Add asserts to check fail_sched_loop() only once

src/rt/rust_task.cpp

@@ -129,11 +129,17 @@ cleanup_task(cleanup_args *args) {
         // assert(task->task_local_data != NULL);
         task->task_local_data_cleanup(task->task_local_data);
         task->task_local_data = NULL;
-    } else if (threw_exception) {
+    } else if (threw_exception && task->id == INIT_TASK_ID) {
         // Edge case: If main never spawns any tasks, but fails anyway, TLS
         // won't be around to take down the kernel (task.rs:kill_taskgroup,
         // rust_task_kill_all). Do it here instead.
+        // (Note that children tasks can not init their TLS if they were
+        // killed too early, so we need to check main's task id too.)
         task->fail_sched_loop();
+        // This must not happen twice.
+        static bool main_task_failed_without_spawning = false;
+        assert(!main_task_failed_without_spawning);
+        main_task_failed_without_spawning = true;
     }
 
     // FIXME (#2676): For performance we should do the annihilator