Define __EXTENSIONS__ on Solaris, too.

[pgsql.git] / src / backend / catalog / storage.c

/*-------------------------------------------------------------------------
 *
 * storage.c
 *        code to create and destroy physical storage for relations
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        src/backend/catalog/storage.c
 *
 * NOTES
 *        Some of this code used to be in storage/smgr/smgr.c, and the
 *        function names still reflect that.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/visibilitymap.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "access/xlogutils.h"
#include "catalog/storage.h"
#include "catalog/storage_xlog.h"
#include "miscadmin.h"
#include "storage/bulk_write.h"
#include "storage/freespace.h"
#include "storage/proc.h"
#include "storage/smgr.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"
#include "utils/rel.h"

/* GUC variables */
int                     wal_skip_threshold = 2048;      /* in kilobytes */

/*
 * We keep a list of all relations (represented as RelFileLocator values)
 * that have been created or deleted in the current transaction.  When
 * a relation is created, we create the physical file immediately, but
 * remember it so that we can delete the file again if the current
 * transaction is aborted.  Conversely, a deletion request is NOT
 * executed immediately, but is just entered in the list.  When and if
 * the transaction commits, we can delete the physical file.
 *
 * To handle subtransactions, every entry is marked with its transaction
 * nesting level.  At subtransaction commit, we reassign the subtransaction's
 * entries to the parent nesting level.  At subtransaction abort, we can
 * immediately execute the abort-time actions for all entries of the current
 * nesting level.
 *
 * NOTE: the list is kept in TopMemoryContext to be sure it won't disappear
 * unbetimes.  It'd probably be OK to keep it in TopTransactionContext,
 * but I'm being paranoid.
 */

typedef struct PendingRelDelete
{
        RelFileLocator rlocator;        /* relation that may need to be deleted */
        ProcNumber      procNumber;             /* INVALID_PROC_NUMBER if not a temp rel */
        bool            atCommit;               /* T=delete at commit; F=delete at abort */
        int                     nestLevel;              /* xact nesting level of request */
        struct PendingRelDelete *next;  /* linked-list link */
} PendingRelDelete;

typedef struct PendingRelSync
{
        RelFileLocator rlocator;
        bool            is_truncated;   /* Has the file experienced truncation? */
} PendingRelSync;

static PendingRelDelete *pendingDeletes = NULL; /* head of linked list */
static HTAB *pendingSyncHash = NULL;


/*
 * AddPendingSync
 *              Queue an at-commit fsync.
 */
static void
AddPendingSync(const RelFileLocator *rlocator)
{
        PendingRelSync *pending;
        bool            found;

        /* create the hash if not yet */
        if (!pendingSyncHash)
        {
                HASHCTL         ctl;

                ctl.keysize = sizeof(RelFileLocator);
                ctl.entrysize = sizeof(PendingRelSync);
                ctl.hcxt = TopTransactionContext;
                pendingSyncHash = hash_create("pending sync hash", 16, &ctl,
                                                                          HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
        }

        pending = hash_search(pendingSyncHash, rlocator, HASH_ENTER, &found);
        Assert(!found);
        pending->is_truncated = false;
}

/*
 * RelationCreateStorage
 *              Create physical storage for a relation.
 *
 * Create the underlying disk file storage for the relation. This only
 * creates the main fork; additional forks are created lazily by the
 * modules that need them.
 *
 * This function is transactional. The creation is WAL-logged, and if the
 * transaction aborts later on, the storage will be destroyed.  A caller
 * that does not want the storage to be destroyed in case of an abort may
 * pass register_delete = false.
 */
SMgrRelation
RelationCreateStorage(RelFileLocator rlocator, char relpersistence,
                                          bool register_delete)
{
        SMgrRelation srel;
        ProcNumber      procNumber;
        bool            needs_wal;

        Assert(!IsInParallelMode());    /* couldn't update pendingSyncHash */

        switch (relpersistence)
        {
                case RELPERSISTENCE_TEMP:
                        procNumber = ProcNumberForTempRelations();
                        needs_wal = false;
                        break;
                case RELPERSISTENCE_UNLOGGED:
                        procNumber = INVALID_PROC_NUMBER;
                        needs_wal = false;
                        break;
                case RELPERSISTENCE_PERMANENT:
                        procNumber = INVALID_PROC_NUMBER;
                        needs_wal = true;
                        break;
                default:
                        elog(ERROR, "invalid relpersistence: %c", relpersistence);
                        return NULL;            /* placate compiler */
        }

        srel = smgropen(rlocator, procNumber);
        smgrcreate(srel, MAIN_FORKNUM, false);

        if (needs_wal)
                log_smgrcreate(&srel->smgr_rlocator.locator, MAIN_FORKNUM);

        /*
         * Add the relation to the list of stuff to delete at abort, if we are
         * asked to do so.
         */
        if (register_delete)
        {
                PendingRelDelete *pending;

                pending = (PendingRelDelete *)
                        MemoryContextAlloc(TopMemoryContext, sizeof(PendingRelDelete));
                pending->rlocator = rlocator;
                pending->procNumber = procNumber;
                pending->atCommit = false;      /* delete if abort */
                pending->nestLevel = GetCurrentTransactionNestLevel();
                pending->next = pendingDeletes;
                pendingDeletes = pending;
        }

        if (relpersistence == RELPERSISTENCE_PERMANENT && !XLogIsNeeded())
        {
                Assert(procNumber == INVALID_PROC_NUMBER);
                AddPendingSync(&rlocator);
        }

        return srel;
}

/*
 * Perform XLogInsert of an XLOG_SMGR_CREATE record to WAL.
 */
void
log_smgrcreate(const RelFileLocator *rlocator, ForkNumber forkNum)
{
        xl_smgr_create xlrec;

        /*
         * Make an XLOG entry reporting the file creation.
         */
        xlrec.rlocator = *rlocator;
        xlrec.forkNum = forkNum;

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, sizeof(xlrec));
        XLogInsert(RM_SMGR_ID, XLOG_SMGR_CREATE | XLR_SPECIAL_REL_UPDATE);
}

/*
 * RelationDropStorage
 *              Schedule unlinking of physical storage at transaction commit.
 */
void
RelationDropStorage(Relation rel)
{
        PendingRelDelete *pending;

        /* Add the relation to the list of stuff to delete at commit */
        pending = (PendingRelDelete *)
                MemoryContextAlloc(TopMemoryContext, sizeof(PendingRelDelete));
        pending->rlocator = rel->rd_locator;
        pending->procNumber = rel->rd_backend;
        pending->atCommit = true;       /* delete if commit */
        pending->nestLevel = GetCurrentTransactionNestLevel();
        pending->next = pendingDeletes;
        pendingDeletes = pending;

        /*
         * NOTE: if the relation was created in this transaction, it will now be
         * present in the pending-delete list twice, once with atCommit true and
         * once with atCommit false.  Hence, it will be physically deleted at end
         * of xact in either case (and the other entry will be ignored by
         * smgrDoPendingDeletes, so no error will occur).  We could instead remove
         * the existing list entry and delete the physical file immediately, but
         * for now I'll keep the logic simple.
         */

        RelationCloseSmgr(rel);
}

/*
 * RelationPreserveStorage
 *              Mark a relation as not to be deleted after all.
 *
 * We need this function because relation mapping changes are committed
 * separately from commit of the whole transaction, so it's still possible
 * for the transaction to abort after the mapping update is done.
 * When a new physical relation is installed in the map, it would be
 * scheduled for delete-on-abort, so we'd delete it, and be in trouble.
 * The relation mapper fixes this by telling us to not delete such relations
 * after all as part of its commit.
 *
 * We also use this to reuse an old build of an index during ALTER TABLE, this
 * time removing the delete-at-commit entry.
 *
 * No-op if the relation is not among those scheduled for deletion.
 */
void
RelationPreserveStorage(RelFileLocator rlocator, bool atCommit)
{
        PendingRelDelete *pending;
        PendingRelDelete *prev;
        PendingRelDelete *next;

        prev = NULL;
        for (pending = pendingDeletes; pending != NULL; pending = next)
        {
                next = pending->next;
                if (RelFileLocatorEquals(rlocator, pending->rlocator)
                        && pending->atCommit == atCommit)
                {
                        /* unlink and delete list entry */
                        if (prev)
                                prev->next = next;
                        else
                                pendingDeletes = next;
                        pfree(pending);
                        /* prev does not change */
                }
                else
                {
                        /* unrelated entry, don't touch it */
                        prev = pending;
                }
        }
}

/*
 * RelationTruncate
 *              Physically truncate a relation to the specified number of blocks.
 *
 * This includes getting rid of any buffers for the blocks that are to be
 * dropped.
 */
void
RelationTruncate(Relation rel, BlockNumber nblocks)
{
        bool            fsm;
        bool            vm;
        bool            need_fsm_vacuum = false;
        ForkNumber      forks[MAX_FORKNUM];
        BlockNumber blocks[MAX_FORKNUM];
        int                     nforks = 0;
        SMgrRelation reln;

        /*
         * Make sure smgr_targblock etc aren't pointing somewhere past new end.
         * (Note: don't rely on this reln pointer below this loop.)
         */
        reln = RelationGetSmgr(rel);
        reln->smgr_targblock = InvalidBlockNumber;
        for (int i = 0; i <= MAX_FORKNUM; ++i)
                reln->smgr_cached_nblocks[i] = InvalidBlockNumber;

        /* Prepare for truncation of MAIN fork of the relation */
        forks[nforks] = MAIN_FORKNUM;
        blocks[nforks] = nblocks;
        nforks++;

        /* Prepare for truncation of the FSM if it exists */
        fsm = smgrexists(RelationGetSmgr(rel), FSM_FORKNUM);
        if (fsm)
        {
                blocks[nforks] = FreeSpaceMapPrepareTruncateRel(rel, nblocks);
                if (BlockNumberIsValid(blocks[nforks]))
                {
                        forks[nforks] = FSM_FORKNUM;
                        nforks++;
                        need_fsm_vacuum = true;
                }
        }

        /* Prepare for truncation of the visibility map too if it exists */
        vm = smgrexists(RelationGetSmgr(rel), VISIBILITYMAP_FORKNUM);
        if (vm)
        {
                blocks[nforks] = visibilitymap_prepare_truncate(rel, nblocks);
                if (BlockNumberIsValid(blocks[nforks]))
                {
                        forks[nforks] = VISIBILITYMAP_FORKNUM;
                        nforks++;
                }
        }

        RelationPreTruncate(rel);

        /*
         * The code which follows can interact with concurrent checkpoints in two
         * separate ways.
         *
         * First, the truncation operation might drop buffers that the checkpoint
         * otherwise would have flushed. If it does, then it's essential that the
         * files actually get truncated on disk before the checkpoint record is
         * written. Otherwise, if reply begins from that checkpoint, the
         * to-be-truncated blocks might still exist on disk but have older
         * contents than expected, which can cause replay to fail. It's OK for the
         * blocks to not exist on disk at all, but not for them to have the wrong
         * contents. For this reason, we need to set DELAY_CHKPT_COMPLETE while
         * this code executes.
         *
         * Second, the call to smgrtruncate() below will in turn call
         * RegisterSyncRequest(). We need the sync request created by that call to
         * be processed before the checkpoint completes. CheckPointGuts() will
         * call ProcessSyncRequests(), but if we register our sync request after
         * that happens, then the WAL record for the truncation could end up
         * preceding the checkpoint record, while the actual sync doesn't happen
         * until the next checkpoint. To prevent that, we need to set
         * DELAY_CHKPT_START here. That way, if the XLOG_SMGR_TRUNCATE precedes
         * the redo pointer of a concurrent checkpoint, we're guaranteed that the
         * corresponding sync request will be processed before the checkpoint
         * completes.
         */
        Assert((MyProc->delayChkptFlags & (DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE)) == 0);
        MyProc->delayChkptFlags |= DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE;

        /*
         * We WAL-log the truncation before actually truncating, which means
         * trouble if the truncation fails. If we then crash, the WAL replay
         * likely isn't going to succeed in the truncation either, and cause a
         * PANIC. It's tempting to put a critical section here, but that cure
         * would be worse than the disease. It would turn a usually harmless
         * failure to truncate, that might spell trouble at WAL replay, into a
         * certain PANIC.
         */
        if (RelationNeedsWAL(rel))
        {
                /*
                 * Make an XLOG entry reporting the file truncation.
                 */
                XLogRecPtr      lsn;
                xl_smgr_truncate xlrec;

                xlrec.blkno = nblocks;
                xlrec.rlocator = rel->rd_locator;
                xlrec.flags = SMGR_TRUNCATE_ALL;

                XLogBeginInsert();
                XLogRegisterData((char *) &xlrec, sizeof(xlrec));

                lsn = XLogInsert(RM_SMGR_ID,
                                                 XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);

                /*
                 * Flush, because otherwise the truncation of the main relation might
                 * hit the disk before the WAL record, and the truncation of the FSM
                 * or visibility map. If we crashed during that window, we'd be left
                 * with a truncated heap, but the FSM or visibility map would still
                 * contain entries for the non-existent heap pages.
                 */
                if (fsm || vm)
                        XLogFlush(lsn);
        }

        /*
         * This will first remove any buffers from the buffer pool that should no
         * longer exist after truncation is complete, and then truncate the
         * corresponding files on disk.
         */
        smgrtruncate(RelationGetSmgr(rel), forks, nforks, blocks);

        /* We've done all the critical work, so checkpoints are OK now. */
        MyProc->delayChkptFlags &= ~(DELAY_CHKPT_START | DELAY_CHKPT_COMPLETE);

        /*
         * Update upper-level FSM pages to account for the truncation. This is
         * important because the just-truncated pages were likely marked as
         * all-free, and would be preferentially selected.
         *
         * NB: There's no point in delaying checkpoints until this is done.
         * Because the FSM is not WAL-logged, we have to be prepared for the
         * possibility of corruption after a crash anyway.
         */
        if (need_fsm_vacuum)
                FreeSpaceMapVacuumRange(rel, nblocks, InvalidBlockNumber);
}

/*
 * RelationPreTruncate
 *              Perform AM-independent work before a physical truncation.
 *
 * If an access method's relation_nontransactional_truncate does not call
 * RelationTruncate(), it must call this before decreasing the table size.
 */
void
RelationPreTruncate(Relation rel)
{
        PendingRelSync *pending;

        if (!pendingSyncHash)
                return;

        pending = hash_search(pendingSyncHash,
                                                  &(RelationGetSmgr(rel)->smgr_rlocator.locator),
                                                  HASH_FIND, NULL);
        if (pending)
                pending->is_truncated = true;
}

/*
 * Copy a fork's data, block by block.
 *
 * Note that this requires that there is no dirty data in shared buffers. If
 * it's possible that there are, callers need to flush those using
 * e.g. FlushRelationBuffers(rel).
 *
 * Also note that this is frequently called via locutions such as
 *              RelationCopyStorage(RelationGetSmgr(rel), ...);
 * That's safe only because we perform only smgr and WAL operations here.
 * If we invoked anything else, a relcache flush could cause our SMgrRelation
 * argument to become a dangling pointer.
 */
void
RelationCopyStorage(SMgrRelation src, SMgrRelation dst,
                                        ForkNumber forkNum, char relpersistence)
{
        bool            use_wal;
        bool            copying_initfork;
        BlockNumber nblocks;
        BlockNumber blkno;
        BulkWriteState *bulkstate;

        /*
         * The init fork for an unlogged relation in many respects has to be
         * treated the same as normal relation, changes need to be WAL logged and
         * it needs to be synced to disk.
         */
        copying_initfork = relpersistence == RELPERSISTENCE_UNLOGGED &&
                forkNum == INIT_FORKNUM;

        /*
         * We need to log the copied data in WAL iff WAL archiving/streaming is
         * enabled AND it's a permanent relation.  This gives the same answer as
         * "RelationNeedsWAL(rel) || copying_initfork", because we know the
         * current operation created new relation storage.
         */
        use_wal = XLogIsNeeded() &&
                (relpersistence == RELPERSISTENCE_PERMANENT || copying_initfork);

        bulkstate = smgr_bulk_start_smgr(dst, forkNum, use_wal);

        nblocks = smgrnblocks(src, forkNum);

        for (blkno = 0; blkno < nblocks; blkno++)
        {
                BulkWriteBuffer buf;

                /* If we got a cancel signal during the copy of the data, quit */
                CHECK_FOR_INTERRUPTS();

                buf = smgr_bulk_get_buf(bulkstate);
                smgrread(src, forkNum, blkno, (Page) buf);

                if (!PageIsVerifiedExtended((Page) buf, blkno,
                                                                        PIV_LOG_WARNING | PIV_REPORT_STAT))
                {
                        /*
                         * For paranoia's sake, capture the file path before invoking the
                         * ereport machinery.  This guards against the possibility of a
                         * relcache flush caused by, e.g., an errcontext callback.
                         * (errcontext callbacks shouldn't be risking any such thing, but
                         * people have been known to forget that rule.)
                         */
                        char       *relpath = relpathbackend(src->smgr_rlocator.locator,
                                                                                                 src->smgr_rlocator.backend,
                                                                                                 forkNum);

                        ereport(ERROR,
                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                         errmsg("invalid page in block %u of relation %s",
                                                        blkno, relpath)));
                }

                /*
                 * Queue the page for WAL-logging and writing out.  Unfortunately we
                 * don't know what kind of a page this is, so we have to log the full
                 * page including any unused space.
                 */
                smgr_bulk_write(bulkstate, blkno, buf, false);
        }
        smgr_bulk_finish(bulkstate);
}

/*
 * RelFileLocatorSkippingWAL
 *              Check if a BM_PERMANENT relfilelocator is using WAL.
 *
 * Changes to certain relations must not write WAL; see "Skipping WAL for
 * New RelFileLocator" in src/backend/access/transam/README.  Though it is
 * known from Relation efficiently, this function is intended for the code
 * paths not having access to Relation.
 */
bool
RelFileLocatorSkippingWAL(RelFileLocator rlocator)
{
        if (!pendingSyncHash ||
                hash_search(pendingSyncHash, &rlocator, HASH_FIND, NULL) == NULL)
                return false;

        return true;
}

/*
 * EstimatePendingSyncsSpace
 *              Estimate space needed to pass syncs to parallel workers.
 */
Size
EstimatePendingSyncsSpace(void)
{
        long            entries;

        entries = pendingSyncHash ? hash_get_num_entries(pendingSyncHash) : 0;
        return mul_size(1 + entries, sizeof(RelFileLocator));
}

/*
 * SerializePendingSyncs
 *              Serialize syncs for parallel workers.
 */
void
SerializePendingSyncs(Size maxSize, char *startAddress)
{
        HTAB       *tmphash;
        HASHCTL         ctl;
        HASH_SEQ_STATUS scan;
        PendingRelSync *sync;
        PendingRelDelete *delete;
        RelFileLocator *src;
        RelFileLocator *dest = (RelFileLocator *) startAddress;

        if (!pendingSyncHash)
                goto terminate;

        /* Create temporary hash to collect active relfilelocators */
        ctl.keysize = sizeof(RelFileLocator);
        ctl.entrysize = sizeof(RelFileLocator);
        ctl.hcxt = CurrentMemoryContext;
        tmphash = hash_create("tmp relfilelocators",
                                                  hash_get_num_entries(pendingSyncHash), &ctl,
                                                  HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

        /* collect all rlocator from pending syncs */
        hash_seq_init(&scan, pendingSyncHash);
        while ((sync = (PendingRelSync *) hash_seq_search(&scan)))
                (void) hash_search(tmphash, &sync->rlocator, HASH_ENTER, NULL);

        /* remove deleted rnodes */
        for (delete = pendingDeletes; delete != NULL; delete = delete->next)
                if (delete->atCommit)
                        (void) hash_search(tmphash, &delete->rlocator,
                                                           HASH_REMOVE, NULL);

        hash_seq_init(&scan, tmphash);
        while ((src = (RelFileLocator *) hash_seq_search(&scan)))
                *dest++ = *src;

        hash_destroy(tmphash);

terminate:
        MemSet(dest, 0, sizeof(RelFileLocator));
}

/*
 * RestorePendingSyncs
 *              Restore syncs within a parallel worker.
 *
 * RelationNeedsWAL() and RelFileLocatorSkippingWAL() must offer the correct
 * answer to parallel workers.  Only smgrDoPendingSyncs() reads the
 * is_truncated field, at end of transaction.  Hence, don't restore it.
 */
void
RestorePendingSyncs(char *startAddress)
{
        RelFileLocator *rlocator;

        Assert(pendingSyncHash == NULL);
        for (rlocator = (RelFileLocator *) startAddress; rlocator->relNumber != 0;
                 rlocator++)
                AddPendingSync(rlocator);
}

/*
 *      smgrDoPendingDeletes() -- Take care of relation deletes at end of xact.
 *
 * This also runs when aborting a subxact; we want to clean up a failed
 * subxact immediately.
 *
 * Note: It's possible that we're being asked to remove a relation that has
 * no physical storage in any fork. In particular, it's possible that we're
 * cleaning up an old temporary relation for which RemovePgTempFiles has
 * already recovered the physical storage.
 */
void
smgrDoPendingDeletes(bool isCommit)
{
        int                     nestLevel = GetCurrentTransactionNestLevel();
        PendingRelDelete *pending;
        PendingRelDelete *prev;
        PendingRelDelete *next;
        int                     nrels = 0,
                                maxrels = 0;
        SMgrRelation *srels = NULL;

        prev = NULL;
        for (pending = pendingDeletes; pending != NULL; pending = next)
        {
                next = pending->next;
                if (pending->nestLevel < nestLevel)
                {
                        /* outer-level entries should not be processed yet */
                        prev = pending;
                }
                else
                {
                        /* unlink list entry first, so we don't retry on failure */
                        if (prev)
                                prev->next = next;
                        else
                                pendingDeletes = next;
                        /* do deletion if called for */
                        if (pending->atCommit == isCommit)
                        {
                                SMgrRelation srel;

                                srel = smgropen(pending->rlocator, pending->procNumber);

                                /* allocate the initial array, or extend it, if needed */
                                if (maxrels == 0)
                                {
                                        maxrels = 8;
                                        srels = palloc(sizeof(SMgrRelation) * maxrels);
                                }
                                else if (maxrels <= nrels)
                                {
                                        maxrels *= 2;
                                        srels = repalloc(srels, sizeof(SMgrRelation) * maxrels);
                                }

                                srels[nrels++] = srel;
                        }
                        /* must explicitly free the list entry */
                        pfree(pending);
                        /* prev does not change */
                }
        }

        if (nrels > 0)
        {
                smgrdounlinkall(srels, nrels, false);

                for (int i = 0; i < nrels; i++)
                        smgrclose(srels[i]);

                pfree(srels);
        }
}

/*
 *      smgrDoPendingSyncs() -- Take care of relation syncs at end of xact.
 */
void
smgrDoPendingSyncs(bool isCommit, bool isParallelWorker)
{
        PendingRelDelete *pending;
        int                     nrels = 0,
                                maxrels = 0;
        SMgrRelation *srels = NULL;
        HASH_SEQ_STATUS scan;
        PendingRelSync *pendingsync;

        Assert(GetCurrentTransactionNestLevel() == 1);

        if (!pendingSyncHash)
                return;                                 /* no relation needs sync */

        /* Abort -- just throw away all pending syncs */
        if (!isCommit)
        {
                pendingSyncHash = NULL;
                return;
        }

        AssertPendingSyncs_RelationCache();

        /* Parallel worker -- just throw away all pending syncs */
        if (isParallelWorker)
        {
                pendingSyncHash = NULL;
                return;
        }

        /* Skip syncing nodes that smgrDoPendingDeletes() will delete. */
        for (pending = pendingDeletes; pending != NULL; pending = pending->next)
                if (pending->atCommit)
                        (void) hash_search(pendingSyncHash, &pending->rlocator,
                                                           HASH_REMOVE, NULL);

        hash_seq_init(&scan, pendingSyncHash);
        while ((pendingsync = (PendingRelSync *) hash_seq_search(&scan)))
        {
                ForkNumber      fork;
                BlockNumber nblocks[MAX_FORKNUM + 1];
                BlockNumber total_blocks = 0;
                SMgrRelation srel;

                srel = smgropen(pendingsync->rlocator, INVALID_PROC_NUMBER);

                /*
                 * We emit newpage WAL records for smaller relations.
                 *
                 * Small WAL records have a chance to be flushed along with other
                 * backends' WAL records.  We emit WAL records instead of syncing for
                 * files that are smaller than a certain threshold, expecting faster
                 * commit.  The threshold is defined by the GUC wal_skip_threshold.
                 */
                if (!pendingsync->is_truncated)
                {
                        for (fork = 0; fork <= MAX_FORKNUM; fork++)
                        {
                                if (smgrexists(srel, fork))
                                {
                                        BlockNumber n = smgrnblocks(srel, fork);

                                        /* we shouldn't come here for unlogged relations */
                                        Assert(fork != INIT_FORKNUM);
                                        nblocks[fork] = n;
                                        total_blocks += n;
                                }
                                else
                                        nblocks[fork] = InvalidBlockNumber;
                        }
                }

                /*
                 * Sync file or emit WAL records for its contents.
                 *
                 * Although we emit WAL record if the file is small enough, do file
                 * sync regardless of the size if the file has experienced a
                 * truncation. It is because the file would be followed by trailing
                 * garbage blocks after a crash recovery if, while a past longer file
                 * had been flushed out, we omitted syncing-out of the file and
                 * emitted WAL instead.  You might think that we could choose WAL if
                 * the current main fork is longer than ever, but there's a case where
                 * main fork is longer than ever but FSM fork gets shorter.
                 */
                if (pendingsync->is_truncated ||
                        total_blocks * BLCKSZ / 1024 >= wal_skip_threshold)
                {
                        /* allocate the initial array, or extend it, if needed */
                        if (maxrels == 0)
                        {
                                maxrels = 8;
                                srels = palloc(sizeof(SMgrRelation) * maxrels);
                        }
                        else if (maxrels <= nrels)
                        {
                                maxrels *= 2;
                                srels = repalloc(srels, sizeof(SMgrRelation) * maxrels);
                        }

                        srels[nrels++] = srel;
                }
                else
                {
                        /* Emit WAL records for all blocks.  The file is small enough. */
                        for (fork = 0; fork <= MAX_FORKNUM; fork++)
                        {
                                int                     n = nblocks[fork];
                                Relation        rel;

                                if (!BlockNumberIsValid(n))
                                        continue;

                                /*
                                 * Emit WAL for the whole file.  Unfortunately we don't know
                                 * what kind of a page this is, so we have to log the full
                                 * page including any unused space.  ReadBufferExtended()
                                 * counts some pgstat events; unfortunately, we discard them.
                                 */
                                rel = CreateFakeRelcacheEntry(srel->smgr_rlocator.locator);
                                log_newpage_range(rel, fork, 0, n, false);
                                FreeFakeRelcacheEntry(rel);
                        }
                }
        }

        pendingSyncHash = NULL;

        if (nrels > 0)
        {
                smgrdosyncall(srels, nrels);
                pfree(srels);
        }
}

/*
 * smgrGetPendingDeletes() -- Get a list of non-temp relations to be deleted.
 *
 * The return value is the number of relations scheduled for termination.
 * *ptr is set to point to a freshly-palloc'd array of RelFileLocators.
 * If there are no relations to be deleted, *ptr is set to NULL.
 *
 * Only non-temporary relations are included in the returned list.  This is OK
 * because the list is used only in contexts where temporary relations don't
 * matter: we're either writing to the two-phase state file (and transactions
 * that have touched temp tables can't be prepared) or we're writing to xlog
 * (and all temporary files will be zapped if we restart anyway, so no need
 * for redo to do it also).
 *
 * Note that the list does not include anything scheduled for termination
 * by upper-level transactions.
 */
int
smgrGetPendingDeletes(bool forCommit, RelFileLocator **ptr)
{
        int                     nestLevel = GetCurrentTransactionNestLevel();
        int                     nrels;
        RelFileLocator *rptr;
        PendingRelDelete *pending;

        nrels = 0;
        for (pending = pendingDeletes; pending != NULL; pending = pending->next)
        {
                if (pending->nestLevel >= nestLevel && pending->atCommit == forCommit
                        && pending->procNumber == INVALID_PROC_NUMBER)
                        nrels++;
        }
        if (nrels == 0)
        {
                *ptr = NULL;
                return 0;
        }
        rptr = (RelFileLocator *) palloc(nrels * sizeof(RelFileLocator));
        *ptr = rptr;
        for (pending = pendingDeletes; pending != NULL; pending = pending->next)
        {
                if (pending->nestLevel >= nestLevel && pending->atCommit == forCommit
                        && pending->procNumber == INVALID_PROC_NUMBER)
                {
                        *rptr = pending->rlocator;
                        rptr++;
                }
        }
        return nrels;
}

/*
 *      PostPrepare_smgr -- Clean up after a successful PREPARE
 *
 * What we have to do here is throw away the in-memory state about pending
 * relation deletes.  It's all been recorded in the 2PC state file and
 * it's no longer smgr's job to worry about it.
 */
void
PostPrepare_smgr(void)
{
        PendingRelDelete *pending;
        PendingRelDelete *next;

        for (pending = pendingDeletes; pending != NULL; pending = next)
        {
                next = pending->next;
                pendingDeletes = next;
                /* must explicitly free the list entry */
                pfree(pending);
        }
}


/*
 * AtSubCommit_smgr() --- Take care of subtransaction commit.
 *
 * Reassign all items in the pending-deletes list to the parent transaction.
 */
void
AtSubCommit_smgr(void)
{
        int                     nestLevel = GetCurrentTransactionNestLevel();
        PendingRelDelete *pending;

        for (pending = pendingDeletes; pending != NULL; pending = pending->next)
        {
                if (pending->nestLevel >= nestLevel)
                        pending->nestLevel = nestLevel - 1;
        }
}

/*
 * AtSubAbort_smgr() --- Take care of subtransaction abort.
 *
 * Delete created relations and forget about deleted relations.
 * We can execute these operations immediately because we know this
 * subtransaction will not commit.
 */
void
AtSubAbort_smgr(void)
{
        smgrDoPendingDeletes(false);
}

void
smgr_redo(XLogReaderState *record)
{
        XLogRecPtr      lsn = record->EndRecPtr;
        uint8           info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;

        /* Backup blocks are not used in smgr records */
        Assert(!XLogRecHasAnyBlockRefs(record));

        if (info == XLOG_SMGR_CREATE)
        {
                xl_smgr_create *xlrec = (xl_smgr_create *) XLogRecGetData(record);
                SMgrRelation reln;

                reln = smgropen(xlrec->rlocator, INVALID_PROC_NUMBER);
                smgrcreate(reln, xlrec->forkNum, true);
        }
        else if (info == XLOG_SMGR_TRUNCATE)
        {
                xl_smgr_truncate *xlrec = (xl_smgr_truncate *) XLogRecGetData(record);
                SMgrRelation reln;
                Relation        rel;
                ForkNumber      forks[MAX_FORKNUM];
                BlockNumber blocks[MAX_FORKNUM];
                int                     nforks = 0;
                bool            need_fsm_vacuum = false;

                reln = smgropen(xlrec->rlocator, INVALID_PROC_NUMBER);

                /*
                 * Forcibly create relation if it doesn't exist (which suggests that
                 * it was dropped somewhere later in the WAL sequence).  As in
                 * XLogReadBufferForRedo, we prefer to recreate the rel and replay the
                 * log as best we can until the drop is seen.
                 */
                smgrcreate(reln, MAIN_FORKNUM, true);

                /*
                 * Before we perform the truncation, update minimum recovery point to
                 * cover this WAL record. Once the relation is truncated, there's no
                 * going back. The buffer manager enforces the WAL-first rule for
                 * normal updates to relation files, so that the minimum recovery
                 * point is always updated before the corresponding change in the data
                 * file is flushed to disk. We have to do the same manually here.
                 *
                 * Doing this before the truncation means that if the truncation fails
                 * for some reason, you cannot start up the system even after restart,
                 * until you fix the underlying situation so that the truncation will
                 * succeed. Alternatively, we could update the minimum recovery point
                 * after truncation, but that would leave a small window where the
                 * WAL-first rule could be violated.
                 */
                XLogFlush(lsn);

                /* Prepare for truncation of MAIN fork */
                if ((xlrec->flags & SMGR_TRUNCATE_HEAP) != 0)
                {
                        forks[nforks] = MAIN_FORKNUM;
                        blocks[nforks] = xlrec->blkno;
                        nforks++;

                        /* Also tell xlogutils.c about it */
                        XLogTruncateRelation(xlrec->rlocator, MAIN_FORKNUM, xlrec->blkno);
                }

                /* Prepare for truncation of FSM and VM too */
                rel = CreateFakeRelcacheEntry(xlrec->rlocator);

                if ((xlrec->flags & SMGR_TRUNCATE_FSM) != 0 &&
                        smgrexists(reln, FSM_FORKNUM))
                {
                        blocks[nforks] = FreeSpaceMapPrepareTruncateRel(rel, xlrec->blkno);
                        if (BlockNumberIsValid(blocks[nforks]))
                        {
                                forks[nforks] = FSM_FORKNUM;
                                nforks++;
                                need_fsm_vacuum = true;
                        }
                }
                if ((xlrec->flags & SMGR_TRUNCATE_VM) != 0 &&
                        smgrexists(reln, VISIBILITYMAP_FORKNUM))
                {
                        blocks[nforks] = visibilitymap_prepare_truncate(rel, xlrec->blkno);
                        if (BlockNumberIsValid(blocks[nforks]))
                        {
                                forks[nforks] = VISIBILITYMAP_FORKNUM;
                                nforks++;
                        }
                }

                /* Do the real work to truncate relation forks */
                if (nforks > 0)
                        smgrtruncate(reln, forks, nforks, blocks);

                /*
                 * Update upper-level FSM pages to account for the truncation. This is
                 * important because the just-truncated pages were likely marked as
                 * all-free, and would be preferentially selected.
                 */
                if (need_fsm_vacuum)
                        FreeSpaceMapVacuumRange(rel, xlrec->blkno,
                                                                        InvalidBlockNumber);

                FreeFakeRelcacheEntry(rel);
        }
        else
                elog(PANIC, "smgr_redo: unknown op code %u", info);
}