PythonAPI sensor actor cleanup "no stream available"

[fpasch]

There seems to be an issue within the cleanup of the sensor actors, which results in the following error messages:

ERROR: Invalid session: no stream available with id <...>
...

This was seen within the CARLA ROS bridge. It seems to happen whenever a sensor, that has a registered callback (self.carla_actor.listen(self._callback_sensor_data)) goes away.

To my knowledge there is no API available to handle that issue from within Python. (e.g. a callback whenever the actor goes away).

So I believe that we need to handle it within the CARLA PythonAPI code.

[nsubiron]

It could be very handy having this callback, I think we can add a list of destroyed actors in the world observer's message, that'd make very easy adding callbacks in the client-side. This would be also very handy for the pedestrian navigation.

Meanwhile a workaround could be checking periodically if the actor is still alive

if world.get_snapshot().find(actor_id) is None:
    # actor was destroyed.

[fpasch]

At the moment there are two tick()-callbacks registered in CARLA ROS bridge:

1. update list of actors (this is done once per seconds) (Code)
2. update time and current state of actors (on every tick) (Code)

Therefore the error-message is only seen for max 1s.

So ideally, we would have two callbacks:

1. whenever a new actor was spawned
2. whenever an actor was deleted

[germanros1987]

@fpasch Is this still a relevant issue?

[fpasch]

Yes.

[germanros1987]

Would it be fine to just keep a list with the deleted actors and offer that information to the client?

[fpasch]

This issue has two aspects:

1. How to find deleted actors
2. Error message from CARLA Client Library

For 1.), although it would be convenient to have a callback on actor-deletion, there is the workaround to compare the actor lists between ticks (see ros-bridge). So this has a low priority.
What should get investigated is 2.), the clean up of sensors with registered callback.

[germanros1987]

Added to the task board.