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

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

src/backend/catalog/pg_subscription.c

@@ -319,7 +319,7 @@ AddSubscriptionRelState(Oid subid, Oid relid, char state,
  */
 void
 UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
-						   XLogRecPtr sublsn)
+						   XLogRecPtr sublsn, bool already_locked)
 {
 	Relation	rel;
 	HeapTuple	tup;
@@ -327,9 +327,24 @@ UpdateSubscriptionRelState(Oid subid, Oid relid, char state,
 	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;
 
+		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,

src/backend/replication/logical/tablesync.c

@@ -316,7 +316,8 @@ process_syncing_tables_for_sync(XLogRecPtr current_lsn)
 		UpdateSubscriptionRelState(MyLogicalRepWorker->subid,
 								   MyLogicalRepWorker->relid,
 								   MyLogicalRepWorker->relstate,
-								   MyLogicalRepWorker->relstate_lsn);
+								   MyLogicalRepWorker->relstate_lsn,
+								   false);
 
 		/*
 		 * End streaming so that LogRepWorkerWalRcvConn can be used to drop
@@ -425,6 +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());
 
@@ -492,7 +494,17 @@ 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,
@@ -504,7 +516,7 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 				 */
 				UpdateSubscriptionRelState(MyLogicalRepWorker->subid,
 										   rstate->relid, rstate->state,
-										   rstate->lsn);
+										   rstate->lsn, true);
 			}
 		}
 		else
@@ -555,7 +567,14 @@ 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);
 					}
@@ -622,6 +641,11 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 		}
 	}
 
+	/* Close table if opened */
+	if (rel)
+		table_close(rel, NoLock);
+
+
 	if (started_tx)
 	{
 		/*
@@ -1413,7 +1437,8 @@ LogicalRepSyncTableStart(XLogRecPtr *origin_startpos)
 	UpdateSubscriptionRelState(MyLogicalRepWorker->subid,
 							   MyLogicalRepWorker->relid,
 							   MyLogicalRepWorker->relstate,
-							   MyLogicalRepWorker->relstate_lsn);
+							   MyLogicalRepWorker->relstate_lsn,
+							   false);
 	CommitTransactionCommand();
 	pgstat_report_stat(true);
 
@@ -1546,7 +1571,8 @@ LogicalRepSyncTableStart(XLogRecPtr *origin_startpos)
 	UpdateSubscriptionRelState(MyLogicalRepWorker->subid,
 							   MyLogicalRepWorker->relid,
 							   SUBREL_STATE_FINISHEDCOPY,
-							   MyLogicalRepWorker->relstate_lsn);
+							   MyLogicalRepWorker->relstate_lsn,
+							   false);
 
 	CommitTransactionCommand();
 

src/include/catalog/pg_subscription_rel.h

@@ -85,7 +85,7 @@ typedef struct SubscriptionRelState
 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);
+									   XLogRecPtr sublsn, bool already_locked);
 extern char GetSubscriptionRelState(Oid subid, Oid relid, XLogRecPtr *sublsn);
 extern void RemoveSubscriptionRel(Oid subid, Oid relid);