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commit    d6a3dbe14f98d867b2fc3faeb99d2d3c2a48ca67
Author:   Jeff Davis <jdavis@postgresql.org>
Date:     Thu Nov 10 22:46:30 2022 +0000
First released in: 16.0, 11.19, 12.14, 13.10, 14.7, 15.2

    Fix theoretical torn page hazard.
    
    The original report was concerned with a possible inconsistency
    between the heap and the visibility map, which I was unable to
    confirm. The concern has been retracted.
    
    However, there did seem to be a torn page hazard when using
    checksums. By not setting the heap page LSN during redo, the
    protections of minRecoveryPoint were bypassed. Fixed, along with a
    misleading comment.
    
    It may have been impossible to hit this problem in practice, because
    it would require a page tear between the checksum and the flags, so I
    am marking this as a theoretical risk. But, as discussed, it did
    violate expectations about the page LSN, so it may have other
    consequences.
    
    Backpatch to all supported versions.
    
    Reported-by: Konstantin Knizhnik
    Reviewed-by: Konstantin Knizhnik
    Discussion: https://postgr.es/m/fed17dac-8cb8-4f5b-d462-1bb4908c029e@garret.ru
    Backpatch-through: 11


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

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 560f1c81a2c..5c8cdfb9b20 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -8823,8 +8823,7 @@ heap_xlog_visible(XLogReaderState *record)
 		/*
 		 * We don't bump the LSN of the heap page when setting the visibility
 		 * map bit (unless checksums or wal_hint_bits is enabled, in which
-		 * case we must), because that would generate an unworkable volume of
-		 * full-page writes.  This exposes us to torn page hazards, but since
+		 * case we must). This exposes us to torn page hazards, but since
 		 * we're not inspecting the existing page contents in any way, we
 		 * don't care.
 		 */
@@ -8832,6 +8831,9 @@ heap_xlog_visible(XLogReaderState *record)
 
 		PageSetAllVisible(page);
 
+		if (XLogHintBitIsNeeded())
+			PageSetLSN(page, lsn);
+
 		MarkBufferDirty(buffer);
 	}
 	else if (action == BLK_RESTORED)


[parent: 3eb8eeccbee3]