TaskGroup's task's exception's participate in a reference cycle, via TaskGroup.create_task -> task -> exception and loop.create_task -> task -> exception

[graingert]

Bug report

Bug description:

the following test fails when run with eager tasks

    async def test_exception_refcycles_parent_task(self):
        """Test that TaskGroup deletes self._parent_task"""
        tg = asyncio.TaskGroup()
        exc = None

        class _Done(Exception):
            pass

        async def coro_fn():
            async with tg:
                raise _Done

        try:
            async with asyncio.TaskGroup() as tg2:
                tg2.create_task(coro_fn())
        except* _Done as excs:
            exc = excs.exceptions[0].exceptions[0]

        self.assertIsInstance(exc, _Done)
        self.assertListEqual(gc.get_referrers(exc), no_other_refs())

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-128552: fix refcycles in eager task creation #128553
• [3.13] gh-128552: fix refcycles in eager task creation (#128553) #128585
• [3.12] gh-128588: gh-128550: gh-128552: fix refcycles in eager task creation (#128553) and remove eager tasks optimization that missed and introduced incorrect cancellations (python#129063) #128586

[graingert]

still waiting for the backports to 3.12 on this

[graingert]

once #129063 is in 3.12 we can merge #128586