Fix, or at least ameliorate, bugs in logicalrep_worker_launch().

If we failed to get a background worker slot, the code just walked
away from the logicalrep-worker slot it already had, leaving that
looking like the worker is still starting up.  This led to an indefinite
hang in subscription startup, as reported by Thomas Munro.  We must
release the slot on failure.

Also fix a thinko: we must capture the worker slot's generation before
releasing LogicalRepWorkerLock the first time, else testing to see if
it's changed is pretty meaningless.

BTW, the CHECK_FOR_INTERRUPTS() in WaitForReplicationWorkerAttach is a
ticking time bomb, even without considering the possibility of elog(ERROR)
in one of the other functions it calls.  Really, this entire business needs
a redesign with some actual thought about error recovery.  But for now
I'm just band-aiding the case observed in testing.

Back-patch to v10 where this code was added.

Discussion: https://postgr.es/m/CAEepm=2bP3TBMFBArP6o20AZaRduWjMnjCjt22hSdnA-EvrtCw@mail.gmail.com

src/backend/replication/logical/launcher.c

@@ -168,14 +168,11 @@ get_subscription_list(void)
  */
 static void
 WaitForReplicationWorkerAttach(LogicalRepWorker *worker,
+							   uint16 generation,
 							   BackgroundWorkerHandle *handle)
 {
 	BgwHandleStatus status;
 	int			rc;
-	uint16		generation;
-
-	/* Remember generation for future identification. */
-	generation = worker->generation;
 
 	for (;;)
 	{
@@ -282,7 +279,7 @@ logicalrep_workers_find(Oid subid, bool only_running)
 }
 
 /*
- * Start new apply background worker.
+ * Start new apply background worker, if possible.
  */
 void
 logicalrep_worker_launch(Oid dbid, Oid subid, const char *subname, Oid userid,
@@ -290,6 +287,7 @@ logicalrep_worker_launch(Oid dbid, Oid subid, const char *subname, Oid userid,
 {
 	BackgroundWorker bgw;
 	BackgroundWorkerHandle *bgw_handle;
+	uint16		generation;
 	int			i;
 	int			slot = 0;
 	LogicalRepWorker *worker = NULL;
@@ -406,6 +404,9 @@ retry:
 	worker->reply_lsn = InvalidXLogRecPtr;
 	TIMESTAMP_NOBEGIN(worker->reply_time);
 
+	/* Before releasing lock, remember generation for future identification. */
+	generation = worker->generation;
+
 	LWLockRelease(LogicalRepWorkerLock);
 
 	/* Register the new dynamic worker. */
@@ -428,6 +429,12 @@ retry:
 
 	if (!RegisterDynamicBackgroundWorker(&bgw, &bgw_handle))
 	{
+		/* Failed to start worker, so clean up the worker slot. */
+		LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);
+		Assert(generation == worker->generation);
+		logicalrep_worker_cleanup(worker);
+		LWLockRelease(LogicalRepWorkerLock);
+
 		ereport(WARNING,
 				(errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
 				 errmsg("out of background worker slots"),
@@ -436,7 +443,7 @@ retry:
 	}
 
 	/* Now wait until it attaches. */
-	WaitForReplicationWorkerAttach(worker, bgw_handle);
+	WaitForReplicationWorkerAttach(worker, generation, bgw_handle);
 }
 
 /*