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gh-155418: Fix TaskGroup hang when a task cancels it before suspending #155421

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gh-155418: Fix TaskGroup hang when a task cancels it before suspending #155421

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gh-155418: Fix TaskGroup hang when a task cancels it before suspending
deadlovelll Aug 9, 2026
dd46221
assert the child did not finish
deadlovelll Aug 9, 2026
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3 changes: 3 additions & 0 deletions Lib/asyncio/taskgroups.py
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Original file line number Diff line number Diff line change
Expand Up @@ -222,6 +222,9 @@ def create_task(self, coro, **kwargs):
# the current task too early. gh-128550, gh-128588
self._tasks.add(task)
task.add_done_callback(self._on_task_done)
# gh-155418: an eager task can cancel the group before joining _tasks
if self._aborting and not task.done():
task.cancel()
try:
return task
finally:
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14 changes: 14 additions & 0 deletions Lib/test/test_asyncio/test_taskgroups.py
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Expand Up @@ -1154,6 +1154,20 @@ async def test_taskgroup_cancel_before_create_task(self):
with self.assertRaises(RuntimeError):
tg.create_task(asyncio.sleep(1))

async def test_taskgroup_cancel_from_child_before_first_suspension(self):
# gh-155418: an eager task can cancel the group before joining _tasks
done = []

async def child(tg):
tg.cancel()
await asyncio.sleep(10)
done.append(True)

async with asyncio.TaskGroup() as tg:

@picnixz picnixz Aug 9, 2026

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I thought the problem was with the eager task factory?

@deadlovelll deadlovelll Aug 9, 2026

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hi! if i understand you well - BaseTestTaskGroup class that stores all of the tests is not inheriting from unittests TestCase https://github.com/deadlovelll/cpython/blob/62ab3f87bdb17aafa97e44fd26386893ead0497b/Lib/test/test_asyncio/test_taskgroups.py#L60.

Runners are TestTaskGroup and TestEagerTaskTaskGroup that inherits from BaseTestTaskGroup and unittest.IsolatedAsyncioTestCase, they are located at the bottom of the file https://github.com/deadlovelll/cpython/blob/62ab3f87bdb17aafa97e44fd26386893ead0497b/Lib/test/test_asyncio/test_taskgroups.py#L1307-L1315

TestEagerTaskTaskGroup already inject eager factory for this

task = tg.create_task(child(tg))

@picnixz picnixz Aug 9, 2026

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Maybe use your reproducer with the print as well? could be useful to catch (instead of print, just use a list and mutate it)

@deadlovelll deadlovelll Aug 9, 2026

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thanks! just pushed the changes

self.assertTrue(task.cancelled())
self.assertEqual(done, [])

async def test_taskgroup_cancel_before_exception(self):
async def raise_exc(parent_tg: asyncio.TaskGroup):
parent_tg.cancel()
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-09-14-27-23.gh-issue-155418.kCXUIG.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix :class:`asyncio.TaskGroup` hang when a task cancels it before
suspending.
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