Prevent logical rep workers with removed subscriptions from starting.

Any logical rep workers must have their subscription entries in
pg_subscription. To ensure this, we need to prevent the launcher
from starting new worker corresponding to the subscription that
DROP SUBSCRIPTION command is removing. To implement this,
previously LogicalRepLauncherLock was introduced and held until
the end of transaction running DROP SUBSCRIPTION. But using
LWLock for that purpose was not valid.

Instead, this commit changes DROP SUBSCRIPTION so that it takes
AccessExclusiveLock on pg_subscription, in order to ensure that
the launcher cannot see any subscriptions being removed. Also this
commit gets rid of LogicalRepLauncherLock.

Patch by me, reviewed by Petr Jelinek

Discussion: https://www.postgresql.org/message-id/CAHGQGwHPi8ky-yANFfe0sgmhKtsYcQLTnKx07bW9S7-Rn1746w@mail.gmail.com

src/backend/replication/logical/launcher.c

@@ -305,17 +305,12 @@ logicalrep_worker_launch(Oid dbid, Oid subid, const char *subname, Oid userid)
 /*
  * Stop the logical replication worker and wait until it detaches from the
  * slot.
- *
- * The caller must hold LogicalRepLauncherLock to ensure that new workers are
- * not being started during this function call.
  */
 void
 logicalrep_worker_stop(Oid subid)
 {
 	LogicalRepWorker *worker;
 
-	Assert(LWLockHeldByMe(LogicalRepLauncherLock));
-
 	LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 
 	worker = logicalrep_worker_find(subid);
@@ -602,9 +597,6 @@ ApplyLauncherMain(Datum main_arg)
 										   ALLOCSET_DEFAULT_MAXSIZE);
 			oldctx = MemoryContextSwitchTo(subctx);
 
-			/* Block any concurrent DROP SUBSCRIPTION. */
-			LWLockAcquire(LogicalRepLauncherLock, LW_EXCLUSIVE);
-
 			/* search for subscriptions to start or stop. */
 			sublist = get_subscription_list();
 
@@ -628,8 +620,6 @@ ApplyLauncherMain(Datum main_arg)
 				}
 			}
 
-			LWLockRelease(LogicalRepLauncherLock);
-
 			/* Switch back to original memory context. */
 			MemoryContextSwitchTo(oldctx);
 			/* Clean the temporary memory. */