amcheck: Distinguish interrupted page deletion from corruption.

nmisch · nmisch · commit e04509f3246d · 2023-10-30T14:46:09.000-07:00
This prevents false-positive reports about "the first child of leftmost target page is not leftmost of its level", "block %u is not leftmost" and "left link/right link pair". They appeared if amcheck ran before VACUUM cleaned things, after a cluster exited recovery between the first-stage and second-stage WAL records of a deletion. Back-patch to v11 (all supported versions). Reviewed by Peter Geoghegan. Discussion: https://postgr.es/m/20231005025232.c7.nmisch@google.com
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
@@ -127,6 +127,9 @@ static void bt_check_every_level(Relation rel, Relation heaprel,
 					 bool readonly, bool heapallindexed);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 							 BtreeLevel level);
+static bool bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
+										   BlockNumber start,
+										   BTPageOpaque start_opaque);
 static void bt_target_page_check(BtreeCheckState *state);
 static ScanKey bt_right_page_check_scankey(BtreeCheckState *state);
 static void bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
@@ -716,7 +719,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 			 */
 			if (state->readonly)
 			{
-				if (!P_LEFTMOST(opaque))
+				if (!bt_leftmost_ignoring_half_dead(state, current, opaque))
 					ereport(ERROR,
 							(errcode(ERRCODE_INDEX_CORRUPTED),
 							 errmsg("block %u is not leftmost in index \"%s\"",
@@ -769,10 +772,14 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 		}
 
 		/*
-		 * readonly mode can only ever land on live pages and half-dead pages,
-		 * so sibling pointers should always be in mutual agreement
+		 * Sibling links should be in mutual agreement.  There arises
+		 * leftcurrent == P_NONE && btpo_prev != P_NONE when the left sibling
+		 * of the parent's low-key downlink is half-dead.  (A half-dead page
+		 * has no downlink from its parent.)  Under heavyweight locking, the
+		 * last bt_leftmost_ignoring_half_dead() validated this btpo_prev.
 		 */
-		if (state->readonly && opaque->btpo_prev != leftcurrent)
+		if (state->readonly &&
+			opaque->btpo_prev != leftcurrent && leftcurrent != P_NONE)
 			ereport(ERROR,
 					(errcode(ERRCODE_INDEX_CORRUPTED),
 					 errmsg("left link/right link pair in index \"%s\" not in agreement",
@@ -822,6 +829,67 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 	return nextleveldown;
 }
 
+/*
+ * Like P_LEFTMOST(start_opaque), but accept an arbitrarily-long chain of
+ * half-dead, sibling-linked pages to the left.  If a half-dead page appears
+ * under state->readonly, the database exited recovery between the first-stage
+ * and second-stage WAL records of a deletion.
+ */
+static bool
+bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
+							   BlockNumber start,
+							   BTPageOpaque start_opaque)
+{
+	BlockNumber reached = start_opaque->btpo_prev,
+				reached_from = start;
+	bool		all_half_dead = true;
+
+	/*
+	 * To handle the !readonly case, we'd need to accept BTP_DELETED pages and
+	 * potentially observe nbtree/README "Page deletion and backwards scans".
+	 */
+	Assert(state->readonly);
+
+	while (reached != P_NONE && all_half_dead)
+	{
+		Page		page = palloc_btree_page(state, reached);
+		BTPageOpaque reached_opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+
+		CHECK_FOR_INTERRUPTS();
+
+		/*
+		 * Try to detect btpo_prev circular links.  _bt_unlink_halfdead_page()
+		 * writes that side-links will continue to point to the siblings.
+		 * Check btpo_next for that property.
+		 */
+		all_half_dead = P_ISHALFDEAD(reached_opaque) &&
+			reached != start &&
+			reached != reached_from &&
+			reached_opaque->btpo_next == reached_from;
+		if (all_half_dead)
+		{
+			XLogRecPtr	pagelsn = PageGetLSN(page);
+
+			/* pagelsn should point to an XLOG_BTREE_MARK_PAGE_HALFDEAD */
+			ereport(DEBUG1,
+					(errcode(ERRCODE_NO_DATA),
+					 errmsg_internal("harmless interrupted page deletion detected in index \"%s\"",
+									 RelationGetRelationName(state->rel)),
+					 errdetail_internal("Block=%u right block=%u page lsn=%X/%X.",
+										reached, reached_from,
+										(uint32) (pagelsn >> 32),
+										(uint32) pagelsn)));
+
+			reached_from = reached;
+			reached = reached_opaque->btpo_prev;
+		}
+
+		pfree(page);
+	}
+
+	return all_half_dead;
+}
+
 /*
  * Function performs the following checks on target page, or pages ancillary to
  * target page:
