Is there a way to propagate panic in actor? How do you write tests?

[nfejzic]

Hello! First of all, thank you for this great crate, appreciate it a lot.

I have a question regarding testing. As far as I can see, the panics are caught and I don't see a way to figure out that an Actor panicked. When using actors in tests, this means that tests pass even though something panicked somewhere. Is there a way to make it so that panic gets propagated and makes the test fail?

[slawlor]

In order to determine if an actor panicked, there's essentially two flows.

Flow 1: Actor panicked in pre_start

In this case, the panic will be caught and wrapped into an SpawnErr::StartupFailed err. So the actor won't complete startup or register into the supervision flow

Flow 2: Actor panics in either post_start, post_stop, handle, or handle_supervisor_evt

In this case, you would need the actor to have a supervisor. If there's a supervisor, the supervisor will have a SupervisionEvent sent to it and it'll be handled by the handle_supervisor_evt() logic. If there is no supervisor then the panic is lost, and someone would need to wait on the JoinHandle to see the actor died and/or poll the actor's ActorStatus on the ActorCell|ActorRef. But you wouldn't be able to capture the panic message without a supervision flow.

[slawlor]

In a test-case, I'd wait for the actor to exit however if it actually failed, there isn't a clear way to capture the panic without a supervisor (and that's somewhat by design).

[nfejzic]

Thank you for your answer. Yeah, the fact that it's hard to crash the system is very nice to be honest. In testing it's a little inconvenient, but that's a good trade-off I would say.

Another question/feature request would be, is there a way to stop the actor but don't stop processing messages already in the queue? The stop_and_wait stops processing immediately, and handle.await seems to just hang if there's no signal to stop the actor. What we do right now is we have a message available in tests only that sends a response. This makes it possible to have a sync point where we wait for the message to process.

A functionality that signals to the actor to stop receiving new messages, finish processing the ones in the queue and then stop would be great!

[slawlor]

There is, it's called drain or drain_and_wait. You might just need to upgrade your version if you don't have it already

[nfejzic]

There is, it's called drain or drain_and_wait. You might just need to upgrade your version if you don't have it already

Ah alright, awesome! I will update and try it out, thank you!