Simplify the cleanup after crash.

No special code path is needed for the scheduler worker. Furthermore, we do
not need a special task and a worker to process that.

Author: Antonin Houska

worker.c

@@ -173,7 +173,6 @@ static void drop_replication_slots(void);
 static void cleanup_after_server_start(void);
 static void cleanup_repl_origins(void);
 static void cleanup_repl_slots(void);
-static void start_cleanup_worker(MemoryContext task_cxt);
 static Snapshot build_historic_snapshot(SnapBuild *builder);
 static void process_task_internal(MemoryContext task_cxt);
 
@@ -941,20 +940,18 @@ squeeze_worker_main(Datum main_arg)
 	 * replication slots and/or origins that other workers could not remove
 	 * due to server crash. Do that while holding the exclusive lock - that
 	 * also ensures that the other workers wait for the cleanup to finish
-	 * instead of complaining about the existing slots / origins.
+	 * before they create new slots / origins, which we might then drop
+	 * accidentally.
+	 *
+	 * If no "standalone" squeeze worker performed the cleanup yet, the
+	 * scheduler must do it now because it'll also create replication slots /
+	 * origins. Those could be dropped by one of the new workers if that
+	 * worker was to perform the cleanup.
 	 */
-	if (!am_i_scheduler && !workerData->cleanup_done)
+	if (!workerData->cleanup_done)
 	{
 		cleanup_after_server_start();
 		workerData->cleanup_done = true;
-
-		/* Are we assigned a "cleanup-only" task? */
-		if (!OidIsValid(MyWorkerTask->dbid))
-		{
-			LWLockRelease(workerData->lock);
-			ereport(DEBUG1, (errmsg("cleanup-only task completed")));
-			goto done;
-		}
 	}
 
 	for (i = 0; i < workerData->nslots; i++)
@@ -1073,27 +1070,12 @@ scheduler_worker_loop(void)
 	long		delay = 0L;
 	int		i;
 	MemoryContext	sched_cxt, old_cxt;
-	bool	cleanup_done;
 
 	/* Context for allocations which cannot be freed too early. */
 	sched_cxt = AllocSetContextCreate(TopMemoryContext,
 									  "pg_squeeze scheduler context",
 									  ALLOCSET_DEFAULT_SIZES);
 
-	/*
-	 * This lock does not eliminate all the possible race conditions: e.g. if
-	 * multiple schedulers (one per database) are launched at the same time,
-	 * multiple clean-up workers can be launched. Nevertheless, it makes sense
-	 * as the worker also uses this lock to examine and set the field.
-	 */
-	LWLockAcquire(workerData->lock, LW_EXCLUSIVE);
-	cleanup_done = workerData->cleanup_done;
-	LWLockRelease(workerData->lock);
-
-	/* Do we need to do cleanup first? */
-	if (!cleanup_done)
-		start_cleanup_worker(sched_cxt);
-
 	while (!got_sigterm)
 	{
 		StringInfoData	query;
@@ -1863,78 +1845,6 @@ cleanup_repl_slots(void)
 	}
 }
 
-static void
-start_cleanup_worker(MemoryContext task_cxt)
-{
-	WorkerTask	*task;
-	bool	task_exists;
-	int		task_idx;
-	NameData	dummy_name;
-	SqueezeWorker	*worker;
-	MemoryContext	old_cxt;
-	bool	registered;
-
-	/*
-	 * Create a worker to perform the initial cleanup.
-	 *
-	 * We must be sure that the cleanup has finished before we start to create
-	 * replication slots for other workers, otherwise the "cleanup worker"
-	 * could drop them too.
-	 */
-	squeezeWorkerCount = 1;
-	squeezeWorkers = (SqueezeWorker *) MemoryContextAllocZero(task_cxt,
-															  squeezeWorkerCount *
-															  sizeof(SqueezeWorker));
-	task = get_unused_task(InvalidOid, NULL, NULL, &task_idx, &task_exists);
-	Assert(!task_exists);
-	if (task == NULL)
-		/*
-		 * This is unlikely to happen, but possible if too many "standalone"
-		 * workers have been started after our check of the 'cleanup_done'
-		 * flag.
-		 */
-		ereport(ERROR,
-				(errmsg("the task queue is currently full")));
-
-	/*
-	 * No specific information needed here. Setting dummy values explicitly
-	 * seem a good practice though.
-	 */
-	NameStr(dummy_name)[0] = '\0';
-	initialize_worker_task(task, -1, &dummy_name, &dummy_name, NULL,
-						   false, false, 0);
-
-	worker = squeezeWorkers;
-	StartTransactionCommand();
-	/*
-	 * The handle (and possibly other allocations) must survive the current
-	 * transaction.
-	 */
-	old_cxt = MemoryContextSwitchTo(task_cxt);
-	registered = start_worker_internal(false, task_idx, &worker->handle);
-	MemoryContextSwitchTo(old_cxt);
-	if (!registered)
-	{
-		/*
-		 * The worker could not even get registered, so it won't set its
-		 * status to WTS_UNUSED. Make sure the task does not leak.
-		 */
-		release_task(worker->task);
-
-		ereport(ERROR,
-				(errmsg("squeeze worker could not start")),
-				(errhint("consider increasing \"max_worker_processes\" or decreasing \"squeeze.workers_per_database\"")));
-
-	}
-	CommitTransactionCommand();
-
-	/* Wait until the cleanup is done. */
-	cleanup_workers_and_tasks(false);
-
-	if (!workerData->cleanup_done)
-		ereport(ERROR, (errmsg("failed to perform the initial cleanup")));
-}
-
 /*
  * Wrapper for SnapBuildInitialSnapshot().
  *