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Commitf9b7fc6

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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
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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"
2122
#include"pgstat.h"
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#include"storage/predicate.h"
@@ -1382,22 +1383,34 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
13821383
{
13831384
/*
13841385
* We only get here if the index is completely empty. Lock relation
1385-
* because nothing finer to lock exists.
1386+
* because nothing finer to lock exists. Without a buffer lock, it's
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* possible for another transaction to insert data between
1388+
* _bt_search() and PredicateLockRelation(). We have to try again
1389+
* after taking the relation-level predicate lock, to close a narrow
1390+
* window where we wouldn't scan concurrently inserted tuples, but the
1391+
* writer wouldn't see our predicate lock.
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*/
1387-
PredicateLockRelation(rel,scan->xs_snapshot);
1388-
1389-
/*
1390-
* mark parallel scan as done, so that all the workers can finish
1391-
* their scan
1392-
*/
1393-
_bt_parallel_done(scan);
1394-
BTScanPosInvalidate(so->currPos);
1393+
if (IsolationIsSerializable())
1394+
{
1395+
PredicateLockRelation(rel,scan->xs_snapshot);
1396+
stack=_bt_search(rel,NULL,&inskey,&buf,BT_READ,
1397+
scan->xs_snapshot);
1398+
_bt_freestack(stack);
1399+
}
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1396-
return false;
1401+
if (!BufferIsValid(buf))
1402+
{
1403+
/*
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* Mark parallel scan as done, so that all the workers can finish
1405+
* their scan.
1406+
*/
1407+
_bt_parallel_done(scan);
1408+
BTScanPosInvalidate(so->currPos);
1409+
return false;
1410+
}
13971411
}
1398-
else
1399-
PredicateLockPage(rel,BufferGetBlockNumber(buf),
1400-
scan->xs_snapshot);
1412+
1413+
PredicateLockPage(rel,BufferGetBlockNumber(buf),scan->xs_snapshot);
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14021415
_bt_initialize_more_data(so,dir);
14031416

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