Fix assertion when decrementing eager scanning success and failure counters.

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

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);
+				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
@@ -1426,20 +1440,11 @@ 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--;
 		}
-		}
 
 		/*
 		 * Now drop the buffer lock and, potentially, update the FSM.