Check if worker registration succeeded

If `RegisterDynamicbackgroundworker` fails the returned handle will be
null, which will cause `WaitForBackgroundWorkerStartup` to fail with a
crash. This can typically happen if there is not enough slots in
`BackgroundWorkerData`. In that case, just return and do not call
`WaitForBackgroundWorkerStartup`.

.unreleased/fix_6240

@@ -0,0 +1 @@
+Fixes: #6240 Check if worker registration succeeded

test/src/bgw/scheduler_mock.c

@@ -222,6 +222,13 @@ ts_bgw_db_scheduler_test_run_and_wait_for_scheduler_finish(PG_FUNCTION_ARGS)
 	worker_handle = start_test_scheduler(PG_GETARG_INT32(0), GetUserId());
 	TestAssertTrue(worker_handle != NULL);
 
+	/*
+	 * If RegisterDynamicbackgroundworker fails, worker_handle will be
+	 * NULL. Since messages have already been printed in, just exit.
+	 */
+	if (!worker_handle)
+		PG_RETURN_VOID();
+
 	BgwHandleStatus status = WaitForBackgroundWorkerStartup(worker_handle, &pid);
 	TestAssertTrue(BGWH_STARTED == status);
 	if (status != BGWH_STARTED)
@@ -248,6 +255,13 @@ ts_bgw_db_scheduler_test_run(PG_FUNCTION_ARGS)
 	current_handle = start_test_scheduler(PG_GETARG_INT32(0), GetUserId());
 	MemoryContextSwitchTo(old_ctx);
 
+	/*
+	 * If RegisterDynamicbackgroundworker fails, current_handle will be
+	 * NULL. Since messages have already been printed in, just exit.
+	 */
+	if (!current_handle)
+		PG_RETURN_VOID();
+
 	status = WaitForBackgroundWorkerStartup(current_handle, &pid);
 	TestAssertTrue(BGWH_STARTED == status);
 	if (status != BGWH_STARTED)