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