[issue42130] AsyncIO's wait_for can hide cancellation in a rare race condition

[Taras Voinarovskyi]

New submission from Taras Voinarovskyi <voyn1991 at gmail.com>:

Hi, during migration to Python 3.8.6 we encountered a behavior change from previous versions: wait_for ignored the request to cancellation and returned instead. After investigation, it seems to be related to the update in bpo-32751 and is only reproduced if the waited task is finished when cancellation of wait_for happens (code mistakes external CancelledError for a timeout).

The following example can reproduce the behavior on both 3.8.6 and 3.9.0 for me:

```

import asyncio


async def inner():
    return

async def with_for_coro():
    await asyncio.wait_for(inner(), timeout=100)
    await asyncio.sleep(1)
    print('End of with_for_coro. Should not be reached!')

async def main():
    task = asyncio.create_task(with_for_coro())
    await asyncio.sleep(0)
    assert not task.done()
    task.cancel()
    print('Called task.cancel()')
    await task  # -> You would expect a CancelledError to be raised.


asyncio.run(main())
```

Changing the wait time before cancellation slightly will return the correct behavior and CancelledError will be raised.

----------
components: asyncio
messages: 379454
nosy: asvetlov, tvoinarovskyi, yselivanov
priority: normal
severity: normal
status: open
title: AsyncIO's wait_for can hide cancellation in a rare race condition
type: behavior
versions: Python 3.10, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42130>
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