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commit    d03d9a2614fd58d4a9e90107f1700952333ce2a2
Author:   Thomas Munro <tmunro@postgresql.org>
Date:     Mon Jul 03 04:18:20 2023 +0000
First released in: 17.0, 11.21, 12.16, 13.12, 14.9, 15.4, 16.0

    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


 src/backend/access/heap/heapam_handler.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 0755be83901..5a17112c91e 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/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;
 
 	/*


[parent: 0cb1fb2c9bf8]