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Fix race condition that could allow two concurrent transactions

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to insert the same key into a supposedly unique index.  The bug is of
low probability, and may not explain any of the recent reports of
duplicated rows; but a bug is a bug.
This commit is contained in:
Tom Lane 2002-01-01 20:32:37 +00:00
parent c546170e77
commit 1ccc67600b
1 changed files with 24 additions and 7 deletions
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31
src/backend/access/nbtree/nbtinsert.c
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@ -8,7 +8,7 @@
* *
* *
* IDENTIFICATION * IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.87 2001/10/25 05:49:21 momjian Exp $ * $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.88 2002/01/01 20:32:37 tgl Exp $
* *
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
@ -75,7 +75,6 @@ static void _bt_pgaddtup(Relation rel, Page page,
static bool _bt_isequal(TupleDesc itupdesc, Page page, OffsetNumber offnum, static bool _bt_isequal(TupleDesc itupdesc, Page page, OffsetNumber offnum,
int keysz, ScanKey scankey); int keysz, ScanKey scankey);
static Relation _xlheapRel; /* temporary hack */
/* /*
* _bt_doinsert() -- Handle insertion of a single btitem in the tree. * _bt_doinsert() -- Handle insertion of a single btitem in the tree.
@ -116,7 +115,21 @@ top:
/* /*
* If we're not allowing duplicates, make sure the key isn't already * If we're not allowing duplicates, make sure the key isn't already
* in the index. XXX this belongs somewhere else, likely * in the index.
*
* NOTE: obviously, _bt_check_unique can only detect keys that are
* already in the index; so it cannot defend against concurrent
* insertions of the same key. We protect against that by means
* of holding a write lock on the target page. Any other would-be
* inserter of the same key must acquire a write lock on the same
* target page, so only one would-be inserter can be making the check
* at one time. Furthermore, once we are past the check we hold
* write locks continuously until we have performed our insertion,
* so no later inserter can fail to see our insertion. (This
* requires some care in _bt_insertonpg.)
*
* If we must wait for another xact, we release the lock while waiting,
* and then must start over completely.
*/ */
if (index_is_unique) if (index_is_unique)
{ {
@ -135,8 +148,6 @@ top:
} }
} }
_xlheapRel = heapRel; /* temporary hack */
/* do the insertion */ /* do the insertion */
res = _bt_insertonpg(rel, buf, stack, natts, itup_scankey, btitem, 0); res = _bt_insertonpg(rel, buf, stack, natts, itup_scankey, btitem, 0);
@ -397,9 +408,16 @@ _bt_insertonpg(Relation rel,
{ {
/* step right one page */ /* step right one page */
BlockNumber rblkno = lpageop->btpo_next; BlockNumber rblkno = lpageop->btpo_next;
Buffer rbuf;
/*
* must write-lock next page before releasing write lock on
* current page; else someone else's _bt_check_unique scan
* could fail to see our insertion.
*/
rbuf = _bt_getbuf(rel, rblkno, BT_WRITE);
_bt_relbuf(rel, buf); _bt_relbuf(rel, buf);
buf = _bt_getbuf(rel, rblkno, BT_WRITE); buf = rbuf;
page = BufferGetPage(buf); page = BufferGetPage(buf);
lpageop = (BTPageOpaque) PageGetSpecialPointer(page); lpageop = (BTPageOpaque) PageGetSpecialPointer(page);
movedright = true; movedright = true;
@ -833,7 +851,6 @@ _bt_split(Relation rel, Buffer buf, OffsetNumber firstright,
* page is not updated yet. Log changes before continuing. * page is not updated yet. Log changes before continuing.
* *
* NO ELOG(ERROR) till right sibling is updated. * NO ELOG(ERROR) till right sibling is updated.
*
*/ */
START_CRIT_SECTION(); START_CRIT_SECTION();
{ {