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gh-155854: Fix asyncio.shield() leaking cancelled waiters #155855

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gh-155854: Fix asyncio.shield() leaking cancelled waiters #155855

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4 changes: 4 additions & 0 deletions Lib/asyncio/tasks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -994,6 +994,10 @@ def _inner_done_callback(inner):
def _outer_done_callback(outer):
if not inner.done():
inner.remove_done_callback(_inner_done_callback)
# gh-155854: waiter is gone but inner lives on, clean up here
if cur_task is not None:
inner.remove_done_callback(_clear_awaited_by_callback)
futures.future_discard_from_awaited_by(inner, cur_task)
# Keep only one callback to log on cancel
inner.remove_done_callback(_log_on_exception)
inner.add_done_callback(_log_on_exception)
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13 changes: 13 additions & 0 deletions Lib/test/test_asyncio/test_tasks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2150,6 +2150,19 @@ def test_shield_duplicate_log_once(self):
test_utils.run_briefly(self.loop)
mock_handler.assert_called_once()

def test_shield_discards_awaited_by_on_outer_cancel(self):
# gh-155854: a cancelled waiter must not stay in inner's await-graph
async def coro():
inner = self.new_future(self.loop)
for _ in range(3):
asyncio.shield(inner).cancel()
await asyncio.sleep(0)
self.assertFalse(inner._asyncio_awaited_by)
self.assertEqual(1, len(inner._callbacks))
inner.cancel()

self.loop.run_until_complete(self.new_task(self.loop, coro()))

def test_shield_shortcut(self):
fut = self.new_future(self.loop)
fut.set_result(42)
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-15-18-30-39.gh-issue-155854.jQYOhY.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix :func:`asyncio.shield` keeping cancelled waiters alive via
``awaited_by``.
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