I'll dig into the existing asyncio unit tests and see what I can come up with. I'm not sure, yet, exactly what I might test for.

The variables involved with reproducing the error are mostly environmental. CPU speed of the host, amount of CPU bound work happening in handler coroutines, and the rate of new connections are the major contributors we've identified. I'm not sure how I might simulate those in a unit test.

Would it be sufficient to add a test that ensures the _accept_connection calls .accept() on the listening socket 'backlog' number of times in event there are no OS errors?