GH-95289: Always call uncancel() when required

[gvanrossum]

In __aexit__() we should always call self._parent_task.uncancel() when we called self._parent_task.cancel() -- even if the exception being received by __aexit__() is not CancelledError (since it may have been suppressed or replaced by another exception).

The logic is a bit contorted since we must propagate a cancellation error in two separate situations:

• if uncancel() is called and it returns a value greater than zero
• if we don't call uncancel() at all

• Issue: asyncio.TaskGroup corrupts uncancel() stack when the parent task replaces the cancelled error #95289

[YvesDup]

Hi,
I ran this code with your changes, for the required test in the 'externally cancel' case.
Here a CancelledError is raised, _parent_cancel_requested is False, so uncancel() is never called. Is the expected behavior ?

import asyncio

class MyException(Exception):
    pass

async def async_fn2():
    try:
        await asyncio.sleep(0.0)
        raise MyException
    finally:
        print("raise done")

async def main():
    task = asyncio.current_task()
    try:
        async with asyncio.TaskGroup() as tg:
            tg.create_task(async_fn2())
            await asyncio.sleep(1)

    except* (MyException, asyncio.CancelledError) as e:
        # How to check a private `_parent_cancel_requested` attr here, outside the class ?
        print(f"{e.exceptions} -> {task.cancelling()=}, {tg._parent_cancel_requested=}")

async def supermain():
    t = asyncio.create_task(main())
    await asyncio.sleep(0)

    t.cancel()
    print("cancel main task")
    await asyncio.sleep(1)

    print(f"{t.cancelling()=} must be 1, confirm ?")

asyncio.run(supermain())

[gvanrossum]

That code looks racy with the sleep(0.0) calls, but it is correct that since your supermain code cancels main, uncancel will not be called. Note that _parent_cancel_requested is not truly private (single underscore) so for debugging can check it.

[YvesDup]

I wonder whether it is important to check if parent task is not cancelling when starts the async with ?
In that case, should we raise a RuntimeError in the __aenter__ ?

[gvanrossum]

I wonder whether it is important to check if parent task is not cancelling when starts the async with ? In that case, should we raise a RuntimeError in the __aenter__ ?

What problem would that solve?

[YvesDup]

I wonder whether it is important to check if parent task is not cancelling when starts the async with ? In that case, should we raise a RuntimeError in the aenter ?

What problem would that solve?

Problem is already solved with the 'externally cancel' case.
Sorry for waste your time.

[kumaraditya303]

Superseded by #95602 which includes tests.