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

1 files changed, 24 insertions, 3 deletions

diff --git a/src/backend/replication/logical/tablesync.c b/src/backend/replication/logical/tablesync.c
index e6159acba00..dc0526e2da2 100644
--- a/src/backend/replication/logical/tablesync.c
+++ b/src/backend/replication/logical/tablesync.c
@@ -379,6 +379,7 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 	static HTAB *last_start_times = NULL;
 	ListCell   *lc;
 	bool		started_tx = false;
+	Relation	rel = NULL;
 
 	Assert(!IsTransactionState());
 
@@ -470,7 +471,16 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 				 * refresh for the subscription where we remove the table
 				 * state and its origin and by this time the origin might be
 				 * already removed. 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);
+
 				ReplicationOriginNameForTablesync(MyLogicalRepWorker->subid,
 												  rstate->relid,
 												  originname,
@@ -480,9 +490,9 @@ 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
@@ -533,7 +543,14 @@ 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);
 					}
@@ -593,6 +610,10 @@ process_syncing_tables_for_apply(XLogRecPtr current_lsn)
 		}
 	}
 
+	/* Close table if opened */
+	if (rel)
+		table_close(rel, NoLock);
+
 	if (started_tx)
 	{
 		CommitTransactionCommand();