refcycles in exceptions raised from asyncio.TaskGroup

[graingert]

Bug report

Bug description:

asyncio.TaskGroup attempts to avoid refcycles in raised exceptions by deleting self._errors but when I reviewed the code it doesn't actually achieve this:

see

cpython/Lib/asyncio/taskgroups.py

Lines 152 to 156 in 5e9e506

	try:
	me = BaseExceptionGroup('unhandled errors in a TaskGroup', self._errors)
	raise me from None
	finally:
	self._errors = None

There's a refcycle in me is me.__traceback__.tb_next.tb_frame.f_locals["me"]

I wrote a few tests to route out all the refcycles in tracebacks

import asyncio
import gc
import unittest

class TestTaskGroup(unittest.IsolatedAsyncioTestCase):

    async def test_exception_refcycles_direct(self):
        """Test that TaskGroup doesn't keep a reference to the raised ExceptionGroup"""
        tg = asyncio.TaskGroup()
        exc = None

        class _Done(Exception):
            pass

        try:
            async with tg:
                raise _Done
        except ExceptionGroup as e:
            exc = e

        self.assertIsNotNone(exc)
        self.assertListEqual(gc.get_referrers(exc), [])


    async def test_exception_refcycles_errors(self):
        """Test that TaskGroup deletes self._errors, and __aexit__ args"""
        tg = asyncio.TaskGroup()
        exc = None

        class _Done(Exception):
            pass

        try:
            async with tg:
                raise _Done
        except* _Done as excs:
            exc = excs.exceptions[0]

        self.assertIsInstance(exc, _Done)
        self.assertListEqual(gc.get_referrers(exc), [])


    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), [])

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

        try:
            async with asyncio.timeout(-1):
                async with tg:
                    await asyncio.sleep(0)
        except TimeoutError as e:
            exc = e.__cause__

        self.assertIsInstance(exc, asyncio.CancelledError)
        self.assertListEqual(gc.get_referrers(exc), [])

    async def test_exception_refcycles_base_error(self):
        """Test that TaskGroup deletes self._base_error"""
        class MyKeyboardInterrupt(KeyboardInterrupt):
            pass

        tg = asyncio.TaskGroup()
        exc = None

        try:
            async with tg:
                raise MyKeyboardInterrupt
        except MyKeyboardInterrupt as e:
            exc = e

        self.assertIsNotNone(exc)
        self.assertListEqual(gc.get_referrers(exc), [])

in writing all these tests I noticed refcycles in PyFuture:

cpython/Lib/asyncio/futures.py

Lines 197 to 198 in 58f7763

	exc = self._make_cancelled_error()
	raise exc

cpython/Lib/asyncio/futures.py

Lines 215 to 216 in 58f7763

	exc = self._make_cancelled_error()
	raise exc

class BaseFutureTests:
    def test_future_cancelled_result_refcycles(self):
        f = self._new_future(loop=self.loop)
        f.cancel()
        exc = None
        try:
            f.result()
        except asyncio.CancelledError as e:
            exc = e
        self.assertIsNotNone(exc)
        self.assertListEqual(gc.get_referrers(exc), [])

    def test_future_cancelled_exception_refcycles(self):
        f = self._new_future(loop=self.loop)
        f.cancel()
        exc = None
        try:
            f.exception()
        except asyncio.CancelledError as e:
            exc = e
        self.assertIsNotNone(exc)
        self.assertListEqual(gc.get_referrers(exc), [])

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

Linux

Linked PRs

• gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles #124959
• [3.13] gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles (GH-124959) #125463
• [3.12] gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles (#124959) #125466
• gh-124958: Revert "gh-125472: Revert "gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles ... #125486