silence-bogus-all-visible-warning-1.patch

diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index 67e0be9..dbd2e8d 100644
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -332,6 +332,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 		Size		freespace;
 		bool		all_visible_according_to_vm = false;
 		bool		all_visible;
+		bool		has_dead_tuples;
 
 		/*
 		 * Skip pages that don't require vacuuming according to the visibility
@@ -475,6 +476,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 		 * requiring freezing.
 		 */
 		all_visible = true;
+		has_dead_tuples = false;
 		nfrozen = 0;
 		hastup = false;
 		prev_dead_count = vacrelstats->num_dead_tuples;
@@ -613,6 +615,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 			{
 				lazy_record_dead_tuple(vacrelstats, &(tuple.t_self));
 				tups_vacuumed += 1;
+				has_dead_tuples = true;
 			}
 			else
 			{
@@ -671,9 +674,22 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 			PageSetAllVisible(page);
 			SetBufferCommitInfoNeedsSave(buf);
 		}
-		else if (PageIsAllVisible(page) && !all_visible)
+		/*
+		 * It's possible for the value returned by GetOldestXmin() to move
+		 * backwards, so it's not wrong for us to see tuples that appear to
+		 * not be visible to everyone yet, while PD_ALL_VISIBLE is already
+		 * set. The real safe xmin value never moves backwards, but
+		 * GetOldestXmin() is conservative and sometimes returns a value
+		 * that's unnecessarily small, so if we see that contradiction it
+		 * just means that the tuples that we think are not visible to
+		 * everyone yet actually are, and the PD_ALL_VISIBLE flag is correct.
+		 *
+		 * There shouldn't be any dead tuples on a page with PD_ALL_VISIBLE
+		 * set, however.
+		 */
+		else if (PageIsAllVisible(page) && has_dead_tuples)
 		{
-			elog(WARNING, "PD_ALL_VISIBLE flag was incorrectly set in relation \"%s\" page %u",
+			elog(WARNING, "page containing dead tuples is marked as all-visible in relation \"%s\" page %u",
 				 relname, blkno);
 			PageClearAllVisible(page);
 			SetBufferCommitInfoNeedsSave(buf);