KeyError triggered by ConnectionError: Connection closed by server.

[JoseKilo]

Hi maintainers, thank you for the package !

This line can fail with a ConnectionError, at least when using the Redis backend, possibly others too:

broadcaster/broadcaster/_base.py

Line 95 in 69cf29a

await self._backend.subscribe(channel)

When that happens, queue is never added to self._subscribers[channel].

But the finally block will try to remove it nonetheless, causing a KeyError and hiding the original issue:

broadcaster/broadcaster/_base.py

Line 102 in 69cf29a

self._subscribers[channel].remove(queue)

Possible solutions:

• Using discard instead of remove
• maybe moving the _backend.subscribe call outside the try block: when that call fails you can probably skip self._backend.unsubscribe. You'd still need to put a None in the queue though.

I could prepare a PR with any of those if they make sense.