nbtpage.c

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/*-------------------------------------------------------------------------
 *
 * nbtpage.c
 *    BTree-specific page management code for the Postgres btree access
 *    method.
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/nbtree/nbtpage.c
 *
 *  NOTES
 *     Postgres btree pages look like ordinary relation pages.  The opaque
 *     data at high addresses includes pointers to left and right siblings
 *     and flag data describing page state.  The first page in a btree, page
 *     zero, is special -- it stores meta-information describing the tree.
 *     Pages one and higher store the actual tree data.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/nbtree.h"
#include "access/transam.h"
#include "miscadmin.h"
#include "storage/indexfsm.h"
#include "storage/lmgr.h"
#include "storage/predicate.h"
#include "utils/inval.h"
#include "utils/snapmgr.h"


/*
 *  _bt_initmetapage() -- Fill a page buffer with a correct metapage image
 */
void
_bt_initmetapage(Page page, BlockNumber rootbknum, uint32 level)
{
    BTMetaPageData *metad;
    BTPageOpaque metaopaque;

    _bt_pageinit(page, BLCKSZ);

    metad = BTPageGetMeta(page);
    metad->btm_magic = BTREE_MAGIC;
    metad->btm_version = BTREE_VERSION;
    metad->btm_root = rootbknum;
    metad->btm_level = level;
    metad->btm_fastroot = rootbknum;
    metad->btm_fastlevel = level;

    metaopaque = (BTPageOpaque) PageGetSpecialPointer(page);
    metaopaque->btpo_flags = BTP_META;

    /*
     * Set pd_lower just past the end of the metadata.  This is not essential
     * but it makes the page look compressible to xlog.c.
     */
    ((PageHeader) page)->pd_lower =
        ((char *) metad + sizeof(BTMetaPageData)) - (char *) page;
}

/*
 *  _bt_getroot() -- Get the root page of the btree.
 *
 *      Since the root page can move around the btree file, we have to read
 *      its location from the metadata page, and then read the root page
 *      itself.  If no root page exists yet, we have to create one.  The
 *      standard class of race conditions exists here; I think I covered
 *      them all in the Hopi Indian rain dance of lock requests below.
 *
 *      The access type parameter (BT_READ or BT_WRITE) controls whether
 *      a new root page will be created or not.  If access = BT_READ,
 *      and no root page exists, we just return InvalidBuffer.  For
 *      BT_WRITE, we try to create the root page if it doesn't exist.
 *      NOTE that the returned root page will have only a read lock set
 *      on it even if access = BT_WRITE!
 *
 *      The returned page is not necessarily the true root --- it could be
 *      a "fast root" (a page that is alone in its level due to deletions).
 *      Also, if the root page is split while we are "in flight" to it,
 *      what we will return is the old root, which is now just the leftmost
 *      page on a probably-not-very-wide level.  For most purposes this is
 *      as good as or better than the true root, so we do not bother to
 *      insist on finding the true root.  We do, however, guarantee to
 *      return a live (not deleted or half-dead) page.
 *
 *      On successful return, the root page is pinned and read-locked.
 *      The metadata page is not locked or pinned on exit.
 */
Buffer
_bt_getroot(Relation rel, int access)
{
    Buffer      metabuf;
    Page        metapg;
    BTPageOpaque metaopaque;
    Buffer      rootbuf;
    Page        rootpage;
    BTPageOpaque rootopaque;
    BlockNumber rootblkno;
    uint32      rootlevel;
    BTMetaPageData *metad;

    /*
     * Try to use previously-cached metapage data to find the root.  This
     * normally saves one buffer access per index search, which is a very
     * helpful savings in bufmgr traffic and hence contention.
     */
    if (rel->rd_amcache != NULL)
    {
        metad = (BTMetaPageData *) rel->rd_amcache;
        /* We shouldn't have cached it if any of these fail */
        Assert(metad->btm_magic == BTREE_MAGIC);
        Assert(metad->btm_version == BTREE_VERSION);
        Assert(metad->btm_root != P_NONE);

        rootblkno = metad->btm_fastroot;
        Assert(rootblkno != P_NONE);
        rootlevel = metad->btm_fastlevel;

        rootbuf = _bt_getbuf(rel, rootblkno, BT_READ);
        rootpage = BufferGetPage(rootbuf);
        rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);

        /*
         * Since the cache might be stale, we check the page more carefully
         * here than normal.  We *must* check that it's not deleted. If it's
         * not alone on its level, then we reject too --- this may be overly
         * paranoid but better safe than sorry.  Note we don't check P_ISROOT,
         * because that's not set in a "fast root".
         */
        if (!P_IGNORE(rootopaque) &&
            rootopaque->btpo.level == rootlevel &&
            P_LEFTMOST(rootopaque) &&
            P_RIGHTMOST(rootopaque))
        {
            /* OK, accept cached page as the root */
            return rootbuf;
        }
        _bt_relbuf(rel, rootbuf);
        /* Cache is stale, throw it away */
        if (rel->rd_amcache)
            pfree(rel->rd_amcache);
        rel->rd_amcache = NULL;
    }

    metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
    metapg = BufferGetPage(metabuf);
    metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
    metad = BTPageGetMeta(metapg);

    /* sanity-check the metapage */
    if (!(metaopaque->btpo_flags & BTP_META) ||
        metad->btm_magic != BTREE_MAGIC)
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("index \"%s\" is not a btree",
                        RelationGetRelationName(rel))));

    if (metad->btm_version != BTREE_VERSION)
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("version mismatch in index \"%s\": file version %d, code version %d",
                        RelationGetRelationName(rel),
                        metad->btm_version, BTREE_VERSION)));

    /* if no root page initialized yet, do it */
    if (metad->btm_root == P_NONE)
    {
        /* If access = BT_READ, caller doesn't want us to create root yet */
        if (access == BT_READ)
        {
            _bt_relbuf(rel, metabuf);
            return InvalidBuffer;
        }

        /* trade in our read lock for a write lock */
        LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
        LockBuffer(metabuf, BT_WRITE);

        /*
         * Race condition:  if someone else initialized the metadata between
         * the time we released the read lock and acquired the write lock, we
         * must avoid doing it again.
         */
        if (metad->btm_root != P_NONE)
        {
            /*
             * Metadata initialized by someone else.  In order to guarantee no
             * deadlocks, we have to release the metadata page and start all
             * over again.  (Is that really true? But it's hardly worth trying
             * to optimize this case.)
             */
            _bt_relbuf(rel, metabuf);
            return _bt_getroot(rel, access);
        }

        /*
         * Get, initialize, write, and leave a lock of the appropriate type on
         * the new root page.  Since this is the first page in the tree, it's
         * a leaf as well as the root.
         */
        rootbuf = _bt_getbuf(rel, P_NEW, BT_WRITE);
        rootblkno = BufferGetBlockNumber(rootbuf);
        rootpage = BufferGetPage(rootbuf);
        rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);
        rootopaque->btpo_prev = rootopaque->btpo_next = P_NONE;
        rootopaque->btpo_flags = (BTP_LEAF | BTP_ROOT);
        rootopaque->btpo.level = 0;
        rootopaque->btpo_cycleid = 0;

        /* NO ELOG(ERROR) till meta is updated */
        START_CRIT_SECTION();

        metad->btm_root = rootblkno;
        metad->btm_level = 0;
        metad->btm_fastroot = rootblkno;
        metad->btm_fastlevel = 0;

        MarkBufferDirty(rootbuf);
        MarkBufferDirty(metabuf);

        /* XLOG stuff */
        if (RelationNeedsWAL(rel))
        {
            xl_btree_newroot xlrec;
            XLogRecPtr  recptr;
            XLogRecData rdata;

            xlrec.node = rel->rd_node;
            xlrec.rootblk = rootblkno;
            xlrec.level = 0;

            rdata.data = (char *) &xlrec;
            rdata.len = SizeOfBtreeNewroot;
            rdata.buffer = InvalidBuffer;
            rdata.next = NULL;

            recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_NEWROOT, &rdata);

            PageSetLSN(rootpage, recptr);
            PageSetLSN(metapg, recptr);
        }

        END_CRIT_SECTION();

        /*
         * Send out relcache inval for metapage change (probably unnecessary
         * here, but let's be safe).
         */
        CacheInvalidateRelcache(rel);

        /*
         * swap root write lock for read lock.  There is no danger of anyone
         * else accessing the new root page while it's unlocked, since no one
         * else knows where it is yet.
         */
        LockBuffer(rootbuf, BUFFER_LOCK_UNLOCK);
        LockBuffer(rootbuf, BT_READ);

        /* okay, metadata is correct, release lock on it */
        _bt_relbuf(rel, metabuf);
    }
    else
    {
        rootblkno = metad->btm_fastroot;
        Assert(rootblkno != P_NONE);
        rootlevel = metad->btm_fastlevel;

        /*
         * Cache the metapage data for next time
         */
        rel->rd_amcache = MemoryContextAlloc(rel->rd_indexcxt,
                                             sizeof(BTMetaPageData));
        memcpy(rel->rd_amcache, metad, sizeof(BTMetaPageData));

        /*
         * We are done with the metapage; arrange to release it via first
         * _bt_relandgetbuf call
         */
        rootbuf = metabuf;

        for (;;)
        {
            rootbuf = _bt_relandgetbuf(rel, rootbuf, rootblkno, BT_READ);
            rootpage = BufferGetPage(rootbuf);
            rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);

            if (!P_IGNORE(rootopaque))
                break;

            /* it's dead, Jim.  step right one page */
            if (P_RIGHTMOST(rootopaque))
                elog(ERROR, "no live root page found in index \"%s\"",
                     RelationGetRelationName(rel));
            rootblkno = rootopaque->btpo_next;
        }

        /* Note: can't check btpo.level on deleted pages */
        if (rootopaque->btpo.level != rootlevel)
            elog(ERROR, "root page %u of index \"%s\" has level %u, expected %u",
                 rootblkno, RelationGetRelationName(rel),
                 rootopaque->btpo.level, rootlevel);
    }

    /*
     * By here, we have a pin and read lock on the root page, and no lock set
     * on the metadata page.  Return the root page's buffer.
     */
    return rootbuf;
}

/*
 *  _bt_gettrueroot() -- Get the true root page of the btree.
 *
 *      This is the same as the BT_READ case of _bt_getroot(), except
 *      we follow the true-root link not the fast-root link.
 *
 * By the time we acquire lock on the root page, it might have been split and
 * not be the true root anymore.  This is okay for the present uses of this
 * routine; we only really need to be able to move up at least one tree level
 * from whatever non-root page we were at.  If we ever do need to lock the
 * one true root page, we could loop here, re-reading the metapage on each
 * failure.  (Note that it wouldn't do to hold the lock on the metapage while
 * moving to the root --- that'd deadlock against any concurrent root split.)
 */
Buffer
_bt_gettrueroot(Relation rel)
{
    Buffer      metabuf;
    Page        metapg;
    BTPageOpaque metaopaque;
    Buffer      rootbuf;
    Page        rootpage;
    BTPageOpaque rootopaque;
    BlockNumber rootblkno;
    uint32      rootlevel;
    BTMetaPageData *metad;

    /*
     * We don't try to use cached metapage data here, since (a) this path is
     * not performance-critical, and (b) if we are here it suggests our cache
     * is out-of-date anyway.  In light of point (b), it's probably safest to
     * actively flush any cached metapage info.
     */
    if (rel->rd_amcache)
        pfree(rel->rd_amcache);
    rel->rd_amcache = NULL;

    metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
    metapg = BufferGetPage(metabuf);
    metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
    metad = BTPageGetMeta(metapg);

    if (!(metaopaque->btpo_flags & BTP_META) ||
        metad->btm_magic != BTREE_MAGIC)
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("index \"%s\" is not a btree",
                        RelationGetRelationName(rel))));

    if (metad->btm_version != BTREE_VERSION)
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("version mismatch in index \"%s\": file version %d, code version %d",
                        RelationGetRelationName(rel),
                        metad->btm_version, BTREE_VERSION)));

    /* if no root page initialized yet, fail */
    if (metad->btm_root == P_NONE)
    {
        _bt_relbuf(rel, metabuf);
        return InvalidBuffer;
    }

    rootblkno = metad->btm_root;
    rootlevel = metad->btm_level;

    /*
     * We are done with the metapage; arrange to release it via first
     * _bt_relandgetbuf call
     */
    rootbuf = metabuf;

    for (;;)
    {
        rootbuf = _bt_relandgetbuf(rel, rootbuf, rootblkno, BT_READ);
        rootpage = BufferGetPage(rootbuf);
        rootopaque = (BTPageOpaque) PageGetSpecialPointer(rootpage);

        if (!P_IGNORE(rootopaque))
            break;

        /* it's dead, Jim.  step right one page */
        if (P_RIGHTMOST(rootopaque))
            elog(ERROR, "no live root page found in index \"%s\"",
                 RelationGetRelationName(rel));
        rootblkno = rootopaque->btpo_next;
    }

    /* Note: can't check btpo.level on deleted pages */
    if (rootopaque->btpo.level != rootlevel)
        elog(ERROR, "root page %u of index \"%s\" has level %u, expected %u",
             rootblkno, RelationGetRelationName(rel),
             rootopaque->btpo.level, rootlevel);

    return rootbuf;
}

/*
 *  _bt_getrootheight() -- Get the height of the btree search tree.
 *
 *      We return the level (counting from zero) of the current fast root.
 *      This represents the number of tree levels we'd have to descend through
 *      to start any btree index search.
 *
 *      This is used by the planner for cost-estimation purposes.  Since it's
 *      only an estimate, slightly-stale data is fine, hence we don't worry
 *      about updating previously cached data.
 */
int
_bt_getrootheight(Relation rel)
{
    BTMetaPageData *metad;

    /*
     * We can get what we need from the cached metapage data.  If it's not
     * cached yet, load it.  Sanity checks here must match _bt_getroot().
     */
    if (rel->rd_amcache == NULL)
    {
        Buffer      metabuf;
        Page        metapg;
        BTPageOpaque metaopaque;

        metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_READ);
        metapg = BufferGetPage(metabuf);
        metaopaque = (BTPageOpaque) PageGetSpecialPointer(metapg);
        metad = BTPageGetMeta(metapg);

        /* sanity-check the metapage */
        if (!(metaopaque->btpo_flags & BTP_META) ||
            metad->btm_magic != BTREE_MAGIC)
            ereport(ERROR,
                    (errcode(ERRCODE_INDEX_CORRUPTED),
                     errmsg("index \"%s\" is not a btree",
                            RelationGetRelationName(rel))));

        if (metad->btm_version != BTREE_VERSION)
            ereport(ERROR,
                    (errcode(ERRCODE_INDEX_CORRUPTED),
                     errmsg("version mismatch in index \"%s\": file version %d, code version %d",
                            RelationGetRelationName(rel),
                            metad->btm_version, BTREE_VERSION)));

        /*
         * If there's no root page yet, _bt_getroot() doesn't expect a cache
         * to be made, so just stop here and report the index height is zero.
         * (XXX perhaps _bt_getroot() should be changed to allow this case.)
         */
        if (metad->btm_root == P_NONE)
        {
            _bt_relbuf(rel, metabuf);
            return 0;
        }

        /*
         * Cache the metapage data for next time
         */
        rel->rd_amcache = MemoryContextAlloc(rel->rd_indexcxt,
                                             sizeof(BTMetaPageData));
        memcpy(rel->rd_amcache, metad, sizeof(BTMetaPageData));

        _bt_relbuf(rel, metabuf);
    }

    metad = (BTMetaPageData *) rel->rd_amcache;
    /* We shouldn't have cached it if any of these fail */
    Assert(metad->btm_magic == BTREE_MAGIC);
    Assert(metad->btm_version == BTREE_VERSION);
    Assert(metad->btm_fastroot != P_NONE);

    return metad->btm_fastlevel;
}

/*
 *  _bt_checkpage() -- Verify that a freshly-read page looks sane.
 */
void
_bt_checkpage(Relation rel, Buffer buf)
{
    Page        page = BufferGetPage(buf);

    /*
     * ReadBuffer verifies that every newly-read page passes
     * PageHeaderIsValid, which means it either contains a reasonably sane
     * page header or is all-zero.  We have to defend against the all-zero
     * case, however.
     */
    if (PageIsNew(page))
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
             errmsg("index \"%s\" contains unexpected zero page at block %u",
                    RelationGetRelationName(rel),
                    BufferGetBlockNumber(buf)),
                 errhint("Please REINDEX it.")));

    /*
     * Additionally check that the special area looks sane.
     */
    if (PageGetSpecialSize(page) != MAXALIGN(sizeof(BTPageOpaqueData)))
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("index \"%s\" contains corrupted page at block %u",
                        RelationGetRelationName(rel),
                        BufferGetBlockNumber(buf)),
                 errhint("Please REINDEX it.")));
}

/*
 * Log the reuse of a page from the FSM.
 */
static void
_bt_log_reuse_page(Relation rel, BlockNumber blkno, TransactionId latestRemovedXid)
{
    if (!RelationNeedsWAL(rel))
        return;

    /* No ereport(ERROR) until changes are logged */
    START_CRIT_SECTION();

    /*
     * We don't do MarkBufferDirty here because we're about to initialise
     * the page, and nobody else can see it yet.
     */

    /* XLOG stuff */
    {
        XLogRecData rdata[1];
        xl_btree_reuse_page xlrec_reuse;

        xlrec_reuse.node = rel->rd_node;
        xlrec_reuse.block = blkno;
        xlrec_reuse.latestRemovedXid = latestRemovedXid;
        rdata[0].data = (char *) &xlrec_reuse;
        rdata[0].len = SizeOfBtreeReusePage;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = NULL;

        XLogInsert(RM_BTREE_ID, XLOG_BTREE_REUSE_PAGE, rdata);

        /*
         * We don't do PageSetLSN here because we're about to initialise
         * the page, so no need.
         */
    }

    END_CRIT_SECTION();
}

/*
 *  _bt_getbuf() -- Get a buffer by block number for read or write.
 *
 *      blkno == P_NEW means to get an unallocated index page.  The page
 *      will be initialized before returning it.
 *
 *      When this routine returns, the appropriate lock is set on the
 *      requested buffer and its reference count has been incremented
 *      (ie, the buffer is "locked and pinned").  Also, we apply
 *      _bt_checkpage to sanity-check the page (except in P_NEW case).
 */
Buffer
_bt_getbuf(Relation rel, BlockNumber blkno, int access)
{
    Buffer      buf;

    if (blkno != P_NEW)
    {
        /* Read an existing block of the relation */
        buf = ReadBuffer(rel, blkno);
        LockBuffer(buf, access);
        _bt_checkpage(rel, buf);
    }
    else
    {
        bool        needLock;
        Page        page;

        Assert(access == BT_WRITE);

        /*
         * First see if the FSM knows of any free pages.
         *
         * We can't trust the FSM's report unreservedly; we have to check that
         * the page is still free.  (For example, an already-free page could
         * have been re-used between the time the last VACUUM scanned it and
         * the time the VACUUM made its FSM updates.)
         *
         * In fact, it's worse than that: we can't even assume that it's safe
         * to take a lock on the reported page.  If somebody else has a lock
         * on it, or even worse our own caller does, we could deadlock.  (The
         * own-caller scenario is actually not improbable. Consider an index
         * on a serial or timestamp column.  Nearly all splits will be at the
         * rightmost page, so it's entirely likely that _bt_split will call us
         * while holding a lock on the page most recently acquired from FSM. A
         * VACUUM running concurrently with the previous split could well have
         * placed that page back in FSM.)
         *
         * To get around that, we ask for only a conditional lock on the
         * reported page.  If we fail, then someone else is using the page,
         * and we may reasonably assume it's not free.  (If we happen to be
         * wrong, the worst consequence is the page will be lost to use till
         * the next VACUUM, which is no big problem.)
         */
        for (;;)
        {
            blkno = GetFreeIndexPage(rel);
            if (blkno == InvalidBlockNumber)
                break;
            buf = ReadBuffer(rel, blkno);
            if (ConditionalLockBuffer(buf))
            {
                page = BufferGetPage(buf);
                if (_bt_page_recyclable(page))
                {
                    /*
                     * If we are generating WAL for Hot Standby then create a
                     * WAL record that will allow us to conflict with queries
                     * running on standby.
                     */
                    if (XLogStandbyInfoActive())
                    {
                        BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);

                        _bt_log_reuse_page(rel, blkno, opaque->btpo.xact);
                    }

                    /* Okay to use page.  Re-initialize and return it */
                    _bt_pageinit(page, BufferGetPageSize(buf));
                    return buf;
                }
                elog(DEBUG2, "FSM returned nonrecyclable page");
                _bt_relbuf(rel, buf);
            }
            else
            {
                elog(DEBUG2, "FSM returned nonlockable page");
                /* couldn't get lock, so just drop pin */
                ReleaseBuffer(buf);
            }
        }

        /*
         * Extend the relation by one page.
         *
         * We have to use a lock to ensure no one else is extending the rel at
         * the same time, else we will both try to initialize the same new
         * page.  We can skip locking for new or temp relations, however,
         * since no one else could be accessing them.
         */
        needLock = !RELATION_IS_LOCAL(rel);

        if (needLock)
            LockRelationForExtension(rel, ExclusiveLock);

        buf = ReadBuffer(rel, P_NEW);

        /* Acquire buffer lock on new page */
        LockBuffer(buf, BT_WRITE);

        /*
         * Release the file-extension lock; it's now OK for someone else to
         * extend the relation some more.  Note that we cannot release this
         * lock before we have buffer lock on the new page, or we risk a race
         * condition against btvacuumscan --- see comments therein.
         */
        if (needLock)
            UnlockRelationForExtension(rel, ExclusiveLock);

        /* Initialize the new page before returning it */
        page = BufferGetPage(buf);
        Assert(PageIsNew(page));
        _bt_pageinit(page, BufferGetPageSize(buf));
    }

    /* ref count and lock type are correct */
    return buf;
}

/*
 *  _bt_relandgetbuf() -- release a locked buffer and get another one.
 *
 * This is equivalent to _bt_relbuf followed by _bt_getbuf, with the
 * exception that blkno may not be P_NEW.  Also, if obuf is InvalidBuffer
 * then it reduces to just _bt_getbuf; allowing this case simplifies some
 * callers.
 *
 * The original motivation for using this was to avoid two entries to the
 * bufmgr when one would do.  However, now it's mainly just a notational
 * convenience.  The only case where it saves work over _bt_relbuf/_bt_getbuf
 * is when the target page is the same one already in the buffer.
 */
Buffer
_bt_relandgetbuf(Relation rel, Buffer obuf, BlockNumber blkno, int access)
{
    Buffer      buf;

    Assert(blkno != P_NEW);
    if (BufferIsValid(obuf))
        LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
    buf = ReleaseAndReadBuffer(obuf, rel, blkno);
    LockBuffer(buf, access);
    _bt_checkpage(rel, buf);
    return buf;
}

/*
 *  _bt_relbuf() -- release a locked buffer.
 *
 * Lock and pin (refcount) are both dropped.
 */
void
_bt_relbuf(Relation rel, Buffer buf)
{
    UnlockReleaseBuffer(buf);
}

/*
 *  _bt_pageinit() -- Initialize a new page.
 *
 * On return, the page header is initialized; data space is empty;
 * special space is zeroed out.
 */
void
_bt_pageinit(Page page, Size size)
{
    PageInit(page, size, sizeof(BTPageOpaqueData));
}

/*
 *  _bt_page_recyclable() -- Is an existing page recyclable?
 *
 * This exists to make sure _bt_getbuf and btvacuumscan have the same
 * policy about whether a page is safe to re-use.
 */
bool
_bt_page_recyclable(Page page)
{
    BTPageOpaque opaque;

    /*
     * It's possible to find an all-zeroes page in an index --- for example, a
     * backend might successfully extend the relation one page and then crash
     * before it is able to make a WAL entry for adding the page. If we find a
     * zeroed page then reclaim it.
     */
    if (PageIsNew(page))
        return true;

    /*
     * Otherwise, recycle if deleted and too old to have any processes
     * interested in it.
     */
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    if (P_ISDELETED(opaque) &&
        TransactionIdPrecedes(opaque->btpo.xact, RecentGlobalXmin))
        return true;
    return false;
}

/*
 * Delete item(s) from a btree page during VACUUM.
 *
 * This must only be used for deleting leaf items.  Deleting an item on a
 * non-leaf page has to be done as part of an atomic action that includes
 * deleting the page it points to.
 *
 * This routine assumes that the caller has pinned and locked the buffer.
 * Also, the given itemnos *must* appear in increasing order in the array.
 *
 * We record VACUUMs and b-tree deletes differently in WAL. InHotStandby
 * we need to be able to pin all of the blocks in the btree in physical
 * order when replaying the effects of a VACUUM, just as we do for the
 * original VACUUM itself. lastBlockVacuumed allows us to tell whether an
 * intermediate range of blocks has had no changes at all by VACUUM,
 * and so must be scanned anyway during replay. We always write a WAL record
 * for the last block in the index, whether or not it contained any items
 * to be removed. This allows us to scan right up to end of index to
 * ensure correct locking.
 */
void
_bt_delitems_vacuum(Relation rel, Buffer buf,
                    OffsetNumber *itemnos, int nitems,
                    BlockNumber lastBlockVacuumed)
{
    Page        page = BufferGetPage(buf);
    BTPageOpaque opaque;

    /* No ereport(ERROR) until changes are logged */
    START_CRIT_SECTION();

    /* Fix the page */
    if (nitems > 0)
        PageIndexMultiDelete(page, itemnos, nitems);

    /*
     * We can clear the vacuum cycle ID since this page has certainly been
     * processed by the current vacuum scan.
     */
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    opaque->btpo_cycleid = 0;

    /*
     * Mark the page as not containing any LP_DEAD items.  This is not
     * certainly true (there might be some that have recently been marked, but
     * weren't included in our target-item list), but it will almost always be
     * true and it doesn't seem worth an additional page scan to check it.
     * Remember that BTP_HAS_GARBAGE is only a hint anyway.
     */
    opaque->btpo_flags &= ~BTP_HAS_GARBAGE;

    MarkBufferDirty(buf);

    /* XLOG stuff */
    if (RelationNeedsWAL(rel))
    {
        XLogRecPtr  recptr;
        XLogRecData rdata[2];
        xl_btree_vacuum xlrec_vacuum;

        xlrec_vacuum.node = rel->rd_node;
        xlrec_vacuum.block = BufferGetBlockNumber(buf);

        xlrec_vacuum.lastBlockVacuumed = lastBlockVacuumed;
        rdata[0].data = (char *) &xlrec_vacuum;
        rdata[0].len = SizeOfBtreeVacuum;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);

        /*
         * The target-offsets array is not in the buffer, but pretend that it
         * is.  When XLogInsert stores the whole buffer, the offsets array
         * need not be stored too.
         */
        if (nitems > 0)
        {
            rdata[1].data = (char *) itemnos;
            rdata[1].len = nitems * sizeof(OffsetNumber);
        }
        else
        {
            rdata[1].data = NULL;
            rdata[1].len = 0;
        }
        rdata[1].buffer = buf;
        rdata[1].buffer_std = true;
        rdata[1].next = NULL;

        recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_VACUUM, rdata);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();
}

/*
 * Delete item(s) from a btree page during single-page cleanup.
 *
 * As above, must only be used on leaf pages.
 *
 * This routine assumes that the caller has pinned and locked the buffer.
 * Also, the given itemnos *must* appear in increasing order in the array.
 *
 * This is nearly the same as _bt_delitems_vacuum as far as what it does to
 * the page, but the WAL logging considerations are quite different.  See
 * comments for _bt_delitems_vacuum.
 */
void
_bt_delitems_delete(Relation rel, Buffer buf,
                    OffsetNumber *itemnos, int nitems,
                    Relation heapRel)
{
    Page        page = BufferGetPage(buf);
    BTPageOpaque opaque;

    /* Shouldn't be called unless there's something to do */
    Assert(nitems > 0);

    /* No ereport(ERROR) until changes are logged */
    START_CRIT_SECTION();

    /* Fix the page */
    PageIndexMultiDelete(page, itemnos, nitems);

    /*
     * Unlike _bt_delitems_vacuum, we *must not* clear the vacuum cycle ID,
     * because this is not called by VACUUM.
     */

    /*
     * Mark the page as not containing any LP_DEAD items.  This is not
     * certainly true (there might be some that have recently been marked, but
     * weren't included in our target-item list), but it will almost always be
     * true and it doesn't seem worth an additional page scan to check it.
     * Remember that BTP_HAS_GARBAGE is only a hint anyway.
     */
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    opaque->btpo_flags &= ~BTP_HAS_GARBAGE;

    MarkBufferDirty(buf);

    /* XLOG stuff */
    if (RelationNeedsWAL(rel))
    {
        XLogRecPtr  recptr;
        XLogRecData rdata[3];
        xl_btree_delete xlrec_delete;

        xlrec_delete.node = rel->rd_node;
        xlrec_delete.hnode = heapRel->rd_node;
        xlrec_delete.block = BufferGetBlockNumber(buf);
        xlrec_delete.nitems = nitems;

        rdata[0].data = (char *) &xlrec_delete;
        rdata[0].len = SizeOfBtreeDelete;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);

        /*
         * We need the target-offsets array whether or not we store the whole
         * buffer, to allow us to find the latestRemovedXid on a standby
         * server.
         */
        rdata[1].data = (char *) itemnos;
        rdata[1].len = nitems * sizeof(OffsetNumber);
        rdata[1].buffer = InvalidBuffer;
        rdata[1].next = &(rdata[2]);

        rdata[2].data = NULL;
        rdata[2].len = 0;
        rdata[2].buffer = buf;
        rdata[2].buffer_std = true;
        rdata[2].next = NULL;

        recptr = XLogInsert(RM_BTREE_ID, XLOG_BTREE_DELETE, rdata);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();
}

/*
 * Subroutine to pre-check whether a page deletion is safe, that is, its
 * parent page would be left in a valid or deletable state.
 *
 * "target" is the page we wish to delete, and "stack" is a search stack
 * leading to it (approximately).  Note that we will update the stack
 * entry(s) to reflect current downlink positions --- this is harmless and
 * indeed saves later search effort in _bt_pagedel.
 *
 * Note: it's OK to release page locks after checking, because a safe
 * deletion can't become unsafe due to concurrent activity.  A non-rightmost
 * page cannot become rightmost unless there's a concurrent page deletion,
 * but only VACUUM does page deletion and we only allow one VACUUM on an index
 * at a time.  An only child could acquire a sibling (of the same parent) only
 * by being split ... but that would make it a non-rightmost child so the
 * deletion is still safe.
 */
static bool
_bt_parent_deletion_safe(Relation rel, BlockNumber target, BTStack stack)
{
    BlockNumber parent;
    OffsetNumber poffset,
                maxoff;
    Buffer      pbuf;
    Page        page;
    BTPageOpaque opaque;

    /*
     * In recovery mode, assume the deletion being replayed is valid.  We
     * can't always check it because we won't have a full search stack, and we
     * should complain if there's a problem, anyway.
     */
    if (InRecovery)
        return true;

    /* Locate the parent's downlink (updating the stack entry if needed) */
    ItemPointerSet(&(stack->bts_btentry.t_tid), target, P_HIKEY);
    pbuf = _bt_getstackbuf(rel, stack, BT_READ);
    if (pbuf == InvalidBuffer)
        elog(ERROR, "failed to re-find parent key in index \"%s\" for deletion target page %u",
             RelationGetRelationName(rel), target);
    parent = stack->bts_blkno;
    poffset = stack->bts_offset;

    page = BufferGetPage(pbuf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    maxoff = PageGetMaxOffsetNumber(page);

    /*
     * If the target is the rightmost child of its parent, then we can't
     * delete, unless it's also the only child.
     */
    if (poffset >= maxoff)
    {
        /* It's rightmost child... */
        if (poffset == P_FIRSTDATAKEY(opaque))
        {
            /*
             * It's only child, so safe if parent would itself be removable.
             * We have to check the parent itself, and then recurse to test
             * the conditions at the parent's parent.
             */
            if (P_RIGHTMOST(opaque) || P_ISROOT(opaque))
            {
                _bt_relbuf(rel, pbuf);
                return false;
            }

            _bt_relbuf(rel, pbuf);
            return _bt_parent_deletion_safe(rel, parent, stack->bts_parent);
        }
        else
        {
            /* Unsafe to delete */
            _bt_relbuf(rel, pbuf);
            return false;
        }
    }
    else
    {
        /* Not rightmost child, so safe to delete */
        _bt_relbuf(rel, pbuf);
        return true;
    }
}

/*
 * _bt_pagedel() -- Delete a page from the b-tree, if legal to do so.
 *
 * This action unlinks the page from the b-tree structure, removing all
 * pointers leading to it --- but not touching its own left and right links.
 * The page cannot be physically reclaimed right away, since other processes
 * may currently be trying to follow links leading to the page; they have to
 * be allowed to use its right-link to recover.  See nbtree/README.
 *
 * On entry, the target buffer must be pinned and locked (either read or write
 * lock is OK).  This lock and pin will be dropped before exiting.
 *
 * The "stack" argument can be a search stack leading (approximately) to the
 * target page, or NULL --- outside callers typically pass NULL since they
 * have not done such a search, but internal recursion cases pass the stack
 * to avoid duplicated search effort.
 *
 * Returns the number of pages successfully deleted (zero if page cannot
 * be deleted now; could be more than one if parent pages were deleted too).
 *
 * NOTE: this leaks memory.  Rather than trying to clean up everything
 * carefully, it's better to run it in a temp context that can be reset
 * frequently.
 */
int
_bt_pagedel(Relation rel, Buffer buf, BTStack stack)
{
    int         result;
    BlockNumber target,
                leftsib,
                rightsib,
                parent;
    OffsetNumber poffset,
                maxoff;
    uint32      targetlevel,
                ilevel;
    ItemId      itemid;
    IndexTuple  targetkey,
                itup;
    ScanKey     itup_scankey;
    Buffer      lbuf,
                rbuf,
                pbuf;
    bool        parent_half_dead;
    bool        parent_one_child;
    bool        rightsib_empty;
    Buffer      metabuf = InvalidBuffer;
    Page        metapg = NULL;
    BTMetaPageData *metad = NULL;
    Page        page;
    BTPageOpaque opaque;

    /*
     * We can never delete rightmost pages nor root pages.  While at it, check
     * that page is not already deleted and is empty.
     */
    page = BufferGetPage(buf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    if (P_RIGHTMOST(opaque) || P_ISROOT(opaque) || P_ISDELETED(opaque) ||
        P_FIRSTDATAKEY(opaque) <= PageGetMaxOffsetNumber(page))
    {
        /* Should never fail to delete a half-dead page */
        Assert(!P_ISHALFDEAD(opaque));

        _bt_relbuf(rel, buf);
        return 0;
    }

    /*
     * Save info about page, including a copy of its high key (it must have
     * one, being non-rightmost).
     */
    target = BufferGetBlockNumber(buf);
    targetlevel = opaque->btpo.level;
    leftsib = opaque->btpo_prev;
    itemid = PageGetItemId(page, P_HIKEY);
    targetkey = CopyIndexTuple((IndexTuple) PageGetItem(page, itemid));

    /*
     * To avoid deadlocks, we'd better drop the target page lock before going
     * further.
     */
    _bt_relbuf(rel, buf);

    /*
     * We need an approximate pointer to the page's parent page.  We use the
     * standard search mechanism to search for the page's high key; this will
     * give us a link to either the current parent or someplace to its left
     * (if there are multiple equal high keys).  In recursion cases, the
     * caller already generated a search stack and we can just re-use that
     * work.
     */
    if (stack == NULL)
    {
        if (!InRecovery)
        {
            /* we need an insertion scan key to do our search, so build one */
            itup_scankey = _bt_mkscankey(rel, targetkey);
            /* find the leftmost leaf page containing this key */
            stack = _bt_search(rel, rel->rd_rel->relnatts, itup_scankey, false,
                               &lbuf, BT_READ);
            /* don't need a pin on that either */
            _bt_relbuf(rel, lbuf);

            /*
             * If we are trying to delete an interior page, _bt_search did
             * more than we needed.  Locate the stack item pointing to our
             * parent level.
             */
            ilevel = 0;
            for (;;)
            {
                if (stack == NULL)
                    elog(ERROR, "not enough stack items");
                if (ilevel == targetlevel)
                    break;
                stack = stack->bts_parent;
                ilevel++;
            }
        }
        else
        {
            /*
             * During WAL recovery, we can't use _bt_search (for one reason,
             * it might invoke user-defined comparison functions that expect
             * facilities not available in recovery mode).  Instead, just set
             * up a dummy stack pointing to the left end of the parent tree
             * level, from which _bt_getstackbuf will walk right to the parent
             * page.  Painful, but we don't care too much about performance in
             * this scenario.
             */
            pbuf = _bt_get_endpoint(rel, targetlevel + 1, false);
            stack = (BTStack) palloc(sizeof(BTStackData));
            stack->bts_blkno = BufferGetBlockNumber(pbuf);
            stack->bts_offset = InvalidOffsetNumber;
            /* bts_btentry will be initialized below */
            stack->bts_parent = NULL;
            _bt_relbuf(rel, pbuf);
        }
    }

    /*
     * We cannot delete a page that is the rightmost child of its immediate
     * parent, unless it is the only child --- in which case the parent has to
     * be deleted too, and the same condition applies recursively to it. We
     * have to check this condition all the way up before trying to delete. We
     * don't need to re-test when deleting a non-leaf page, though.
     */
    if (targetlevel == 0 &&
        !_bt_parent_deletion_safe(rel, target, stack))
        return 0;

    /*
     * We have to lock the pages we need to modify in the standard order:
     * moving right, then up.  Else we will deadlock against other writers.
     *
     * So, we need to find and write-lock the current left sibling of the
     * target page.  The sibling that was current a moment ago could have
     * split, so we may have to move right.  This search could fail if either
     * the sibling or the target page was deleted by someone else meanwhile;
     * if so, give up.  (Right now, that should never happen, since page
     * deletion is only done in VACUUM and there shouldn't be multiple VACUUMs
     * concurrently on the same table.)
     */
    if (leftsib != P_NONE)
    {
        lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
        page = BufferGetPage(lbuf);
        opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        while (P_ISDELETED(opaque) || opaque->btpo_next != target)
        {
            /* step right one page */
            leftsib = opaque->btpo_next;
            _bt_relbuf(rel, lbuf);
            if (leftsib == P_NONE)
            {
                elog(LOG, "no left sibling (concurrent deletion?) in \"%s\"",
                     RelationGetRelationName(rel));
                return 0;
            }
            lbuf = _bt_getbuf(rel, leftsib, BT_WRITE);
            page = BufferGetPage(lbuf);
            opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        }
    }
    else
        lbuf = InvalidBuffer;

    /*
     * Next write-lock the target page itself.  It should be okay to take just
     * a write lock not a superexclusive lock, since no scans would stop on an
     * empty page.
     */
    buf = _bt_getbuf(rel, target, BT_WRITE);
    page = BufferGetPage(buf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);

    /*
     * Check page is still empty etc, else abandon deletion.  The empty check
     * is necessary since someone else might have inserted into it while we
     * didn't have it locked; the others are just for paranoia's sake.
     */
    if (P_RIGHTMOST(opaque) || P_ISROOT(opaque) || P_ISDELETED(opaque) ||
        P_FIRSTDATAKEY(opaque) <= PageGetMaxOffsetNumber(page))
    {
        _bt_relbuf(rel, buf);
        if (BufferIsValid(lbuf))
            _bt_relbuf(rel, lbuf);
        return 0;
    }
    if (opaque->btpo_prev != leftsib)
        elog(ERROR, "left link changed unexpectedly in block %u of index \"%s\"",
             target, RelationGetRelationName(rel));

    /*
     * And next write-lock the (current) right sibling.
     */
    rightsib = opaque->btpo_next;
    rbuf = _bt_getbuf(rel, rightsib, BT_WRITE);
    page = BufferGetPage(rbuf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    if (opaque->btpo_prev != target)
        elog(ERROR, "right sibling's left-link doesn't match: "
             "block %u links to %u instead of expected %u in index \"%s\"",
             rightsib, opaque->btpo_prev, target,
             RelationGetRelationName(rel));

    /*
     * Any insert which would have gone on the target block will now go to the
     * right sibling block.
     */
    PredicateLockPageCombine(rel, target, rightsib);

    /*
     * Next find and write-lock the current parent of the target page. This is
     * essentially the same as the corresponding step of splitting.
     */
    ItemPointerSet(&(stack->bts_btentry.t_tid), target, P_HIKEY);
    pbuf = _bt_getstackbuf(rel, stack, BT_WRITE);
    if (pbuf == InvalidBuffer)
        elog(ERROR, "failed to re-find parent key in index \"%s\" for deletion target page %u",
             RelationGetRelationName(rel), target);
    parent = stack->bts_blkno;
    poffset = stack->bts_offset;

    /*
     * If the target is the rightmost child of its parent, then we can't
     * delete, unless it's also the only child --- in which case the parent
     * changes to half-dead status.  The "can't delete" case should have been
     * detected by _bt_parent_deletion_safe, so complain if we see it now.
     */
    page = BufferGetPage(pbuf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    maxoff = PageGetMaxOffsetNumber(page);
    parent_half_dead = false;
    parent_one_child = false;
    if (poffset >= maxoff)
    {
        if (poffset == P_FIRSTDATAKEY(opaque))
            parent_half_dead = true;
        else
            elog(ERROR, "failed to delete rightmost child %u of block %u in index \"%s\"",
                 target, parent, RelationGetRelationName(rel));
    }
    else
    {
        /* Will there be exactly one child left in this parent? */
        if (OffsetNumberNext(P_FIRSTDATAKEY(opaque)) == maxoff)
            parent_one_child = true;
    }

    /*
     * If we are deleting the next-to-last page on the target's level, then
     * the rightsib is a candidate to become the new fast root. (In theory, it
     * might be possible to push the fast root even further down, but the odds
     * of doing so are slim, and the locking considerations daunting.)
     *
     * We don't support handling this in the case where the parent is becoming
     * half-dead, even though it theoretically could occur.
     *
     * We can safely acquire a lock on the metapage here --- see comments for
     * _bt_newroot().
     */
    if (leftsib == P_NONE && !parent_half_dead)
    {
        page = BufferGetPage(rbuf);
        opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        Assert(opaque->btpo.level == targetlevel);
        if (P_RIGHTMOST(opaque))
        {
            /* rightsib will be the only one left on the level */
            metabuf = _bt_getbuf(rel, BTREE_METAPAGE, BT_WRITE);
            metapg = BufferGetPage(metabuf);
            metad = BTPageGetMeta(metapg);

            /*
             * The expected case here is btm_fastlevel == targetlevel+1; if
             * the fastlevel is <= targetlevel, something is wrong, and we
             * choose to overwrite it to fix it.
             */
            if (metad->btm_fastlevel > targetlevel + 1)
            {
                /* no update wanted */
                _bt_relbuf(rel, metabuf);
                metabuf = InvalidBuffer;
            }
        }
    }

    /*
     * Check that the parent-page index items we're about to delete/overwrite
     * contain what we expect.  This can fail if the index has become corrupt
     * for some reason.  We want to throw any error before entering the
     * critical section --- otherwise it'd be a PANIC.
     *
     * The test on the target item is just an Assert because _bt_getstackbuf
     * should have guaranteed it has the expected contents.  The test on the
     * next-child downlink is known to sometimes fail in the field, though.
     */
    page = BufferGetPage(pbuf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);

#ifdef USE_ASSERT_CHECKING
    itemid = PageGetItemId(page, poffset);
    itup = (IndexTuple) PageGetItem(page, itemid);
    Assert(ItemPointerGetBlockNumber(&(itup->t_tid)) == target);
#endif

    if (!parent_half_dead)
    {
        OffsetNumber nextoffset;

        nextoffset = OffsetNumberNext(poffset);
        itemid = PageGetItemId(page, nextoffset);
        itup = (IndexTuple) PageGetItem(page, itemid);
        if (ItemPointerGetBlockNumber(&(itup->t_tid)) != rightsib)
            elog(ERROR, "right sibling %u of block %u is not next child %u of block %u in index \"%s\"",
                 rightsib, target, ItemPointerGetBlockNumber(&(itup->t_tid)),
                 parent, RelationGetRelationName(rel));
    }

    /*
     * Here we begin doing the deletion.
     */

    /* No ereport(ERROR) until changes are logged */
    START_CRIT_SECTION();

    /*
     * Update parent.  The normal case is a tad tricky because we want to
     * delete the target's downlink and the *following* key.  Easiest way is
     * to copy the right sibling's downlink over the target downlink, and then
     * delete the following item.
     */
    if (parent_half_dead)
    {
        PageIndexTupleDelete(page, poffset);
        opaque->btpo_flags |= BTP_HALF_DEAD;
    }
    else
    {
        OffsetNumber nextoffset;

        itemid = PageGetItemId(page, poffset);
        itup = (IndexTuple) PageGetItem(page, itemid);
        ItemPointerSet(&(itup->t_tid), rightsib, P_HIKEY);

        nextoffset = OffsetNumberNext(poffset);
        PageIndexTupleDelete(page, nextoffset);
    }

    /*
     * Update siblings' side-links.  Note the target page's side-links will
     * continue to point to the siblings.  Asserts here are just rechecking
     * things we already verified above.
     */
    if (BufferIsValid(lbuf))
    {
        page = BufferGetPage(lbuf);
        opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        Assert(opaque->btpo_next == target);
        opaque->btpo_next = rightsib;
    }
    page = BufferGetPage(rbuf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    Assert(opaque->btpo_prev == target);
    opaque->btpo_prev = leftsib;
    rightsib_empty = (P_FIRSTDATAKEY(opaque) > PageGetMaxOffsetNumber(page));

    /*
     * Mark the page itself deleted.  It can be recycled when all current
     * transactions are gone.  Storing GetTopTransactionId() would work, but
     * we're in VACUUM and would not otherwise have an XID.  Having already
     * updated links to the target, ReadNewTransactionId() suffices as an
     * upper bound.  Any scan having retained a now-stale link is advertising
     * in its PGXACT an xmin less than or equal to the value we read here.  It
     * will continue to do so, holding back RecentGlobalXmin, for the duration
     * of that scan.
     */
    page = BufferGetPage(buf);
    opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    opaque->btpo_flags &= ~BTP_HALF_DEAD;
    opaque->btpo_flags |= BTP_DELETED;
    opaque->btpo.xact = ReadNewTransactionId();

    /* And update the metapage, if needed */
    if (BufferIsValid(metabuf))
    {
        metad->btm_fastroot = rightsib;
        metad->btm_fastlevel = targetlevel;
        MarkBufferDirty(metabuf);
    }

    /* Must mark buffers dirty before XLogInsert */
    MarkBufferDirty(pbuf);
    MarkBufferDirty(rbuf);
    MarkBufferDirty(buf);
    if (BufferIsValid(lbuf))
        MarkBufferDirty(lbuf);

    /* XLOG stuff */
    if (RelationNeedsWAL(rel))
    {
        xl_btree_delete_page xlrec;
        xl_btree_metadata xlmeta;
        uint8       xlinfo;
        XLogRecPtr  recptr;
        XLogRecData rdata[5];
        XLogRecData *nextrdata;

        xlrec.target.node = rel->rd_node;
        ItemPointerSet(&(xlrec.target.tid), parent, poffset);
        xlrec.deadblk = target;
        xlrec.leftblk = leftsib;
        xlrec.rightblk = rightsib;
        xlrec.btpo_xact = opaque->btpo.xact;

        rdata[0].data = (char *) &xlrec;
        rdata[0].len = SizeOfBtreeDeletePage;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = nextrdata = &(rdata[1]);

        if (BufferIsValid(metabuf))
        {
            xlmeta.root = metad->btm_root;
            xlmeta.level = metad->btm_level;
            xlmeta.fastroot = metad->btm_fastroot;
            xlmeta.fastlevel = metad->btm_fastlevel;

            nextrdata->data = (char *) &xlmeta;
            nextrdata->len = sizeof(xl_btree_metadata);
            nextrdata->buffer = InvalidBuffer;
            nextrdata->next = nextrdata + 1;
            nextrdata++;
            xlinfo = XLOG_BTREE_DELETE_PAGE_META;
        }
        else if (parent_half_dead)
            xlinfo = XLOG_BTREE_DELETE_PAGE_HALF;
        else
            xlinfo = XLOG_BTREE_DELETE_PAGE;

        nextrdata->data = NULL;
        nextrdata->len = 0;
        nextrdata->next = nextrdata + 1;
        nextrdata->buffer = pbuf;
        nextrdata->buffer_std = true;
        nextrdata++;

        nextrdata->data = NULL;
        nextrdata->len = 0;
        nextrdata->buffer = rbuf;
        nextrdata->buffer_std = true;
        nextrdata->next = NULL;

        if (BufferIsValid(lbuf))
        {
            nextrdata->next = nextrdata + 1;
            nextrdata++;
            nextrdata->data = NULL;
            nextrdata->len = 0;
            nextrdata->buffer = lbuf;
            nextrdata->buffer_std = true;
            nextrdata->next = NULL;
        }

        recptr = XLogInsert(RM_BTREE_ID, xlinfo, rdata);

        if (BufferIsValid(metabuf))
        {
            PageSetLSN(metapg, recptr);
        }
        page = BufferGetPage(pbuf);
        PageSetLSN(page, recptr);
        page = BufferGetPage(rbuf);
        PageSetLSN(page, recptr);
        page = BufferGetPage(buf);
        PageSetLSN(page, recptr);
        if (BufferIsValid(lbuf))
        {
            page = BufferGetPage(lbuf);
            PageSetLSN(page, recptr);
        }
    }

    END_CRIT_SECTION();

    /* release metapage; send out relcache inval if metapage changed */
    if (BufferIsValid(metabuf))
    {
        CacheInvalidateRelcache(rel);
        _bt_relbuf(rel, metabuf);
    }
    /* can always release leftsib immediately */
    if (BufferIsValid(lbuf))
        _bt_relbuf(rel, lbuf);

    /*
     * If parent became half dead, recurse to delete it. Otherwise, if right
     * sibling is empty and is now the last child of the parent, recurse to
     * try to delete it.  (These cases cannot apply at the same time, though
     * the second case might itself recurse to the first.)
     *
     * When recursing to parent, we hold the lock on the target page until
     * done.  This delays any insertions into the keyspace that was just
     * effectively reassigned to the parent's right sibling.  If we allowed
     * that, and there were enough such insertions before we finish deleting
     * the parent, page splits within that keyspace could lead to inserting
     * out-of-order keys into the grandparent level.  It is thought that that
     * wouldn't have any serious consequences, but it still seems like a
     * pretty bad idea.
     */
    if (parent_half_dead)
    {
        /* recursive call will release pbuf */
        _bt_relbuf(rel, rbuf);
        result = _bt_pagedel(rel, pbuf, stack->bts_parent) + 1;
        _bt_relbuf(rel, buf);
    }
    else if (parent_one_child && rightsib_empty)
    {
        _bt_relbuf(rel, pbuf);
        _bt_relbuf(rel, buf);
        /* recursive call will release rbuf */
        result = _bt_pagedel(rel, rbuf, stack) + 1;
    }
    else
    {
        _bt_relbuf(rel, pbuf);
        _bt_relbuf(rel, buf);
        _bt_relbuf(rel, rbuf);
        result = 1;
    }

    return result;
}