Fix race in SSI interaction with empty btrees.

When predicate-locking btrees, we have a special case for completely
empty btrees, since there is no page to lock.  This was racy, because,
without buffer lock held, a matching key could be inserted between the
_bt_search() and the PredicateLockRelation() calls.

Fix, by rechecking _bt_search() after taking the relation-level SIREAD
lock, if using SERIALIZABLE isolation and an empty btree is discovered.

Back-patch to all supported releases.  Fixes one aspect of bug #17949.

Reported-by: Artem Anisimov <artem.anisimov.255@gmail.com>
Reviewed-by: Dmitry Dolgov <9erthalion6@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/17949-a0f17035294a55e2%40postgresql.org

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 7e05e58676895baee69c0fc31b46fb671f756d1c..3230b3b894066703467b6f20f3259706e48c631a 100644 (file)
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -17,6 +17,7 @@
 
 #include "access/nbtree.h"
 #include "access/relscan.h"
+#include "access/xact.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "storage/predicate.h"
@@ -1382,22 +1383,34 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
    {
        /*
         * We only get here if the index is completely empty. Lock relation
-        * because nothing finer to lock exists.
+        * because nothing finer to lock exists.  Without a buffer lock, it's
+        * possible for another transaction to insert data between
+        * _bt_search() and PredicateLockRelation().  We have to try again
+        * after taking the relation-level predicate lock, to close a narrow
+        * window where we wouldn't scan concurrently inserted tuples, but the
+        * writer wouldn't see our predicate lock.
         */
-       PredicateLockRelation(rel, scan->xs_snapshot);
-
-       /*
-        * mark parallel scan as done, so that all the workers can finish
-        * their scan
-        */
-       _bt_parallel_done(scan);
-       BTScanPosInvalidate(so->currPos);
+       if (IsolationIsSerializable())
+       {
+           PredicateLockRelation(rel, scan->xs_snapshot);
+           stack = _bt_search(rel, NULL, &inskey, &buf, BT_READ,
+                              scan->xs_snapshot);
+           _bt_freestack(stack);
+       }
 
-       return false;
+       if (!BufferIsValid(buf))
+       {
+           /*
+            * Mark parallel scan as done, so that all the workers can finish
+            * their scan.
+            */
+           _bt_parallel_done(scan);
+           BTScanPosInvalidate(so->currPos);
+           return false;
+       }
    }
-   else
-       PredicateLockPage(rel, BufferGetBlockNumber(buf),
-                         scan->xs_snapshot);
+
+   PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
 
    _bt_initialize_more_data(so, dir);