Fix assertion when decrementing eager scanning success and failure counters.

MasahikoSawada · MasahikoSawada · commit 4c08ecd1618e · 2025-05-27T11:42:36.000-07:00
Previously, we asserted that the eager scan's success and failure counters were positive before decrementing them. However, this assumption was incorrect, as it's possible that some blocks have already been eagerly scanned by the time eager scanning is disabled. This commit replaces the assertions with guards to handle this scenario gracefully. With this change, we continue to allow read-ahead operations by the read stream that exceed the success and failure caps. While there is a possibility that overruns will trigger eager scans of additional pages, this does not pose a practical concern as the overruns will not be substantial and remain within an acceptable range. Reviewed-by: Melanie Plageman <melanieplageman@gmail.com> Discussion: https://postgr.es/m/CAD21AoConf6tkVCv-=JhQJj56kYsDwo4jG5+WqgT+ukSkYomSQ@mail.gmail.com
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
@@ -1413,11 +1413,25 @@ lazy_scan_heap(LVRelState *vacrel)
 
 			if (vm_page_frozen)
 			{
-				Assert(vacrel->eager_scan_remaining_successes > 0);
-				vacrel->eager_scan_remaining_successes--;
+				if (vacrel->eager_scan_remaining_successes > 0)
+					vacrel->eager_scan_remaining_successes--;
 
 				if (vacrel->eager_scan_remaining_successes == 0)
 				{
+					/*
+					 * Report only once that we disabled eager scanning. We
+					 * may eagerly read ahead blocks in excess of the success
+					 * or failure caps before attempting to freeze them, so we
+					 * could reach here even after disabling additional eager
+					 * scanning.
+					 */
+					if (vacrel->eager_scan_max_fails_per_region > 0)
+						ereport(vacrel->verbose ? INFO : DEBUG2,
+								(errmsg("disabling eager scanning after freezing %u eagerly scanned blocks of \"%s.%s.%s\"",
+										orig_eager_scan_success_limit,
+										vacrel->dbname, vacrel->relnamespace,
+										vacrel->relname)));
+
 					/*
 					 * If we hit our success cap, permanently disable eager
 					 * scanning by setting the other eager scan management
@@ -1426,19 +1440,10 @@ lazy_scan_heap(LVRelState *vacrel)
 					vacrel->eager_scan_remaining_fails = 0;
 					vacrel->next_eager_scan_region_start = InvalidBlockNumber;
 					vacrel->eager_scan_max_fails_per_region = 0;
-
-					ereport(vacrel->verbose ? INFO : DEBUG2,
-							(errmsg("disabling eager scanning after freezing %u eagerly scanned blocks of \"%s.%s.%s\"",
-									orig_eager_scan_success_limit,
-									vacrel->dbname, vacrel->relnamespace,
-									vacrel->relname)));
 				}
 			}
-			else
-			{
-				Assert(vacrel->eager_scan_remaining_fails > 0);
+			else if (vacrel->eager_scan_remaining_fails > 0)
 				vacrel->eager_scan_remaining_fails--;
-			}
 		}
 
 		/*
