Fix race in SSI interaction with bitmap heap scan.

When performing a bitmap heap scan, we don't want to miss concurrent
writes that occurred after we observed the heap's rs_nblocks, but before
we took predicate locks on index pages.  Therefore, we can't skip
fetching any heap tuples that are referenced by the index, because we
need to test them all with CheckForSerializableConflictOut().  The
old optimization that would ignore any references to blocks >=
rs_nblocks gets in the way of that requirement, because it means that
concurrent writes in that window are ignored.

Removing that optimization shouldn't affect correctness at any isolation
level, because any new tuples shouldn't be visible to an MVCC snapshot.
There also shouldn't be any error-causing references to heap blocks past
the end, because we should have held at least an AccessShareLock on the
table before the index scan.  It can't get smaller while our transaction
is running.  For now, though, we'll keep the optimization at lower
levels to avoid making unnecessary changes in a bug fix.

Back-patch to all supported releases.  In release 11, the code is in a
different place but not fundamentally different.  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

Author: macdice

src/backend/access/heap/heapam_handler.c

@@ -2132,9 +2132,11 @@ heapam_scan_bitmap_next_block(TableScanDesc scan,
 	 * Ignore any claimed entries past what we think is the end of the
 	 * relation. It may have been extended after the start of our scan (we
 	 * only hold an AccessShareLock, and it could be inserts from this
-	 * backend).
+	 * backend).  We don't take this optimization in SERIALIZABLE isolation
+	 * though, as we need to examine all invisible tuples reachable by the
+	 * index.
 	 */
-	if (block >= hscan->rs_nblocks)
+	if (!IsolationIsSerializable() && block >= hscan->rs_nblocks)
 		return false;
 
 	/*