pybricks/tools/pb_type_async: Don't close if already scheduled.

When forgetting <await> in one or more tasks that use the same resource, the
resource could be scheduled for cancellation but the awaitable is not awaited
and closed, so race=True will trigger the other task to stop and thus try to
cancel it again. But the parent object is already set to MP_OBJ_SENTINEL at
this point, so evaluating close(parent) segfaults.

The block code generator should be fixed to include await, but we also
shouldn't segfault in case the user forgets await in their own user code.

See pybricks/support#2621

Author: laurensvalk

CHANGELOG.md

@@ -15,10 +15,14 @@
 ### Fixed
 - Fixed EV3 light animation stopping when screen is used ([support#2599]).
 - Fixed Powered Up motors not resetting to the absolute value ([support#2620]).
+- Fixed segfault when user or block code generator forgets `await` in one or
+  more tasks that use the same resource (like a `DriveBase`) along
+  with `race=True` ([support#2621]).
 
 [support#2599]: https://github.com/pybricks/support/issues/2599
 [support#2603]: https://github.com/pybricks/support/issues/2603
 [support#2620]: https://github.com/pybricks/support/issues/2620
+[support#2621]: https://github.com/pybricks/support/issues/2621
 
 ## [4.0.0b7] - 2026-02-19
 

pybricks/tools/pb_type_async.c

@@ -36,7 +36,9 @@ void pb_type_async_schedule_stop_iteration(pb_type_async_t *iter) {
 
 mp_obj_t pb_type_async_close(mp_obj_t iter_in) {
     pb_type_async_t *iter = MP_OBJ_TO_PTR(iter_in);
-    if (iter->close && iter->parent_obj != MP_OBJ_NULL) {
+    // Close only if there is a close function and we aren't already closing
+    // or scheduled it to close next time.
+    if (iter->close && iter->parent_obj != MP_OBJ_NULL && iter->parent_obj != MP_OBJ_SENTINEL) {
         iter->close(iter->parent_obj);
     }
     // Closing is stronger than cancellation. In case of close, we expect that

pybricks/tools/pb_type_async.h

@@ -49,7 +49,7 @@ typedef struct {
      *
      * Special values:
      *      MP_OBJ_NULL: This iterable has been fully exhausted and can be reused.
-     *      MP_OBJ_SENTINEL: This iterable is will raise StopIteration when it is iterated again.
+     *      MP_OBJ_SENTINEL: This iterable will raise StopIteration when it is iterated again.
      */
     mp_obj_t parent_obj;
     /**