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Commit8f705d7

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Fix race in SSI interaction with empty btrees.
When predicate-locking btrees, we have a special case for completelyempty btrees, since there is no page to lock. This was racy, because,without buffer lock held, a matching key could be inserted between the_bt_search() and the PredicateLockRelation() calls.Fix, by rechecking _bt_search() after taking the relation-level SIREADlock, if using SERIALIZABLE isolation and an empty btree is discovered.Back-patch to all supported releases. Fixes one aspect of bug #17949.Reported-by: Artem Anisimov <artem.anisimov.255@gmail.com>Reviewed-by: Dmitry Dolgov <9erthalion6@gmail.com>Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>Discussion:https://postgr.es/m/17949-a0f17035294a55e2%40postgresql.org
1 parent753f20c commit8f705d7

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‎src/backend/access/nbtree/nbtsearch.c

Lines changed: 26 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -17,6 +17,7 @@
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#include"access/nbtree.h"
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#include"access/relscan.h"
20+
#include"access/xact.h"
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#include"miscadmin.h"
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#include"pgstat.h"
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#include"storage/predicate.h"
@@ -1381,22 +1382,34 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
13811382
{
13821383
/*
13831384
* We only get here if the index is completely empty. Lock relation
1384-
* because nothing finer to lock exists.
1385+
* because nothing finer to lock exists. Without a buffer lock, it's
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* possible for another transaction to insert data between
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* _bt_search() and PredicateLockRelation(). We have to try again
1388+
* after taking the relation-level predicate lock, to close a narrow
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* window where we wouldn't scan concurrently inserted tuples, but the
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* writer wouldn't see our predicate lock.
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*/
1386-
PredicateLockRelation(rel,scan->xs_snapshot);
1387-
1388-
/*
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* mark parallel scan as done, so that all the workers can finish
1390-
* their scan
1391-
*/
1392-
_bt_parallel_done(scan);
1393-
BTScanPosInvalidate(so->currPos);
1392+
if (IsolationIsSerializable())
1393+
{
1394+
PredicateLockRelation(rel,scan->xs_snapshot);
1395+
stack=_bt_search(rel,&inskey,&buf,BT_READ,
1396+
scan->xs_snapshot);
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_bt_freestack(stack);
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}
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1395-
return false;
1400+
if (!BufferIsValid(buf))
1401+
{
1402+
/*
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* Mark parallel scan as done, so that all the workers can finish
1404+
* their scan.
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*/
1406+
_bt_parallel_done(scan);
1407+
BTScanPosInvalidate(so->currPos);
1408+
return false;
1409+
}
13961410
}
1397-
else
1398-
PredicateLockPage(rel,BufferGetBlockNumber(buf),
1399-
scan->xs_snapshot);
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1412+
PredicateLockPage(rel,BufferGetBlockNumber(buf),scan->xs_snapshot);
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14011414
_bt_initialize_more_data(so,dir);
14021415

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