TaskGroup fails to cancel tasks pending creation, if that task starts a failing task

[graingert]

Bug report

Bug description:

this 'deadlocks' on eager tasks, but raises an EG as expected on regular 'lazy' tasks.

import sys
import asyncio


if sys.version_info >= (3, 13):
    from asyncio import EventLoop
elif sys.platform == "win32":
    from asyncio import ProactorEventLoop as EventLoop
else:
    from asyncio import SelectorEventLoop as EventLoop


def loop_factory():
    loop = EventLoop()
    loop.set_task_factory(asyncio.eager_task_factory)
    return loop


async def main():
    async with asyncio.TaskGroup() as tg:
        async def third_task():
            raise RuntimeError("third task failed")

        async def second_task():
            tg.create_task(third_task())
            await asyncio.Event().wait()

        tg.create_task(second_task())

# asyncio.run(main())
asyncio.run(main(), loop_factory=loop_factory)

CPython versions tested on:

3.12, 3.13, 3.14

Operating systems tested on:

Linux

Linked PRs

• gh-128550: TaskGroup: cancel tasks added to set after aborting #128551

[kumaraditya303]

This is happening because as per the following code, we skip scheduling the done callback to the event loop incase the task completes eagerly, I would lean towards fixing this by removing this case and always add the done callback which would be simpler and fix this.

cpython/Lib/asyncio/taskgroups.py

Lines 200 to 207 in a30277a

	# optimization: Immediately call the done callback if the task is
	# already done (e.g. if the coro was able to complete eagerly),
	# and skip scheduling a done callback
	if task.done():
	self._on_task_done(task)
	else:
	self._tasks.add(task)
	task.add_done_callback(self._on_task_done)

[graingert]

@kumaraditya303 do we want this removed on 3.14, 3.13 and 3.12? or just 3.12?

[kumaraditya303]

Yes, all the branches.

[graingert]

ok will get that setup,

[graingert]

ok working on news now, looks like it will fix #128588 also