return values of _send_command and _wait_for_answer_async in cri_controller.py #48

While reading cri_controller.py I noticed two things I'd like to discuss.

1. _send_command() returns only a message_id

In most call sites the returned message_id is only used to look up the asyncio.Event in _wait_for_answer() / _wait_for_answer_async(), which has to take the lock and search answer_events again.

Proposal: return the event together with the id, e.g.

class SentCommand(NamedTuple):
    message_id: int
    answer_event: asyncio.Event | None  # None if register_answer=False

The wait functions could then use the event directly and skip the lookup.

2. _wait_for_answer_async() uses None for "success"

The docstring says None means "an answer was received with no error". However, the function also returns None here:

with self.answer_events_lock:
    if message_id not in self.answer_events:
        return None
    wait_event = self.answer_events[message_id]

So if someone calls _send_command(command) and forgets register_answer=True, _wait_for_answer_async() returns None and the caller treats the command as successfully acknowledged, although nothing was waited for.

It also makes call sites hard to read, because is not None means "error":

msg_id = self._send_command(command, True)
if (error_msg := await self._wait_for_answer_async(msg_id, timeout=30.0)) is not None:
    logger.debug("Error in Move Joints command: %s", error_msg)
    return False

Possible directions:

• A. Raise on failure (e.g. CommandRejectedError, AnswerTimeoutError, ValueError for an unregistered id) and return the answer on success.
• B. Return a small result object (ok: bool, message: str | None).
• C. Keep the current return type, but at least raise for an unregistered message_id, since that is a programming error rather than a robot answer.

Combined with point 1, the "unregistered id" case could disappear entirely (no event, nothing to wait on).