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Fix race in SSI interaction with empty btrees.

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When predicate-locking btrees, we have a special case for completely
empty 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 SIREAD
lock, 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
This commit is contained in:
Thomas Munro 2023-07-03 16:16:27 +12:00
parent 5396b188c9
commit ae6d536ed0
1 changed files with 26 additions and 13 deletions
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39
src/backend/access/nbtree/nbtsearch.c
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@ -17,6 +17,7 @@
#include "access/nbtree.h" #include "access/nbtree.h"
#include "access/relscan.h" #include "access/relscan.h"
#include "access/xact.h"
#include "miscadmin.h" #include "miscadmin.h"
#include "pgstat.h" #include "pgstat.h"
#include "storage/predicate.h" #include "storage/predicate.h"
@ -1377,22 +1378,34 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
{ {
/* /*
* We only get here if the index is completely empty. Lock relation * We only get here if the index is completely empty. Lock relation
* because nothing finer to lock exists. * because nothing finer to lock exists. Without a buffer lock, it's
* possible for another transaction to insert data between
* _bt_search() and PredicateLockRelation(). We have to try again
* after taking the relation-level predicate lock, to close a narrow
* window where we wouldn't scan concurrently inserted tuples, but the
* writer wouldn't see our predicate lock.
*/ */
PredicateLockRelation(rel, scan->xs_snapshot); if (IsolationIsSerializable())
{
PredicateLockRelation(rel, scan->xs_snapshot);
stack = _bt_search(rel, &inskey, &buf, BT_READ,
scan->xs_snapshot);
_bt_freestack(stack);
}
/* if (!BufferIsValid(buf))
* mark parallel scan as done, so that all the workers can finish {
* their scan /*
*/ * Mark parallel scan as done, so that all the workers can finish
_bt_parallel_done(scan); * their scan.
BTScanPosInvalidate(so->currPos); */
_bt_parallel_done(scan);
return false; BTScanPosInvalidate(so->currPos);
return false;
}
} }
else
PredicateLockPage(rel, BufferGetBlockNumber(buf), PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
scan->xs_snapshot);
_bt_initialize_more_data(so, dir); _bt_initialize_more_data(so, dir);