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Fix another empty nbtree index SSI race.

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Commit f9b7fc65 fixed a race when predicate-locking completely empty
btrees: without a buffer lock held, a matching key could be inserted
between _bt_search and the PredicateLockRelation call, so the scan would
miss concurrently inserted tuples while the writer wouldn't see the
reader's predicate lock.  That commit only fixed _bt_first's _bt_search
path, though.  Scans without useful insertion scan keys return early
from _bt_first via _bt_endpoint, which still didn't recheck if the
relation was empty.

To fix, add handling to _bt_endpoint that is analogous to the handling
added to _bt_search by commit f9b7fc65.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkNoTn3yXY0iGkSuavJ+sL8EROf+kitW+_2v2tJVWuKmA@mail.gmail.com
Backpatch-through: 14
This commit is contained in:
Peter Geoghegan
2026-07-25 12:01:32 -04:00
parent f2d6cf8802
commit d560e730e8
1 changed files with 14 additions and 5 deletions
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19
src/backend/access/nbtree/nbtsearch.c
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@@ -2714,12 +2714,21 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir)
if (!BufferIsValid(so->currPos.buf))
{
/*
* Empty index. Lock the whole relation, as nothing finer to lock
* exists.
* Empty index. Lock the whole relation using the approach explained
* at the same point in the _bt_first path.
*/
PredicateLockRelation(rel, scan->xs_snapshot);
_bt_parallel_done(scan);
return false;
if (IsolationIsSerializable())
{
PredicateLockRelation(rel, scan->xs_snapshot);
so->currPos.buf = _bt_get_endpoint(rel, 0,
ScanDirectionIsBackward(dir));
}
if (!BufferIsValid(so->currPos.buf))
{
_bt_parallel_done(scan);
return false;
}
}
page = BufferGetPage(so->currPos.buf);