v4-0001-Fix-DROP-SUBSCRIPTION-deadlock-with-new-database-V16.patch

From 87935136c739ac64d390f324085fc82c0ef65caf Mon Sep 17 00:00:00 2001
From: Dilip Kumar <dilipkumarb@google.com>
Date: Thu, 14 Aug 2025 07:06:54 +0000
Subject: [PATCH v4] Fix DROP SUBSCRIPTION deadlock with new database creation

DROP SUBSCRIPTION previously acquired an AccessExclusiveLock on the
pg_subscription catalog to prevent the replication launcher from starting
a new worker. However, this caused a deadlock. New database creation also
need to acquire an AccessShareLock on this same catalog during their
initialization phase. This created a lock conflict where DROP SUBSCRIPTION
would block this, while simultaneously waiting for a connection to
complete for dropping the replication slot.

This commit resolves the deadlock by having DROP SUBSCRIPTION acquire a less
restrictive RowExclusiveLock on the catalog instead.

To address the original concern of orphaned workers, a new check is
implemented.  The replication worker now takes a shared object lock on the
subscription itself.

If a worker starts for a subscription that no longer exists, it immediately detects
this condition and exits. This ensures that no orphan workers are created without
the need for an overly broad AccessExclusiveLock on the system catalog.
---
 src/backend/commands/subscriptioncmds.c  | 8 +++++---
 src/backend/replication/logical/worker.c | 7 +++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/backend/commands/subscriptioncmds.c b/src/backend/commands/subscriptioncmds.c
index e1260fc0e90..aebfaecd9d8 100644
--- a/src/backend/commands/subscriptioncmds.c
+++ b/src/backend/commands/subscriptioncmds.c
@@ -1491,10 +1491,12 @@ DropSubscription(DropSubscriptionStmt *stmt, bool isTopLevel)
 	bool		must_use_password;
 
 	/*
-	 * Lock pg_subscription with AccessExclusiveLock to ensure that the
-	 * launcher doesn't restart new worker during dropping the subscription
+	 * The launcher may concurrently start a new worker for this subscription.
+	 * During initialization, the worker checks for subscription validity and
+	 * exits if the subscription has already been dropped. See
+	 * InitializeLogRepWorker.
 	 */
-	rel = table_open(SubscriptionRelationId, AccessExclusiveLock);
+	rel = table_open(SubscriptionRelationId, RowExclusiveLock);
 
 	tup = SearchSysCache2(SUBSCRIPTIONNAME, MyDatabaseId,
 						  CStringGetDatum(stmt->subname));
diff --git a/src/backend/replication/logical/worker.c b/src/backend/replication/logical/worker.c
index 4df0a594dc9..9b5c641941f 100644
--- a/src/backend/replication/logical/worker.c
+++ b/src/backend/replication/logical/worker.c
@@ -4492,6 +4492,13 @@ InitializeApplyWorker(void)
 	StartTransactionCommand();
 	oldctx = MemoryContextSwitchTo(ApplyContext);
 
+	/*
+	 * Lock the subscription to prevent it from being concurrently dropped,
+	 * then re-verify its existence. After the initialization, the worker will
+	 * be terminated gracefully if the subscription is dropped.
+	 */
+	LockSharedObject(SubscriptionRelationId, MyLogicalRepWorker->subid, 0,
+					 AccessShareLock);
 	MySubscription = GetSubscription(MyLogicalRepWorker->subid, true);
 	if (!MySubscription)
 	{
-- 
2.51.0.rc0.215.g125493bb4a-goog