postgres.git / summary / log / commit / refs

commit    d560e730e813343b8d3f4a336244f2bc09ca84fc
Author:   Peter Geoghegan <pg@bowt.ie>
Date:     Sat Jul 25 16:01:32 2026 +0000
First released in: 14.24, 15.19, 16.15, 17.11, 18.6

    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


 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 47b6552d67c..d47e4b20783 100644
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -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);


[parent: f2d6cf880240]