Fix theoretical torn page hazard.

jeff-davis · jeff-davis · commit 7bf713dd2d07 · 2022-11-11T12:46:11.000-08:00
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 The concern has been retracted.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
diff --git 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.
 		 *
@@ -8838,6 +8837,9 @@ heap_xlog_visible(XLogReaderState *record)
 
 		PageSetAllVisible(page);
 
+		if (XLogHintBitIsNeeded())
+			PageSetLSN(page, lsn);
+
 		MarkBufferDirty(buffer);
 	}
 	else if (action == BLK_RESTORED)
