Proactor and selector event loops behave differently when call_soon_threadsafe races with a concurrent call to loop.close(). In a selector event loop, call_soon_threadsafe will either succeed or raise a RuntimeError("Event loop is closed"). In a proactor event loop, it could raise this RuntimeError, but it can also raise an AttributeError due to an unguarded access to self._csock. 

https://github.com/python/cpython/blob/1ed61617a4a6632905ad6a0b440cd2cafb8b6414/Lib/asyncio/proactor_events.py#L785-L787

Comments in BaseSelectorEventLoop._write_to_self indicate that this is deliberate, so the `csock is not None` check here should probably be copied to the proactor event loop version.

https://github.com/python/cpython/blob/1ed61617a4a6632905ad6a0b440cd2cafb8b6414/Lib/asyncio/selector_events.py#L129-L136


I'd also accept an answer that the exact behavior of this race is undefined and it's up to the application to either arrange for all calls to call_soon_threadsafe to stop before closing the loop. However, I've had users of Tornado argue that they use the equivalent of call_soon_threadsafe in contexts where this coordination would be difficult, and I've decided that tornado's version of this method would never raise, even if there is a concurrent close. So if asyncio declines to specify which exceptions are allowed in this case, tornado will need to add a blanket `except Exception:` around calls to call_soon_threadsafe.

Good catch. We should fix this for Python 3.8.3.

Is this related to bpo-39010 too?

No, this is unrelated to bpo-39010.

This sadly missed 3.8.3 but I want this addressed for 3.8.4.

We have trouble finding a Windows expert with available time to address this :/ This is missing 3.8.4 as well.

New changeset 1b0f0e3d7d03155da1cf9769a847874d559e57e3 by Victor Stinner in branch 'master':
bpo-39651: Fix asyncio proactor _write_to_self() (GH-22197)
https://github.com/python/cpython/commit/1b0f0e3d7d03155da1cf9769a847874d559e57e3

Thanks for the bug report Ben Darnell, it's now fixed.

New changeset 530d1105ed7b0aa5ef76e3116918fe39bc6a4823 by Miss Islington (bot) in branch '3.8':
bpo-39651: Fix asyncio proactor _write_to_self() (GH-22197)
https://github.com/python/cpython/commit/530d1105ed7b0aa5ef76e3116918fe39bc6a4823

New changeset 7dfcc8e0795cce6a8cb42ae2a7f159a38da6b38a by Miss Islington (bot) in branch '3.9':
bpo-39651: Fix asyncio proactor _write_to_self() (GH-22197)
https://github.com/python/cpython/commit/7dfcc8e0795cce6a8cb42ae2a7f159a38da6b38a