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gh-116720: Fix corner cases of taskgroups (#117407)

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This prevents external cancellations of a task group's parent task to
be dropped when an internal cancellation happens at the same time.
Also strengthen the semantics of uncancel() to clear self._must_cancel
when the cancellation count reaches zero.

Co-Authored-By: Tin Tvrtković <tinchester@gmail.com>
Co-Authored-By: Arthur Tacca
This commit is contained in:
Guido van Rossum 2024-04-09 08:17:28 -07:00 • committed by GitHub
parent 22b25d1eba
commit fa58e75a86
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8 changed files with 183 additions and 13 deletions
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19
Lib/asyncio/taskgroups.py
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@ -77,12 +77,6 @@ class TaskGroup:
propagate_cancellation_error = exc
else:
propagate_cancellation_error = None
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
if et is not None:
if not self._aborting:
@ -130,6 +124,13 @@ class TaskGroup:
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:
@ -139,6 +140,12 @@ class TaskGroup:
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()
# Exceptions are heavy objects that can have object
# cycles (bad for GC); let's not keep a reference to
# a bunch of them.