Fix a deadlock during ALTER SUBSCRIPTION ... DROP PUBLICATION.

Amit Kapila · Amit Kapila · commit 8c298324a4f3 · 2025-08-01T06:53:16.000Z
A deadlock can occur when the DDL command and the apply worker acquire catalog locks in different orders while dropping replication origins. The issue is rare in PG16 and higher branches because, in most cases, the tablesync worker performs the origin drop in those branches, and its locking sequence does not conflict with DDL operations. This patch ensures consistent lock acquisition to prevent such deadlocks. As per buildfarm. Reported-by: Alexander Lakhin <exclusion@gmail.com> Author: Ajin Cherian <itsajin@gmail.com> Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com> Reviewed-by: vignesh C <vignesh21@gmail.com> Reviewed-by: Amit Kapila <amit.kapila16@gmail.com> Backpatch-through: 14, where it was introduced Discussion: https://postgr.es/m/bab95e12-6cc5-4ebb-80a8-3e41956aa297@gmail.com
diff --git a/src/backend/catalog/pg_subscription.c b/src/backend/catalog/pg_subscription.c
@@ -287,18 +287,33 @@ AddSubscriptionRelState(Oid subid, Oid relid, char state,
  * Update the state of a subscription table.
  */
 void
-UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
-						   XLogRecPtr sublsn)
+UpdateSubscriptionRelStateEx(Oid subid, Oid relid, char state,
+							 XLogRecPtr sublsn, bool already_locked)
 {
 	Relation	rel;
 	HeapTuple	tup;
 	bool		nulls[Natts_pg_subscription_rel];
 	Datum		values[Natts_pg_subscription_rel];
 	bool		replaces[Natts_pg_subscription_rel];
 
-	LockSharedObject(SubscriptionRelationId, subid, 0, AccessShareLock);
+	if (already_locked)
+	{
+#ifdef USE_ASSERT_CHECKING
+		LOCKTAG		tag;
 
-	rel = table_open(SubscriptionRelRelationId, RowExclusiveLock);
+		Assert(CheckRelationOidLockedByMe(SubscriptionRelRelationId,
+										  RowExclusiveLock, true));
+		SET_LOCKTAG_OBJECT(tag, InvalidOid, SubscriptionRelationId, subid, 0);
+		Assert(LockHeldByMe(&tag, AccessShareLock, true));
+#endif
+
+		rel = table_open(SubscriptionRelRelationId, NoLock);
+	}
+	else
+	{
+		LockSharedObject(SubscriptionRelationId, subid, 0, AccessShareLock);
+		rel = table_open(SubscriptionRelRelationId, RowExclusiveLock);
+	}
 
 	/* Try finding existing mapping. */
 	tup = SearchSysCacheCopy2(SUBSCRIPTIONRELMAP,
@@ -332,6 +347,16 @@ UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
 	table_close(rel, NoLock);
 }
 
+/*
+ * Update the state of a subscription table.
+ */
+void
+UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
+						   XLogRecPtr sublsn)
+{
+	UpdateSubscriptionRelStateEx(subid, relid, state, sublsn, false);
+}
+
 /*
  * Get state of subscription table.
  *
diff --git a/src/backend/replication/logical/tablesync.c b/src/backend/replication/logical/tablesync.c
@@ -426,6 +426,7 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 	ListCell   *lc;
 	bool		started_tx = false;
 	bool		should_exit = false;
+	Relation	rel = NULL;
 
 	Assert(!IsTransactionState());
 
@@ -493,7 +494,16 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 				 * worker to remove the origin tracking as if there is any
 				 * error while dropping we won't restart it to drop the
 				 * origin. So passing missing_ok = true.
+				 *
+				 * Lock the subscription and origin in the same order as we
+				 * are doing during DDL commands to avoid deadlocks. See
+				 * AlterSubscription_refresh.
 				 */
+				LockSharedObject(SubscriptionRelationId, MyLogicalRepWorker->subid,
+								 0, AccessShareLock);
+				if (!rel)
+					rel = table_open(SubscriptionRelRelationId, RowExclusiveLock);
+
 				ReplicationOriginNameForLogicalRep(MyLogicalRepWorker->subid,
 												   rstate->relid,
 												   originname,
@@ -503,9 +513,9 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 				/*
 				 * Update the state to READY only after the origin cleanup.
 				 */
-				UpdateSubscriptionRelState(MyLogicalRepWorker->subid,
-										   rstate->relid, rstate->state,
-										   rstate->lsn);
+				UpdateSubscriptionRelStateEx(MyLogicalRepWorker->subid,
+											 rstate->relid, rstate->state,
+											 rstate->lsn, true);
 			}
 		}
 		else
@@ -556,7 +566,14 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 						 * This is required to avoid any undetected deadlocks
 						 * due to any existing lock as deadlock detector won't
 						 * be able to detect the waits on the latch.
+						 *
+						 * Also close any tables prior to the commit.
 						 */
+						if (rel)
+						{
+							table_close(rel, NoLock);
+							rel = NULL;
+						}
 						CommitTransactionCommand();
 						pgstat_report_stat(false);
 					}
@@ -623,6 +640,10 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 		}
 	}
 
+	/* Close table if opened */
+	if (rel)
+		table_close(rel, NoLock);
+
 	if (started_tx)
 	{
 		/*
diff --git a/src/include/catalog/pg_subscription_rel.h b/src/include/catalog/pg_subscription_rel.h
@@ -86,6 +86,8 @@ extern void AddSubscriptionRelState(Oid subid, Oid relid, char state,
 									XLogRecPtr sublsn, bool retain_lock);
 extern void UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
 									   XLogRecPtr sublsn);
+extern void UpdateSubscriptionRelStateEx(Oid subid, Oid relid, char state,
+										 XLogRecPtr sublsn, bool already_locked);
 extern char GetSubscriptionRelState(Oid subid, Oid relid, XLogRecPtr *sublsn);
 extern void RemoveSubscriptionRel(Oid subid, Oid relid);
 
