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gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles by graingert · Pull Request #124959 · python/cpython · GitHub
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gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles #124959

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Merged
1st1 merged 4 commits into from
Oct 14, 2024
Merged

gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles #124959

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1a4f2b5
gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles
graingert Oct 4, 2024
33ddc70
remove detrititus from testing PyFuture/PyTask
graingert Oct 4, 2024
5c5c057
📜🤖 Added by blurb_it.
blurb-it[bot] Oct 4, 2024
c9e0b73
move finally into utility function
graingert Oct 5, 2024
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gh-124958: fix asyncio.TaskGroup and _PyFuture refcycles
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@graingert
graingert committed Oct 4, 2024
commit 1a4f2b577650753b4e3af4310daf4858b6f9c896
6 changes: 2 additions & 4 deletions Lib/asyncio/futures.py
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Original file line number Diff line number Diff line change
Expand Up @@ -190,8 +190,7 @@ def result(self):
the future is done and has an exception set, this exception is raised.
"""
if self._state == _CANCELLED:
exc = self._make_cancelled_error()
raise exc
raise self._make_cancelled_error()
if self._state != _FINISHED:
raise exceptions.InvalidStateError('Result is not ready.')
self.__log_traceback = False
Expand All @@ -208,8 +207,7 @@ def exception(self):
InvalidStateError.
"""
if self._state == _CANCELLED:
exc = self._make_cancelled_error()
raise exc
raise self._make_cancelled_error()
if self._state != _FINISHED:
raise exceptions.InvalidStateError('Exception is not set.')
self.__log_traceback = False
Expand Down
161 changes: 85 additions & 76 deletions Lib/asyncio/taskgroups.py
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Original file line number Diff line number Diff line change
Expand Up @@ -66,94 +66,103 @@ async def __aenter__(self):
return self

async def __aexit__(self, et, exc, tb):
self._exiting = True
try:

@1st1 1st1 Oct 4, 2024

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I don't like this huge diff. Can you instead factor out the code of __aexit__ in a helper function (so that code keeps all of the existing indentation) and nest a call to it in try..finally in __aexit__?

@graingert graingert Oct 4, 2024

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It needs to delete locals as well as instance attributes so it's not possible

@1st1 1st1 Oct 4, 2024

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I think it is. Instance attributes are not a problem at all. Locals are solvable too:

try:
    raise propagate_cancellation_error
finally:
    del propagate_cancellation_error

@graingert graingert Oct 5, 2024 •
edited
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I pushed the changes but it introduces a bunch of extra places to delete things, so I prefer the one big try/finally. You can also just view the diff with whitespace hidden: 1a4f2b5?w=1 which should look like this:
image

self._exiting = True

if (exc is not None and
self._is_base_error(exc) and
self._base_error is None):
self._base_error = exc
if (exc is not None and
self._is_base_error(exc) and
self._base_error is None):
self._base_error = exc

if et is not None and issubclass(et, exceptions.CancelledError):
propagate_cancellation_error = exc
else:
propagate_cancellation_error = None
if et is not None and issubclass(et, exceptions.CancelledError):
propagate_cancellation_error = exc
else:
propagate_cancellation_error = None

if et is not None:
if not self._aborting:
# Our parent task is being cancelled:
#
# async with TaskGroup() as g:
# g.create_task(...)
# await ... # <- CancelledError
#
# or there's an exception in "async with":
#
# async with TaskGroup() as g:
# g.create_task(...)
# 1 / 0
#
self._abort()

# We use while-loop here because "self._on_completed_fut"
# can be cancelled multiple times if our parent task
# is being cancelled repeatedly (or even once, when
# our own cancellation is already in progress)
while self._tasks:
if self._on_completed_fut is None:
self._on_completed_fut = self._loop.create_future()

try:
await self._on_completed_fut
except exceptions.CancelledError as ex:
if et is not None:
if not self._aborting:
# Our parent task is being cancelled:
#
# async def wrapper():
# async with TaskGroup() as g:
# g.create_task(foo)
# async with TaskGroup() as g:
# g.create_task(...)
# await ... # <- CancelledError
#
# or there's an exception in "async with":
#
# async with TaskGroup() as g:
# g.create_task(...)
# 1 / 0
#
# "wrapper" is being cancelled while "foo" is
# still running.
propagate_cancellation_error = ex
self._abort()

self._on_completed_fut = None

assert not self._tasks

if self._base_error is not None:
raise self._base_error

if self._parent_cancel_requested:
# If this flag is set we *must* call uncancel().
if self._parent_task.uncancel() == 0:
# If there are no pending cancellations left,
# don't propagate CancelledError.
propagate_cancellation_error = None

# Propagate CancelledError if there is one, except if there
# are other errors -- those have priority.
if propagate_cancellation_error is not None and not self._errors:
raise propagate_cancellation_error

if et is not None and not issubclass(et, exceptions.CancelledError):
self._errors.append(exc)

if self._errors:
# If the parent task is being cancelled from the outside
# of the taskgroup, un-cancel and re-cancel the parent task,
# which will keep the cancel count stable.
if self._parent_task.cancelling():
self._parent_task.uncancel()
self._parent_task.cancel()
# We use while-loop here because "self._on_completed_fut"
# can be cancelled multiple times if our parent task
# is being cancelled repeatedly (or even once, when
# our own cancellation is already in progress)
while self._tasks:
if self._on_completed_fut is None:
self._on_completed_fut = self._loop.create_future()

try:
await self._on_completed_fut
except exceptions.CancelledError as ex:
if not self._aborting:
# Our parent task is being cancelled:
#
# async def wrapper():
# async with TaskGroup() as g:
# g.create_task(foo)
#
# "wrapper" is being cancelled while "foo" is
# still running.
propagate_cancellation_error = ex
self._abort()

self._on_completed_fut = None

assert not self._tasks

if self._base_error is not None:
raise self._base_error

if self._parent_cancel_requested:
# If this flag is set we *must* call uncancel().
if self._parent_task.uncancel() == 0:
# If there are no pending cancellations left,
# don't propagate CancelledError.
propagate_cancellation_error = None

# Propagate CancelledError if there is one, except if there
# are other errors -- those have priority.
if propagate_cancellation_error is not None and not self._errors:
raise propagate_cancellation_error

if et is not None and not issubclass(et, exceptions.CancelledError):
self._errors.append(exc)

if self._errors:
# If the parent task is being cancelled from the outside
# of the taskgroup, un-cancel and re-cancel the parent task,
# which will keep the cancel count stable.
if self._parent_task.cancelling():
self._parent_task.uncancel()
self._parent_task.cancel()
raise BaseExceptionGroup(
'unhandled errors in a TaskGroup',
self._errors,
) from None
finally:
# Exceptions are heavy objects that can have object
# cycles (bad for GC); let's not keep a reference to
# a bunch of them.
try:
me = BaseExceptionGroup('unhandled errors in a TaskGroup', self._errors)
raise me from None
finally:
self._errors = None
propagate_cancellation_error = None
self._parent_task = None
self._errors = None
self._base_error = None
et = None
exc = None
tb = None


def create_task(self, coro, *, name=None, context=None):
"""Create a new task in this group and return it.
Expand Down
22 changes: 22 additions & 0 deletions Lib/test/test_asyncio/test_futures.py
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Original file line number Diff line number Diff line change
Expand Up @@ -659,6 +659,28 @@ def __del__(self):
fut = self._new_future(loop=self.loop)
fut.set_result(Evil())

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), [])


@unittest.skipUnless(hasattr(futures, '_CFuture'),
'requires the C _asyncio module')
Expand Down
95 changes: 94 additions & 1 deletion Lib/test/test_asyncio/test_taskgroups.py
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
# Adapted with permission from the EdgeDB project;
# license: PSFL.


import gc
import asyncio
import contextvars
import contextlib
Expand All @@ -11,6 +11,10 @@

from test.test_asyncio.utils import await_without_task

if False:
asyncio.Task = asyncio.tasks._CTask = asyncio.tasks.Task = asyncio.tasks._PyTask
asyncio.Future = asyncio.futures._CFuture = asyncio.futures.Future = asyncio.futures._PyFuture


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Outdated
# To prevent a warning "test altered the execution environment"
def tearDownModule():
Expand Down Expand Up @@ -899,6 +903,95 @@ async def outer():

await outer()

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), [])


if __name__ == "__main__":
unittest.main()