From 2fc3e1b44a4b4bb9b01adced7b040b51dc9606a0 Mon Sep 17 00:00:00 2001 From: Peter Geoghegan Date: Sat, 25 Jul 2026 12:01:24 -0400 Subject: [PATCH] Fix another empty nbtree index SSI race. 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 Discussion: https://postgr.es/m/CAH2-WzkNoTn3yXY0iGkSuavJ+sL8EROf+kitW+_2v2tJVWuKmA@mail.gmail.com Backpatch-through: 14 --- src/backend/access/nbtree/nbtsearch.c | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-) diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c index eba47c2f0cf..40d5bab4f2c 100644 --- a/src/backend/access/nbtree/nbtsearch.c +++ b/src/backend/access/nbtree/nbtsearch.c @@ -2426,12 +2426,21 @@ _bt_endpoint(IndexScanDesc scan, ScanDirection dir) if (!BufferIsValid(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); - BTScanPosInvalidate(so->currPos); - return false; + if (IsolationIsSerializable()) + { + PredicateLockRelation(rel, scan->xs_snapshot); + buf = _bt_get_endpoint(rel, 0, ScanDirectionIsBackward(dir), + scan->xs_snapshot); + } + + if (!BufferIsValid(buf)) + { + BTScanPosInvalidate(so->currPos); + return false; + } } PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);