storage.c

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/*-------------------------------------------------------------------------
 *
 * storage.c
 *    code to create and destroy physical storage for relations
 *
 * Portions Copyright (c) 1996-2021, 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/parallel.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/freespace.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 RelFileNode 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
{
    RelFileNode relnode;        /* relation that may need to be deleted */
    BackendId   backend;        /* InvalidBackendId 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
{
    RelFileNode rnode;
    bool        is_truncated;   /* Has the file experienced truncation? */
} PendingRelSync;

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


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

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

        ctl.keysize = sizeof(RelFileNode);
        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, rnode, 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.
 */
SMgrRelation
RelationCreateStorage(RelFileNode rnode, char relpersistence)
{
    PendingRelDelete *pending;
    SMgrRelation srel;
    BackendId   backend;
    bool        needs_wal;

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

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

    srel = smgropen(rnode, backend);
    smgrcreate(srel, MAIN_FORKNUM, false);

    if (needs_wal)
        log_smgrcreate(&srel->smgr_rnode.node, MAIN_FORKNUM);

    /* Add the relation to the list of stuff to delete at abort */
    pending = (PendingRelDelete *)
        MemoryContextAlloc(TopMemoryContext, sizeof(PendingRelDelete));
    pending->relnode = rnode;
    pending->backend = backend;
    pending->atCommit = false;  /* delete if abort */
    pending->nestLevel = GetCurrentTransactionNestLevel();
    pending->next = pendingDeletes;
    pendingDeletes = pending;

    if (relpersistence == RELPERSISTENCE_PERMANENT && !XLogIsNeeded())
    {
        Assert(backend == InvalidBackendId);
        AddPendingSync(&rnode);
    }

    return srel;
}

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

    /*
     * Make an XLOG entry reporting the file creation.
     */
    xlrec.rnode = *rnode;
    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->relnode = rel->rd_node;
    pending->backend = 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(RelFileNode rnode, bool atCommit)
{
    PendingRelDelete *pending;
    PendingRelDelete *prev;
    PendingRelDelete *next;

    prev = NULL;
    for (pending = pendingDeletes; pending != NULL; pending = next)
    {
        next = pending->next;
        if (RelFileNodeEquals(rnode, pending->relnode)
            && 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);

    /*
     * 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.rnode = rel->rd_node;
        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);
    }

    /* Do the real work to truncate relation forks */
    smgrtruncate(RelationGetSmgr(rel), 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, 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_rnode.node),
                          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)
{
    PGAlignedBlock buf;
    Page        page;
    bool        use_wal;
    bool        copying_initfork;
    BlockNumber nblocks;
    BlockNumber blkno;

    page = (Page) buf.data;

    /*
     * 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 a new relfilenode.
     */
    use_wal = XLogIsNeeded() &&
        (relpersistence == RELPERSISTENCE_PERMANENT || copying_initfork);

    nblocks = smgrnblocks(src, forkNum);

    for (blkno = 0; blkno < nblocks; blkno++)
    {
        /* If we got a cancel signal during the copy of the data, quit */
        CHECK_FOR_INTERRUPTS();

        smgrread(src, forkNum, blkno, buf.data);

        if (!PageIsVerifiedExtended(page, 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_rnode.node,
                                                 src->smgr_rnode.backend,
                                                 forkNum);

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

        /*
         * WAL-log the copied page. Unfortunately we don't know what kind of a
         * page this is, so we have to log the full page including any unused
         * space.
         */
        if (use_wal)
            log_newpage(&dst->smgr_rnode.node, forkNum, blkno, page, false);

        PageSetChecksumInplace(page, blkno);

        /*
         * Now write the page.  We say skipFsync = true because there's no
         * need for smgr to schedule an fsync for this write; we'll do it
         * ourselves below.
         */
        smgrextend(dst, forkNum, blkno, buf.data, true);
    }

    /*
     * When we WAL-logged rel pages, we must nonetheless fsync them.  The
     * reason is that since we're copying outside shared buffers, a CHECKPOINT
     * occurring during the copy has no way to flush the previously written
     * data to disk (indeed it won't know the new rel even exists).  A crash
     * later on would replay WAL from the checkpoint, therefore it wouldn't
     * replay our earlier WAL entries. If we do not fsync those pages here,
     * they might still not be on disk when the crash occurs.
     */
    if (use_wal || copying_initfork)
        smgrimmedsync(dst, forkNum);
}

/*
 * RelFileNodeSkippingWAL
 *      Check if a BM_PERMANENT relfilenode is using WAL.
 *
 * Changes of certain relfilenodes must not write WAL; see "Skipping WAL for
 * New RelFileNode" 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
RelFileNodeSkippingWAL(RelFileNode rnode)
{
    if (!pendingSyncHash ||
        hash_search(pendingSyncHash, &rnode, 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(RelFileNode));
}

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

    if (!pendingSyncHash)
        goto terminate;

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

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

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

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

    hash_destroy(tmphash);

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

/*
 * RestorePendingSyncs
 *      Restore syncs within a parallel worker.
 *
 * RelationNeedsWAL() and RelFileNodeSkippingWAL() 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)
{
    RelFileNode *rnode;

    Assert(pendingSyncHash == NULL);
    for (rnode = (RelFileNode *) startAddress; rnode->relNode != 0; rnode++)
        AddPendingSync(rnode);
}

/*
 *  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->relnode, pending->backend);

                /* 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, (void *) &pending->relnode,
                               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->rnode, InvalidBackendId);

        /*
         * We emit newpage WAL records for smaller relations.
         *
         * Small WAL records have a chance to be emitted 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_rnode.node);
                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 RelFileNodes.
 * 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, RelFileNode **ptr)
{
    int         nestLevel = GetCurrentTransactionNestLevel();
    int         nrels;
    RelFileNode *rptr;
    PendingRelDelete *pending;

    nrels = 0;
    for (pending = pendingDeletes; pending != NULL; pending = pending->next)
    {
        if (pending->nestLevel >= nestLevel && pending->atCommit == forCommit
            && pending->backend == InvalidBackendId)
            nrels++;
    }
    if (nrels == 0)
    {
        *ptr = NULL;
        return 0;
    }
    rptr = (RelFileNode *) palloc(nrels * sizeof(RelFileNode));
    *ptr = rptr;
    for (pending = pendingDeletes; pending != NULL; pending = pending->next)
    {
        if (pending->nestLevel >= nestLevel && pending->atCommit == forCommit
            && pending->backend == InvalidBackendId)
        {
            *rptr = pending->relnode;
            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->rnode, InvalidBackendId);
        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->rnode, InvalidBackendId);

        /*
         * 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->rnode, MAIN_FORKNUM, xlrec->blkno);
        }

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

        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);
}