From: Peter Geoghegan <pg@bowt.ie>
Date: Sat, 25 Jul 2026 16:01:35 +0000 (-0400)
Subject: Fix another empty nbtree index SSI race.
X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=ce3f19e2;p=thirdparty%2Fpostgresql.git

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 <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkNoTn3yXY0iGkSuavJ+sL8EROf+kitW+_2v2tJVWuKmA@mail.gmail.com
Backpatch-through: 14
---

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index aae6acb7f57..dfcdd2d4cec 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -2195,12 +2195,21 @@ _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);