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 aae6acb7f57ddd0c6a4215a4c344197e637f550c..dfcdd2d4cec077a7ffc8adbb185be00fb8578b74 100644 (file)
--- 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);