Fix xslt_process() to ensure that it inserts a NULL terminator after the

[PostgreSQL.git] / src / backend / access / nbtree / nbtpage.c

/*-------------------------------------------------------------------------
 *
 * nbtpage.c
 *        BTree-specific page management code for the Postgres btree access
 *        method.
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *      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/bufmgr.h"
#include "storage/freespace.h"
#include "storage/indexfsm.h"
#include "storage/lmgr.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 (!rel->rd_istemp)
                {
                        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);
                        PageSetTLI(rootpage, ThisTimeLineID);
                        PageSetLSN(metapg, recptr);
                        PageSetTLI(metapg, ThisTimeLineID);
                }

                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_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.")));
}

/*
 *      _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))
                                {
                                        /* 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) &&
                TransactionIdPrecedesOrEquals(opaque->btpo.xact, RecentXmin))
                return true;
        return false;
}

/*
 * Delete item(s) from a btree page.
 *
 * 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.
 */
void
_bt_delitems(Relation rel, Buffer buf,
                         OffsetNumber *itemnos, int nitems)
{
        Page            page = BufferGetPage(buf);
        BTPageOpaque opaque;

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

        /* Fix the page */
        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 (!rel->rd_istemp)
        {
                xl_btree_delete xlrec;
                XLogRecPtr      recptr;
                XLogRecData rdata[2];

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

                rdata[0].data = (char *) &xlrec;
                rdata[0].len = SizeOfBtreeDelete;
                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_DELETE, rdata);

                PageSetLSN(page, recptr);
                PageSetTLI(page, ThisTimeLineID);
        }

        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, bool vacuum_full)
{
        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);

        /*
         * 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;
                        }
                }
        }

        /*
         * 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.
         */
        page = BufferGetPage(pbuf);
        opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        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);
                Assert(ItemPointerGetBlockNumber(&(itup->t_tid)) == target);
                ItemPointerSet(&(itup->t_tid), rightsib, P_HIKEY);

                nextoffset = OffsetNumberNext(poffset);
                /* This part is just for double-checking */
                itemid = PageGetItemId(page, nextoffset);
                itup = (IndexTuple) PageGetItem(page, itemid);
                if (ItemPointerGetBlockNumber(&(itup->t_tid)) != rightsib)
                        elog(PANIC, "right sibling %u of block %u is not next child of %u in index \"%s\"",
                                 rightsib, target, BufferGetBlockNumber(pbuf),
                                 RelationGetRelationName(rel));
                PageIndexTupleDelete(page, nextoffset);
        }

        /*
         * Update siblings' side-links.  Note the target page's side-links will
         * continue to point to the siblings.
         */
        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; or immediately if we're doing VACUUM FULL.
         */
        page = BufferGetPage(buf);
        opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        opaque->btpo_flags &= ~BTP_HALF_DEAD;
        opaque->btpo_flags |= BTP_DELETED;
        opaque->btpo.xact =
                vacuum_full ? FrozenTransactionId : 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 (!rel->rd_istemp)
        {
                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;

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

        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, vacuum_full) + 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, vacuum_full) + 1;
        }
        else
        {
                _bt_relbuf(rel, pbuf);
                _bt_relbuf(rel, buf);
                _bt_relbuf(rel, rbuf);
                result = 1;
        }

        return result;
}