[Bug] - CancelledError swallowed (with solution)

[svaraborut]

Version: 4.2.2

Platform: any

Description:

I had various issues related to task cancellation when using Redis due to asyncio.CancelledError disappearing sometimes. Probably related to #2028

The bug is difficult to reproduce as cancel() needs to be called on a task exactly when is waiting within an async with async_timeout.timeout(timeout): block.

I tracked down the issue to async_timeout used inside redis.asyncio.connection. This module has this known issue that is not being addressed.

I patched the library externally with this code

import async_timeout


class _Timeout(async_timeout.Timeout):

    RANDOM_TOKEN = '0f0dd596-373b-42df-aa0b-682d046c5d24'

    def __exit__(self, exc_type, exc_val, exc_tb):
        self._do_exit(exc_type, exc_val)
        return None

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        self._do_exit(exc_type, exc_val)
        return None

    def _do_exit(self, exc_type, exc_val):
        if exc_type is asyncio.CancelledError and str(exc_val) == _Timeout.RANDOM_TOKEN \
                and self._state == async_timeout._State.TIMEOUT:
            self._timeout_handler = None
            raise asyncio.TimeoutError
        # timeout has not expired
        self._state = async_timeout._State.EXIT
        self._reject()

    def _on_timeout(self, task: "asyncio.Task[None]") -> None:
        task.cancel(_Timeout.RANDOM_TOKEN)
        self._state = async_timeout._State.TIMEOUT
        # drop the reference early
        self._timeout_handler = None


async_timeout.Timeout = _Timeout

but connection.py should be rewritten using asyncio.timeout as this library may lead to strange behavior as mentioned here.

[dvora-h]

Closing as I assume this issue was fixed with all the async and CancelledError fixes.

@svaraborut Feel free to re-open it if this still happen to you.

[svaraborut]

The fix does only work on py >= 3.11. Will the underlying issue ever be addressed? Currently it is even worse as the patch does not work with async_timeout=4.0.3 but only with async_timeout=4.0.2