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Commitae6d536

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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 parent5396b18 commitae6d536

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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"
2021
#include"miscadmin.h"
2122
#include"pgstat.h"
2223
#include"storage/predicate.h"
@@ -1377,22 +1378,34 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
13771378
{
13781379
/*
13791380
* We only get here if the index is completely empty. Lock relation
1380-
* because nothing finer to lock exists.
1381+
* because nothing finer to lock exists. Without a buffer lock, it's
1382+
* possible for another transaction to insert data between
1383+
* _bt_search() and PredicateLockRelation(). We have to try again
1384+
* after taking the relation-level predicate lock, to close a narrow
1385+
* window where we wouldn't scan concurrently inserted tuples, but the
1386+
* writer wouldn't see our predicate lock.
13811387
*/
1382-
PredicateLockRelation(rel,scan->xs_snapshot);
1383-
1384-
/*
1385-
* mark parallel scan as done, so that all the workers can finish
1386-
* their scan
1387-
*/
1388-
_bt_parallel_done(scan);
1389-
BTScanPosInvalidate(so->currPos);
1388+
if (IsolationIsSerializable())
1389+
{
1390+
PredicateLockRelation(rel,scan->xs_snapshot);
1391+
stack=_bt_search(rel,&inskey,&buf,BT_READ,
1392+
scan->xs_snapshot);
1393+
_bt_freestack(stack);
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}
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1391-
return false;
1396+
if (!BufferIsValid(buf))
1397+
{
1398+
/*
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* Mark parallel scan as done, so that all the workers can finish
1400+
* their scan.
1401+
*/
1402+
_bt_parallel_done(scan);
1403+
BTScanPosInvalidate(so->currPos);
1404+
return false;
1405+
}
13921406
}
1393-
else
1394-
PredicateLockPage(rel,BufferGetBlockNumber(buf),
1395-
scan->xs_snapshot);
1407+
1408+
PredicateLockPage(rel,BufferGetBlockNumber(buf),scan->xs_snapshot);
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13971410
_bt_initialize_more_data(so,dir);
13981411

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