diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 22a1307b38d..da9d941f4a1 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3408,21 +3408,22 @@ static void
 start_repack_decoding_worker(Oid relid)
 {
 	Size		size;
-	dsm_segment *seg;
 	DecodingWorkerShared *shared;
 	shm_mq	   *mq;
-	shm_mq_handle *mqh;
 	BackgroundWorker bgw;
 
+	decoding_worker = palloc0_object(DecodingWorker);
+
 	/* Setup shared memory. */
 	size = BUFFERALIGN(offsetof(DecodingWorkerShared, error_queue)) +
 		BUFFERALIGN(REPACK_ERROR_QUEUE_SIZE);
-	seg = dsm_create(size, 0);
-	shared = (DecodingWorkerShared *) dsm_segment_address(seg);
+	decoding_worker->seg = dsm_create(size, 0);
+
+	shared = (DecodingWorkerShared *) dsm_segment_address(decoding_worker->seg);
 	shared->initialized = false;
 	shared->lsn_upto = InvalidXLogRecPtr;
 	shared->done = false;
-	SharedFileSetInit(&shared->sfs, seg);
+	SharedFileSetInit(&shared->sfs, decoding_worker->seg);
 	shared->last_exported = -1;
 	SpinLockInit(&shared->mutex);
 	shared->dbid = MyDatabaseId;
@@ -3441,7 +3442,8 @@ start_repack_decoding_worker(Oid relid)
 	mq = shm_mq_create((char *) BUFFERALIGN(shared->error_queue),
 					   REPACK_ERROR_QUEUE_SIZE);
 	shm_mq_set_receiver(mq, MyProc);
-	mqh = shm_mq_attach(mq, seg, NULL);
+
+	decoding_worker->error_mqh = shm_mq_attach(mq, decoding_worker->seg, NULL);
 
 	memset(&bgw, 0, sizeof(bgw));
 	snprintf(bgw.bgw_name, BGW_MAXLEN,
@@ -3454,19 +3456,15 @@ start_repack_decoding_worker(Oid relid)
 	bgw.bgw_restart_time = BGW_NEVER_RESTART;
 	snprintf(bgw.bgw_library_name, MAXPGPATH, "postgres");
 	snprintf(bgw.bgw_function_name, BGW_MAXLEN, "RepackWorkerMain");
-	bgw.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(seg));
+	bgw.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(decoding_worker->seg));
 	bgw.bgw_notify_pid = MyProcPid;
 
-	decoding_worker = palloc0_object(DecodingWorker);
 	if (!RegisterDynamicBackgroundWorker(&bgw, &decoding_worker->handle))
 		ereport(ERROR,
 				errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
 				errmsg("out of background worker slots"),
 				errhint("You might need to increase \"%s\".", "max_worker_processes"));
 
-	decoding_worker->seg = seg;
-	decoding_worker->error_mqh = mqh;
-
 	/*
 	 * The decoding setup must be done before the caller can have XID assigned
 	 * for any reason, otherwise the worker might end up in a deadlock,
@@ -3499,38 +3497,43 @@ start_repack_decoding_worker(Oid relid)
 static void
 stop_repack_decoding_worker(void)
 {
-	BgwHandleStatus status;
-
-	/* Haven't reached the worker startup? */
+	/* Nothing to do if no worker was set up. */
 	if (decoding_worker == NULL)
 		return;
 
-	/* Could not register the worker? */
-	if (decoding_worker->handle == NULL)
-		return;
+	/* Terminate the worker process, if one is running. */
+	if (decoding_worker->handle != NULL)
+	{
+		BgwHandleStatus status;
 
-	TerminateBackgroundWorker(decoding_worker->handle);
-	/* The worker should really exit before the REPACK command does. */
-	HOLD_INTERRUPTS();
-	status = WaitForBackgroundWorkerShutdown(decoding_worker->handle);
-	RESUME_INTERRUPTS();
+		TerminateBackgroundWorker(decoding_worker->handle);
+		/* The worker should really exit before the REPACK command does. */
+		HOLD_INTERRUPTS();
+		status = WaitForBackgroundWorkerShutdown(decoding_worker->handle);
+		RESUME_INTERRUPTS();
 
-	if (status == BGWH_POSTMASTER_DIED)
-		ereport(FATAL,
-				errcode(ERRCODE_ADMIN_SHUTDOWN),
-				errmsg("postmaster exited during REPACK command"));
-
-	shm_mq_detach(decoding_worker->error_mqh);
+		if (status == BGWH_POSTMASTER_DIED)
+			ereport(FATAL,
+					errcode(ERRCODE_ADMIN_SHUTDOWN),
+					errmsg("postmaster exited during REPACK command"));
+	}
 
 	/*
-	 * If we could not cancel the current sleep due to ERROR, do that before
-	 * we detach from the shared memory the condition variable is located in.
-	 * If we did not, the bgworker ERROR handling code would try and fail
-	 * badly.
+	 * Now detach from our shared memory segment.  In error cases there might
+	 * still be messages from the worker in the queue, which ProcessInterrupts
+	 * would try to read; this is pointless (and causes an assertion failure),
+	 * so set the global pointer to NULL to have ProcessRepackMessages ignore
+	 * them.
+	 *
+	 * We must also cancel the current sleep, if one is still set up.  This is
+	 * critical because the CV lives in the DSM that we're about to detach, so
+	 * if we omit it, later automatic cleanup tries to clear freed memory.
 	 */
+	if (decoding_worker->error_mqh != NULL)
+		shm_mq_detach(decoding_worker->error_mqh);
 	ConditionVariableCancelSleep();
-
-	dsm_detach(decoding_worker->seg);
+	if (decoding_worker->seg != NULL)
+		dsm_detach(decoding_worker->seg);
 	pfree(decoding_worker);
 	decoding_worker = NULL;
 }