Cancelling TaskGroup.start() cancels TaskGroup itself (only for asyncio backend)

[jonathanslenders]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

AnyIO version

4.3.0

Python version

3.11.6

What happened?

Consider this snippet:

from anyio import create_task_group, run, sleep_forever, move_on_after


async def in_task_group(task_status):
    await sleep_forever()


async def main():
    async with create_task_group() as tg:
        with move_on_after(1):
            await tg.start(in_task_group)

        print(tg.cancel_scope.cancel_called). # prints 'True'


run(main, backend='asyncio')

Here we spawn a task using TaskGroup.start() then we cancel the .start() call using move_on_after. It turns out this cancels the task group as a whole.

I'd expect this to cancel only that particular task.

For the trio backend, this works fine, and does not cancel the task group.

How can we reproduce the bug?

(see snippet above)

[agronholm]

Isn't this a duplicate of #685?

[jonathanslenders]

It might be, I don't really understand whether or not it's exactly the same.

[agronholm]

I think it probably at least stems from the same issue which I intend to fix for the next release.

[jonathanslenders]

Thanks! That would be awesome. Feel free to close this or copy over the snippet as an additional test case if you prefer that.