brin_pageops.c

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/*
 * brin_pageops.c
 *      Page-handling routines for BRIN indexes
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/brin/brin_pageops.c
 */
#include "postgres.h"

#include "access/brin_pageops.h"
#include "access/brin_page.h"
#include "access/brin_revmap.h"
#include "access/brin_xlog.h"
#include "access/xloginsert.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
#include "utils/rel.h"


/*
 * Maximum size of an entry in a BRIN_PAGETYPE_REGULAR page.  We can tolerate
 * a single item per page, unlike other index AMs.
 */
#define BrinMaxItemSize \
    MAXALIGN_DOWN(BLCKSZ - \
                  (MAXALIGN(SizeOfPageHeaderData + \
                            sizeof(ItemIdData)) + \
                   MAXALIGN(sizeof(BrinSpecialSpace))))

static Buffer brin_getinsertbuffer(Relation irel, Buffer oldbuf, Size itemsz,
                     bool *extended);
static Size br_page_get_freespace(Page page);
static void brin_initialize_empty_new_buffer(Relation idxrel, Buffer buffer);


/*
 * Update tuple origtup (size origsz), located in offset oldoff of buffer
 * oldbuf, to newtup (size newsz) as summary tuple for the page range starting
 * at heapBlk.  oldbuf must not be locked on entry, and is not locked at exit.
 *
 * If samepage is true, attempt to put the new tuple in the same page, but if
 * there's no room, use some other one.
 *
 * If the update is successful, return true; the revmap is updated to point to
 * the new tuple.  If the update is not done for whatever reason, return false.
 * Caller may retry the update if this happens.
 */
bool
brin_doupdate(Relation idxrel, BlockNumber pagesPerRange,
              BrinRevmap *revmap, BlockNumber heapBlk,
              Buffer oldbuf, OffsetNumber oldoff,
              const BrinTuple *origtup, Size origsz,
              const BrinTuple *newtup, Size newsz,
              bool samepage)
{
    Page        oldpage;
    ItemId      oldlp;
    BrinTuple  *oldtup;
    Size        oldsz;
    Buffer      newbuf;
    BlockNumber newblk = InvalidBlockNumber;
    bool        extended;

    Assert(newsz == MAXALIGN(newsz));

    /* If the item is oversized, don't bother. */
    if (newsz > BrinMaxItemSize)
    {
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("index row size %zu exceeds maximum %zu for index \"%s\"",
                        newsz, BrinMaxItemSize, RelationGetRelationName(idxrel))));
        return false;           /* keep compiler quiet */
    }

    /* make sure the revmap is long enough to contain the entry we need */
    brinRevmapExtend(revmap, heapBlk);

    if (!samepage)
    {
        /* need a page on which to put the item */
        newbuf = brin_getinsertbuffer(idxrel, oldbuf, newsz, &extended);
        if (!BufferIsValid(newbuf))
        {
            Assert(!extended);
            return false;
        }

        /*
         * Note: it's possible (though unlikely) that the returned newbuf is
         * the same as oldbuf, if brin_getinsertbuffer determined that the old
         * buffer does in fact have enough space.
         */
        if (newbuf == oldbuf)
        {
            Assert(!extended);
            newbuf = InvalidBuffer;
        }
        else
            newblk = BufferGetBlockNumber(newbuf);
    }
    else
    {
        LockBuffer(oldbuf, BUFFER_LOCK_EXCLUSIVE);
        newbuf = InvalidBuffer;
        extended = false;
    }
    oldpage = BufferGetPage(oldbuf);
    oldlp = PageGetItemId(oldpage, oldoff);

    /*
     * Check that the old tuple wasn't updated concurrently: it might have
     * moved someplace else entirely, and for that matter the whole page
     * might've become a revmap page.  Note that in the first two cases
     * checked here, the "oldlp" we just calculated is garbage; but
     * PageGetItemId() is simple enough that it was safe to do that
     * calculation anyway.
     */
    if (!BRIN_IS_REGULAR_PAGE(oldpage) ||
        oldoff > PageGetMaxOffsetNumber(oldpage) ||
        !ItemIdIsNormal(oldlp))
    {
        LockBuffer(oldbuf, BUFFER_LOCK_UNLOCK);

        /*
         * If this happens, and the new buffer was obtained by extending the
         * relation, then we need to ensure we don't leave it uninitialized or
         * forget about it.
         */
        if (BufferIsValid(newbuf))
        {
            if (extended)
                brin_initialize_empty_new_buffer(idxrel, newbuf);
            UnlockReleaseBuffer(newbuf);
            if (extended)
                FreeSpaceMapVacuumRange(idxrel, newblk, newblk + 1);
        }
        return false;
    }

    oldsz = ItemIdGetLength(oldlp);
    oldtup = (BrinTuple *) PageGetItem(oldpage, oldlp);

    /*
     * ... or it might have been updated in place to different contents.
     */
    if (!brin_tuples_equal(oldtup, oldsz, origtup, origsz))
    {
        LockBuffer(oldbuf, BUFFER_LOCK_UNLOCK);
        if (BufferIsValid(newbuf))
        {
            /* As above, initialize and record new page if we got one */
            if (extended)
                brin_initialize_empty_new_buffer(idxrel, newbuf);
            UnlockReleaseBuffer(newbuf);
            if (extended)
                FreeSpaceMapVacuumRange(idxrel, newblk, newblk + 1);
        }
        return false;
    }

    /*
     * Great, the old tuple is intact.  We can proceed with the update.
     *
     * If there's enough room in the old page for the new tuple, replace it.
     *
     * Note that there might now be enough space on the page even though the
     * caller told us there isn't, if a concurrent update moved another tuple
     * elsewhere or replaced a tuple with a smaller one.
     */
    if (((BrinPageFlags(oldpage) & BRIN_EVACUATE_PAGE) == 0) &&
        brin_can_do_samepage_update(oldbuf, origsz, newsz))
    {
        START_CRIT_SECTION();
        if (!PageIndexTupleOverwrite(oldpage, oldoff, (Item) newtup, newsz))
            elog(ERROR, "failed to replace BRIN tuple");
        MarkBufferDirty(oldbuf);

        /* XLOG stuff */
        if (RelationNeedsWAL(idxrel))
        {
            xl_brin_samepage_update xlrec;
            XLogRecPtr  recptr;
            uint8       info = XLOG_BRIN_SAMEPAGE_UPDATE;

            xlrec.offnum = oldoff;

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

            XLogRegisterBuffer(0, oldbuf, REGBUF_STANDARD);
            XLogRegisterBufData(0, (char *) newtup, newsz);

            recptr = XLogInsert(RM_BRIN_ID, info);

            PageSetLSN(oldpage, recptr);
        }

        END_CRIT_SECTION();

        LockBuffer(oldbuf, BUFFER_LOCK_UNLOCK);

        if (BufferIsValid(newbuf))
        {
            /* As above, initialize and record new page if we got one */
            if (extended)
                brin_initialize_empty_new_buffer(idxrel, newbuf);
            UnlockReleaseBuffer(newbuf);
            if (extended)
                FreeSpaceMapVacuumRange(idxrel, newblk, newblk + 1);
        }

        return true;
    }
    else if (newbuf == InvalidBuffer)
    {
        /*
         * Not enough space, but caller said that there was. Tell them to
         * start over.
         */
        LockBuffer(oldbuf, BUFFER_LOCK_UNLOCK);
        return false;
    }
    else
    {
        /*
         * Not enough free space on the oldpage. Put the new tuple on the new
         * page, and update the revmap.
         */
        Page        newpage = BufferGetPage(newbuf);
        Buffer      revmapbuf;
        ItemPointerData newtid;
        OffsetNumber newoff;
        Size        freespace = 0;

        revmapbuf = brinLockRevmapPageForUpdate(revmap, heapBlk);

        START_CRIT_SECTION();

        /*
         * We need to initialize the page if it's newly obtained.  Note we
         * will WAL-log the initialization as part of the update, so we don't
         * need to do that here.
         */
        if (extended)
            brin_page_init(newpage, BRIN_PAGETYPE_REGULAR);

        PageIndexTupleDeleteNoCompact(oldpage, oldoff);
        newoff = PageAddItem(newpage, (Item) newtup, newsz,
                             InvalidOffsetNumber, false, false);
        if (newoff == InvalidOffsetNumber)
            elog(ERROR, "failed to add BRIN tuple to new page");
        MarkBufferDirty(oldbuf);
        MarkBufferDirty(newbuf);

        /* needed to update FSM below */
        if (extended)
            freespace = br_page_get_freespace(newpage);

        ItemPointerSet(&newtid, newblk, newoff);
        brinSetHeapBlockItemptr(revmapbuf, pagesPerRange, heapBlk, newtid);
        MarkBufferDirty(revmapbuf);

        /* XLOG stuff */
        if (RelationNeedsWAL(idxrel))
        {
            xl_brin_update xlrec;
            XLogRecPtr  recptr;
            uint8       info;

            info = XLOG_BRIN_UPDATE | (extended ? XLOG_BRIN_INIT_PAGE : 0);

            xlrec.insert.offnum = newoff;
            xlrec.insert.heapBlk = heapBlk;
            xlrec.insert.pagesPerRange = pagesPerRange;
            xlrec.oldOffnum = oldoff;

            XLogBeginInsert();

            /* new page */
            XLogRegisterData((char *) &xlrec, SizeOfBrinUpdate);

            XLogRegisterBuffer(0, newbuf, REGBUF_STANDARD | (extended ? REGBUF_WILL_INIT : 0));
            XLogRegisterBufData(0, (char *) newtup, newsz);

            /* revmap page */
            XLogRegisterBuffer(1, revmapbuf, 0);

            /* old page */
            XLogRegisterBuffer(2, oldbuf, REGBUF_STANDARD);

            recptr = XLogInsert(RM_BRIN_ID, info);

            PageSetLSN(oldpage, recptr);
            PageSetLSN(newpage, recptr);
            PageSetLSN(BufferGetPage(revmapbuf), recptr);
        }

        END_CRIT_SECTION();

        LockBuffer(revmapbuf, BUFFER_LOCK_UNLOCK);
        LockBuffer(oldbuf, BUFFER_LOCK_UNLOCK);
        UnlockReleaseBuffer(newbuf);

        if (extended)
        {
            RecordPageWithFreeSpace(idxrel, newblk, freespace);
            FreeSpaceMapVacuumRange(idxrel, newblk, newblk + 1);
        }

        return true;
    }
}

/*
 * Return whether brin_doupdate can do a samepage update.
 */
bool
brin_can_do_samepage_update(Buffer buffer, Size origsz, Size newsz)
{
    return
        ((newsz <= origsz) ||
         PageGetExactFreeSpace(BufferGetPage(buffer)) >= (newsz - origsz));
}

/*
 * Insert an index tuple into the index relation.  The revmap is updated to
 * mark the range containing the given page as pointing to the inserted entry.
 * A WAL record is written.
 *
 * The buffer, if valid, is first checked for free space to insert the new
 * entry; if there isn't enough, a new buffer is obtained and pinned.  No
 * buffer lock must be held on entry, no buffer lock is held on exit.
 *
 * Return value is the offset number where the tuple was inserted.
 */
OffsetNumber
brin_doinsert(Relation idxrel, BlockNumber pagesPerRange,
              BrinRevmap *revmap, Buffer *buffer, BlockNumber heapBlk,
              BrinTuple *tup, Size itemsz)
{
    Page        page;
    BlockNumber blk;
    OffsetNumber off;
    Size        freespace = 0;
    Buffer      revmapbuf;
    ItemPointerData tid;
    bool        extended;

    Assert(itemsz == MAXALIGN(itemsz));

    /* If the item is oversized, don't even bother. */
    if (itemsz > BrinMaxItemSize)
    {
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                 errmsg("index row size %zu exceeds maximum %zu for index \"%s\"",
                        itemsz, BrinMaxItemSize, RelationGetRelationName(idxrel))));
        return InvalidOffsetNumber; /* keep compiler quiet */
    }

    /* Make sure the revmap is long enough to contain the entry we need */
    brinRevmapExtend(revmap, heapBlk);

    /*
     * Acquire lock on buffer supplied by caller, if any.  If it doesn't have
     * enough space, unpin it to obtain a new one below.
     */
    if (BufferIsValid(*buffer))
    {
        /*
         * It's possible that another backend (or ourselves!) extended the
         * revmap over the page we held a pin on, so we cannot assume that
         * it's still a regular page.
         */
        LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
        if (br_page_get_freespace(BufferGetPage(*buffer)) < itemsz)
        {
            UnlockReleaseBuffer(*buffer);
            *buffer = InvalidBuffer;
        }
    }

    /*
     * If we still don't have a usable buffer, have brin_getinsertbuffer
     * obtain one for us.
     */
    if (!BufferIsValid(*buffer))
    {
        do
            *buffer = brin_getinsertbuffer(idxrel, InvalidBuffer, itemsz, &extended);
        while (!BufferIsValid(*buffer));
    }
    else
        extended = false;

    /* Now obtain lock on revmap buffer */
    revmapbuf = brinLockRevmapPageForUpdate(revmap, heapBlk);

    page = BufferGetPage(*buffer);
    blk = BufferGetBlockNumber(*buffer);

    /* Execute the actual insertion */
    START_CRIT_SECTION();
    if (extended)
        brin_page_init(page, BRIN_PAGETYPE_REGULAR);
    off = PageAddItem(page, (Item) tup, itemsz, InvalidOffsetNumber,
                      false, false);
    if (off == InvalidOffsetNumber)
        elog(ERROR, "failed to add BRIN tuple to new page");
    MarkBufferDirty(*buffer);

    /* needed to update FSM below */
    if (extended)
        freespace = br_page_get_freespace(page);

    ItemPointerSet(&tid, blk, off);
    brinSetHeapBlockItemptr(revmapbuf, pagesPerRange, heapBlk, tid);
    MarkBufferDirty(revmapbuf);

    /* XLOG stuff */
    if (RelationNeedsWAL(idxrel))
    {
        xl_brin_insert xlrec;
        XLogRecPtr  recptr;
        uint8       info;

        info = XLOG_BRIN_INSERT | (extended ? XLOG_BRIN_INIT_PAGE : 0);
        xlrec.heapBlk = heapBlk;
        xlrec.pagesPerRange = pagesPerRange;
        xlrec.offnum = off;

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

        XLogRegisterBuffer(0, *buffer, REGBUF_STANDARD | (extended ? REGBUF_WILL_INIT : 0));
        XLogRegisterBufData(0, (char *) tup, itemsz);

        XLogRegisterBuffer(1, revmapbuf, 0);

        recptr = XLogInsert(RM_BRIN_ID, info);

        PageSetLSN(page, recptr);
        PageSetLSN(BufferGetPage(revmapbuf), recptr);
    }

    END_CRIT_SECTION();

    /* Tuple is firmly on buffer; we can release our locks */
    LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
    LockBuffer(revmapbuf, BUFFER_LOCK_UNLOCK);

    BRIN_elog((DEBUG2, "inserted tuple (%u,%u) for range starting at %u",
               blk, off, heapBlk));

    if (extended)
    {
        RecordPageWithFreeSpace(idxrel, blk, freespace);
        FreeSpaceMapVacuumRange(idxrel, blk, blk + 1);
    }

    return off;
}

/*
 * Initialize a page with the given type.
 *
 * Caller is responsible for marking it dirty, as appropriate.
 */
void
brin_page_init(Page page, uint16 type)
{
    PageInit(page, BLCKSZ, sizeof(BrinSpecialSpace));

    BrinPageType(page) = type;
}

/*
 * Initialize a new BRIN index's metapage.
 */
void
brin_metapage_init(Page page, BlockNumber pagesPerRange, uint16 version)
{
    BrinMetaPageData *metadata;

    brin_page_init(page, BRIN_PAGETYPE_META);

    metadata = (BrinMetaPageData *) PageGetContents(page);

    metadata->brinMagic = BRIN_META_MAGIC;
    metadata->brinVersion = version;
    metadata->pagesPerRange = pagesPerRange;

    /*
     * Note we cheat here a little.  0 is not a valid revmap block number
     * (because it's the metapage buffer), but doing this enables the first
     * revmap page to be created when the index is.
     */
    metadata->lastRevmapPage = 0;

    /*
     * Set pd_lower just past the end of the metadata.  This is essential,
     * because without doing so, metadata will be lost if xlog.c compresses
     * the page.
     */
    ((PageHeader) page)->pd_lower =
        ((char *) metadata + sizeof(BrinMetaPageData)) - (char *) page;
}

/*
 * Initiate page evacuation protocol.
 *
 * The page must be locked in exclusive mode by the caller.
 *
 * If the page is not yet initialized or empty, return false without doing
 * anything; it can be used for revmap without any further changes.  If it
 * contains tuples, mark it for evacuation and return true.
 */
bool
brin_start_evacuating_page(Relation idxRel, Buffer buf)
{
    OffsetNumber off;
    OffsetNumber maxoff;
    Page        page;

    page = BufferGetPage(buf);

    if (PageIsNew(page))
        return false;

    maxoff = PageGetMaxOffsetNumber(page);
    for (off = FirstOffsetNumber; off <= maxoff; off++)
    {
        ItemId      lp;

        lp = PageGetItemId(page, off);
        if (ItemIdIsUsed(lp))
        {
            /* prevent other backends from adding more stuff to this page */
            BrinPageFlags(page) |= BRIN_EVACUATE_PAGE;
            MarkBufferDirtyHint(buf, true);

            return true;
        }
    }
    return false;
}

/*
 * Move all tuples out of a page.
 *
 * The caller must hold lock on the page. The lock and pin are released.
 */
void
brin_evacuate_page(Relation idxRel, BlockNumber pagesPerRange,
                   BrinRevmap *revmap, Buffer buf)
{
    OffsetNumber off;
    OffsetNumber maxoff;
    Page        page;
    BrinTuple  *btup = NULL;
    Size        btupsz = 0;

    page = BufferGetPage(buf);

    Assert(BrinPageFlags(page) & BRIN_EVACUATE_PAGE);

    maxoff = PageGetMaxOffsetNumber(page);
    for (off = FirstOffsetNumber; off <= maxoff; off++)
    {
        BrinTuple  *tup;
        Size        sz;
        ItemId      lp;

        CHECK_FOR_INTERRUPTS();

        lp = PageGetItemId(page, off);
        if (ItemIdIsUsed(lp))
        {
            sz = ItemIdGetLength(lp);
            tup = (BrinTuple *) PageGetItem(page, lp);
            tup = brin_copy_tuple(tup, sz, btup, &btupsz);

            LockBuffer(buf, BUFFER_LOCK_UNLOCK);

            if (!brin_doupdate(idxRel, pagesPerRange, revmap, tup->bt_blkno,
                               buf, off, tup, sz, tup, sz, false))
                off--;          /* retry */

            LockBuffer(buf, BUFFER_LOCK_SHARE);

            /* It's possible that someone extended the revmap over this page */
            if (!BRIN_IS_REGULAR_PAGE(page))
                break;
        }
    }

    UnlockReleaseBuffer(buf);
}

/*
 * Given a BRIN index page, initialize it if necessary, and record its
 * current free space in the FSM.
 *
 * The main use for this is when, during vacuuming, an uninitialized page is
 * found, which could be the result of relation extension followed by a crash
 * before the page can be used.
 *
 * Here, we don't bother to update upper FSM pages, instead expecting that our
 * caller (brin_vacuum_scan) will fix them at the end of the scan.  Elsewhere
 * in this file, it's generally a good idea to propagate additions of free
 * space into the upper FSM pages immediately.
 */
void
brin_page_cleanup(Relation idxrel, Buffer buf)
{
    Page        page = BufferGetPage(buf);

    /*
     * If a page was left uninitialized, initialize it now; also record it in
     * FSM.
     *
     * Somebody else might be extending the relation concurrently.  To avoid
     * re-initializing the page before they can grab the buffer lock, we
     * acquire the extension lock momentarily.  Since they hold the extension
     * lock from before getting the page and after its been initialized, we're
     * sure to see their initialization.
     */
    if (PageIsNew(page))
    {
        LockRelationForExtension(idxrel, ShareLock);
        UnlockRelationForExtension(idxrel, ShareLock);

        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
        if (PageIsNew(page))
        {
            brin_initialize_empty_new_buffer(idxrel, buf);
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
            return;
        }
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
    }

    /* Nothing to be done for non-regular index pages */
    if (BRIN_IS_META_PAGE(BufferGetPage(buf)) ||
        BRIN_IS_REVMAP_PAGE(BufferGetPage(buf)))
        return;

    /* Measure free space and record it */
    RecordPageWithFreeSpace(idxrel, BufferGetBlockNumber(buf),
                            br_page_get_freespace(page));
}

/*
 * Return a pinned and exclusively locked buffer which can be used to insert an
 * index item of size itemsz (caller must ensure not to request sizes
 * impossible to fulfill).  If oldbuf is a valid buffer, it is also locked (in
 * an order determined to avoid deadlocks).
 *
 * If we find that the old page is no longer a regular index page (because
 * of a revmap extension), the old buffer is unlocked and we return
 * InvalidBuffer.
 *
 * If there's no existing page with enough free space to accommodate the new
 * item, the relation is extended.  If this happens, *extended is set to true,
 * and it is the caller's responsibility to initialize the page (and WAL-log
 * that fact) prior to use.  The caller should also update the FSM with the
 * page's remaining free space after the insertion.
 *
 * Note that the caller is not expected to update FSM unless *extended is set
 * true.  This policy means that we'll update FSM when a page is created, and
 * when it's found to have too little space for a desired tuple insertion,
 * but not every single time we add a tuple to the page.
 *
 * Note that in some corner cases it is possible for this routine to extend
 * the relation and then not return the new page.  It is this routine's
 * responsibility to WAL-log the page initialization and to record the page in
 * FSM if that happens, since the caller certainly can't do it.
 */
static Buffer
brin_getinsertbuffer(Relation irel, Buffer oldbuf, Size itemsz,
                     bool *extended)
{
    BlockNumber oldblk;
    BlockNumber newblk;
    Page        page;
    Size        freespace;

    /* callers must have checked */
    Assert(itemsz <= BrinMaxItemSize);

    if (BufferIsValid(oldbuf))
        oldblk = BufferGetBlockNumber(oldbuf);
    else
        oldblk = InvalidBlockNumber;

    /* Choose initial target page, re-using existing target if known */
    newblk = RelationGetTargetBlock(irel);
    if (newblk == InvalidBlockNumber)
        newblk = GetPageWithFreeSpace(irel, itemsz);

    /*
     * Loop until we find a page with sufficient free space.  By the time we
     * return to caller out of this loop, both buffers are valid and locked;
     * if we have to restart here, neither page is locked and newblk isn't
     * pinned (if it's even valid).
     */
    for (;;)
    {
        Buffer      buf;
        bool        extensionLockHeld = false;

        CHECK_FOR_INTERRUPTS();

        *extended = false;

        if (newblk == InvalidBlockNumber)
        {
            /*
             * There's not enough free space in any existing index page,
             * according to the FSM: extend the relation to obtain a shiny new
             * page.
             */
            if (!RELATION_IS_LOCAL(irel))
            {
                LockRelationForExtension(irel, ExclusiveLock);
                extensionLockHeld = true;
            }
            buf = ReadBuffer(irel, P_NEW);
            newblk = BufferGetBlockNumber(buf);
            *extended = true;

            BRIN_elog((DEBUG2, "brin_getinsertbuffer: extending to page %u",
                       BufferGetBlockNumber(buf)));
        }
        else if (newblk == oldblk)
        {
            /*
             * There's an odd corner-case here where the FSM is out-of-date,
             * and gave us the old page.
             */
            buf = oldbuf;
        }
        else
        {
            buf = ReadBuffer(irel, newblk);
        }

        /*
         * We lock the old buffer first, if it's earlier than the new one; but
         * then we need to check that it hasn't been turned into a revmap page
         * concurrently.  If we detect that that happened, give up and tell
         * caller to start over.
         */
        if (BufferIsValid(oldbuf) && oldblk < newblk)
        {
            LockBuffer(oldbuf, BUFFER_LOCK_EXCLUSIVE);
            if (!BRIN_IS_REGULAR_PAGE(BufferGetPage(oldbuf)))
            {
                LockBuffer(oldbuf, BUFFER_LOCK_UNLOCK);

                /*
                 * It is possible that the new page was obtained from
                 * extending the relation.  In that case, we must be sure to
                 * record it in the FSM before leaving, because otherwise the
                 * space would be lost forever.  However, we cannot let an
                 * uninitialized page get in the FSM, so we need to initialize
                 * it first.
                 */
                if (*extended)
                    brin_initialize_empty_new_buffer(irel, buf);

                if (extensionLockHeld)
                    UnlockRelationForExtension(irel, ExclusiveLock);

                ReleaseBuffer(buf);

                if (*extended)
                {
                    FreeSpaceMapVacuumRange(irel, newblk, newblk + 1);
                    /* shouldn't matter, but don't confuse caller */
                    *extended = false;
                }

                return InvalidBuffer;
            }
        }

        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

        if (extensionLockHeld)
            UnlockRelationForExtension(irel, ExclusiveLock);

        page = BufferGetPage(buf);

        /*
         * We have a new buffer to insert into.  Check that the new page has
         * enough free space, and return it if it does; otherwise start over.
         * (br_page_get_freespace also checks that the FSM didn't hand us a
         * page that has since been repurposed for the revmap.)
         */
        freespace = *extended ?
            BrinMaxItemSize : br_page_get_freespace(page);
        if (freespace >= itemsz)
        {
            RelationSetTargetBlock(irel, newblk);

            /*
             * Lock the old buffer if not locked already.  Note that in this
             * case we know for sure it's a regular page: it's later than the
             * new page we just got, which is not a revmap page, and revmap
             * pages are always consecutive.
             */
            if (BufferIsValid(oldbuf) && oldblk > newblk)
            {
                LockBuffer(oldbuf, BUFFER_LOCK_EXCLUSIVE);
                Assert(BRIN_IS_REGULAR_PAGE(BufferGetPage(oldbuf)));
            }

            return buf;
        }

        /* This page is no good. */

        /*
         * If an entirely new page does not contain enough free space for the
         * new item, then surely that item is oversized.  Complain loudly; but
         * first make sure we initialize the page and record it as free, for
         * next time.
         */
        if (*extended)
        {
            brin_initialize_empty_new_buffer(irel, buf);
            /* since this should not happen, skip FreeSpaceMapVacuum */

            ereport(ERROR,
                    (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                     errmsg("index row size %zu exceeds maximum %zu for index \"%s\"",
                            itemsz, freespace, RelationGetRelationName(irel))));
            return InvalidBuffer;   /* keep compiler quiet */
        }

        if (newblk != oldblk)
            UnlockReleaseBuffer(buf);
        if (BufferIsValid(oldbuf) && oldblk <= newblk)
            LockBuffer(oldbuf, BUFFER_LOCK_UNLOCK);

        /*
         * Update the FSM with the new, presumably smaller, freespace value
         * for this page, then search for a new target page.
         */
        newblk = RecordAndGetPageWithFreeSpace(irel, newblk, freespace, itemsz);
    }
}

/*
 * Initialize a page as an empty regular BRIN page, WAL-log this, and record
 * the page in FSM.
 *
 * There are several corner situations in which we extend the relation to
 * obtain a new page and later find that we cannot use it immediately.  When
 * that happens, we don't want to leave the page go unrecorded in FSM, because
 * there is no mechanism to get the space back and the index would bloat.
 * Also, because we would not WAL-log the action that would initialize the
 * page, the page would go uninitialized in a standby (or after recovery).
 *
 * While we record the page in FSM here, caller is responsible for doing FSM
 * upper-page update if that seems appropriate.
 */
static void
brin_initialize_empty_new_buffer(Relation idxrel, Buffer buffer)
{
    Page        page;

    BRIN_elog((DEBUG2,
               "brin_initialize_empty_new_buffer: initializing blank page %u",
               BufferGetBlockNumber(buffer)));

    START_CRIT_SECTION();
    page = BufferGetPage(buffer);
    brin_page_init(page, BRIN_PAGETYPE_REGULAR);
    MarkBufferDirty(buffer);
    log_newpage_buffer(buffer, true);
    END_CRIT_SECTION();

    /*
     * We update the FSM for this page, but this is not WAL-logged.  This is
     * acceptable because VACUUM will scan the index and update the FSM with
     * pages whose FSM records were forgotten in a crash.
     */
    RecordPageWithFreeSpace(idxrel, BufferGetBlockNumber(buffer),
                            br_page_get_freespace(page));
}


/*
 * Return the amount of free space on a regular BRIN index page.
 *
 * If the page is not a regular page, or has been marked with the
 * BRIN_EVACUATE_PAGE flag, returns 0.
 */
static Size
br_page_get_freespace(Page page)
{
    if (!BRIN_IS_REGULAR_PAGE(page) ||
        (BrinPageFlags(page) & BRIN_EVACUATE_PAGE) != 0)
        return 0;
    else
        return PageGetFreeSpace(page);
}