock);

        for (i = 0; i < nxids; i++)
        {
                TransactionId xid = xids[i];
                int                     fd;

                TwoPhaseFilePath(path, xid);

                fd = BasicOpenFile(path, O_RDWR | PG_BINARY, 0);
                if (fd < 0)
                {
                        if (errno == ENOENT)
                        {
                                /* OK if gxact is no longer valid */
                                if (!TransactionIdIsPrepared(xid))
                                        continue;
                                /* Restore errno in case it was changed */
                                errno = ENOENT;
                        }
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not open two-phase state file \"%s\": %m",
                                                        path)));
                }

                if (pg_fsync(fd) != 0)
                {
                        close(fd);
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not fsync two-phase state file \"%s\": %m",
                                                        path)));
                }

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

        pfree(xids);

        TRACE_POSTGRESQL_TWOPHASE_CHECKPOINT_DONE();
}

/*
 * PrescanPreparedTransactions
 *
 * Scan the pg_twophase directory 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.
 *
 * 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.
 */
TransactionId
PrescanPreparedTransactions(void)
{
        TransactionId origNextXid = ShmemVariableCache->nextXid;
        TransactionId result = origNextXid;
        DIR                *cldir;
        struct dirent *clde;

        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;
                        TwoPhaseFileHeader *hdr;
                        TransactionId *subxids;
                        int                     i;

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

                        /* Reject XID if too new */
                        if (TransactionIdFollowsOrEquals(xid, origNextXid))
                        {
                                ereport(WARNING,
                                                (errmsg("removing future two-phase state file \"%s\"",
                                                                clde->d_name)));
                                RemoveTwoPhaseFile(xid, true);
                                continue;
                        }

                        /*
                         * Note: we can't check if already processed because clog
                         * subsystem isn't up yet.
                         */

                        /* Read and validate file */
                        buf = ReadTwoPhaseFile(xid);
                        if (buf == NULL)
                        {
                                ereport(WARNING,
                                          (errmsg("removing corrupt two-phase state file \"%s\"",
                                                          clde->d_name)));
                                RemoveTwoPhaseFile(xid, true);
                                continue;
                        }

                        /* Deconstruct header */
                        hdr = (TwoPhaseFileHeader *) buf;
                        if (!TransactionIdEquals(hdr->xid, xid))
                        {
                                ereport(WARNING,
                                          (errmsg("removing corrupt two-phase state file \"%s\"",
                                                          clde->d_name)));
                                RemoveTwoPhaseFile(xid, true);
                                pfree(buf);
                                continue;
                        }

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

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

                                Assert(TransactionIdFollows(subxid, xid));
                                if (TransactionIdFollowsOrEquals(subxid,
                                                                                                 ShmemVariableCache->nextXid))
                                {
                                        ShmemVariableCache->nextXid = subxid;
                                        TransactionIdAdvance(ShmemVariableCache->nextXid);
                                }
                        }

                        pfree(buf);
                }
        }
        FreeDir(cldir);

        return result;
}

/*
 * RecoverPreparedTransactions
 *
 * Scan the pg_twophase directory and reload shared-memory state for each
 * prepared transaction (reacquire locks, etc).  This is run during database
 * startup.
 */
void
RecoverPreparedTransactions(void)
{
        char            dir[MAXPGPATH];
        DIR                *cldir;
        struct dirent *clde;

        snprintf(dir, MAXPGPATH, "%s", TWOPHASE_DIR);

        cldir = AllocateDir(dir);
        while ((clde = ReadDir(cldir, dir)) != NULL)
        {
                if (strlen(clde->d_name) == 8 &&
                        strspn(clde->d_name, "0123456789ABCDEF") == 8)
                {
                        TransactionId xid;
                        char       *buf;
                        char       *bufptr;
                        TwoPhaseFileHeader *hdr;
                        TransactionId *subxids;
                        GlobalTransaction gxact;
                        int                     i;

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

                        /* Already processed? */
                        if (TransactionIdDidCommit(xid) || TransactionIdDidAbort(xid))
                        {
                                ereport(WARNING,
                                                (errmsg("removing stale two-phase state file \"%s\"",
                                                                clde->d_name)));
                                RemoveTwoPhaseFile(xid, true);
                                continue;
                        }

                        /* Read and validate file */
                        buf = ReadTwoPhaseFile(xid);
                        if (buf == NULL)
                        {
                                ereport(WARNING,
                                          (errmsg("removing corrupt two-phase state file \"%s\"",
                                                          clde->d_name)));
                                RemoveTwoPhaseFile(xid, true);
                                continue;
                        }

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

                        /* Deconstruct header */
                        hdr = (TwoPhaseFileHeader *) buf;
                        Assert(TransactionIdEquals(hdr->xid, xid));
                        bufptr = buf + MAXALIGN(sizeof(TwoPhaseFileHeader));
                        subxids = (TransactionId *) bufptr;
                        bufptr += MAXALIGN(hdr->nsubxacts * sizeof(TransactionId));
                        bufptr += MAXALIGN(hdr->ncommitrels * sizeof(RelFileNode));
                        bufptr += MAXALIGN(hdr->nabortrels * sizeof(RelFileNode));

                        /*
                         * 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.
                         */
                        for (i = 0; i < hdr->nsubxacts; i++)
                                SubTransSetParent(subxids[i], xid);

                        /*
                         * Recreate its GXACT and dummy PGPROC
                         *
                         * Note: since we don't have the PREPARE record's WAL location at
                         * hand, we leave prepare_lsn zeroes.  This means the GXACT will
                         * be fsync'd on every future checkpoint.  We assume this
                         * situation is infrequent enough that the performance cost is
                         * negligible (especially since we know the state file has already
                         * been fsynced).
                         */
                        gxact = MarkAsPreparing(xid, hdr->gid,
                                                                        hdr->prepared_at,
                                                                        hdr->owner, hdr->database);
                        GXactLoadSubxactData(gxact, hdr->nsubxacts, subxids);
                        MarkAsPrepared(gxact);

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

                        pfree(buf);
                }
        }
        FreeDir(cldir);
}

/*
 *      RecordTransactionCommitPrepared
 *
 * This is basically the same as RecordTransactionCommit: in particular,
 * we must set the inCommit flag 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,
                                                                RelFileNode *rels)
{
        XLogRecData rdata[3];
        int                     lastrdata = 0;
        xl_xact_commit_prepared xlrec;
        XLogRecPtr      recptr;

        START_CRIT_SECTION();

        /* See notes in RecordTransactionCommit */
        MyProc->inCommit = true;

        /* Emit the XLOG commit record */
        xlrec.xid = xid;
        xlrec.crec.xact_time = GetCurrentTimestamp();
        xlrec.crec.nrels = nrels;
        xlrec.crec.nsubxacts = nchildren;
        rdata[0].data = (char *) (&xlrec);
        rdata[0].len = MinSizeOfXactCommitPrepared;
        rdata[0].buffer = InvalidBuffer;
        /* dump rels to delete */
        if (nrels > 0)
        {
                rdata[0].next = &(rdata[1]);
                rdata[1].data = (char *) rels;
                rdata[1].len = nrels * sizeof(RelFileNode);
                rdata[1].buffer = InvalidBuffer;
                lastrdata = 1;
        }
        /* dump committed child Xids */
        if (nchildren > 0)
        {
                rdata[lastrdata].next = &(rdata[2]);
                rdata[2].data = (char *) children;
                rdata[2].len = nchildren * sizeof(TransactionId);
                rdata[2].buffer = InvalidBuffer;
                lastrdata = 2;
        }
        rdata[lastrdata].next = NULL;

        recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_COMMIT_PREPARED, rdata);

        /*
         * 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_clog */
        TransactionIdCommitTree(xid, nchildren, children);

        /* Checkpoint can proceed now */
        MyProc->inCommit = false;

        END_CRIT_SECTION();
}

/*
 *      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,
                                                           RelFileNode *rels)
{
        XLogRecData rdata[3];
        int                     lastrdata = 0;
        xl_xact_abort_prepared xlrec;
        XLogRecPtr      recptr;

        /*
         * 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 abort record */
        xlrec.xid = xid;
        xlrec.arec.xact_time = GetCurrentTimestamp();
        xlrec.arec.nrels = nrels;
        xlrec.arec.nsubxacts = nchildren;
        rdata[0].data = (char *) (&xlrec);
        rdata[0].len = MinSizeOfXactAbortPrepared;
        rdata[0].buffer = InvalidBuffer;
        /* dump rels to delete */
        if (nrels > 0)
        {
                rdata[0].next = &(rdata[1]);
                rdata[1].data = (char *) rels;
                rdata[1].len = nrels * sizeof(RelFileNode);
                rdata[1].buffer = InvalidBuffer;
                lastrdata = 1;
        }
        /* dump committed child Xids */
        if (nchildren > 0)
        {
                rdata[lastrdata].next = &(rdata[2]);
                rdata[2].data = (char *) children;
                rdata[2].len = nchildren * sizeof(TransactionId);
                rdata[2].buffer = InvalidBuffer;
                lastrdata = 2;
        }
        rdata[lastrdata].next = NULL;

        recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_ABORT_PREPARED, rdata);

        /* 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();
}