stream() no longer suppresses CancelledError

[lithp]

What was wrong?

Trinity had unclean shutdowns. A major contributor to them was lines like this:

ERROR  08-30 17:40:45               asyncio  Task exception was never retrieved
future: <Task finished coro=<<async_generator_asend without __name__>()> exception=StopAsyncIteration()>
StopAsyncIteration

The problem was happening within cancellable_wait. When it tries to cancel the pending tasks (line 142), it assumes that the tasks are successfully cancelled. Prior to this PR, Endpoint.stream() would catch the asyncio.CancelledError and then return. However, AsyncGenerators don't simply return, they signal the end of iteration by raising a StopAsyncIteration. So, the call to cancel() causes this coroutine to emit an exception, and python then complains that the exception is never retrieved.

How was it fixed?

This coroutine no longer catches CancelledError and instead propagates it to the calling coroutine, as the docs suggest every coroutine should.

Cute Animal Picture

[cburgdorf]

OMG! Epic! So happy we have someone on the team who really manages to make sense of all the asyncio mess 😂