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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
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Guido van Rossum 2024-04-09 08:17:28 -07:00 • committed by GitHub
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Doc/library/asyncio-task.rst
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@ -392,6 +392,27 @@ is also included in the exception group.
The same special case is made for
:exc:`KeyboardInterrupt` and :exc:`SystemExit` as in the previous paragraph.
Task groups are careful not to mix up the internal cancellation used to
"wake up" their :meth:`~object.__aexit__` with cancellation requests
for the task in which they are running made by other parties.
In particular, when one task group is syntactically nested in another,
and both experience an exception in one of their child tasks simultaneously,
the inner task group will process its exceptions, and then the outer task group
will receive another cancellation and process its own exceptions.
In the case where a task group is cancelled externally and also must
raise an :exc:`ExceptionGroup`, it will call the parent task's
:meth:`~asyncio.Task.cancel` method. This ensures that a
:exc:`asyncio.CancelledError` will be raised at the next
:keyword:`await`, so the cancellation is not lost.
Task groups preserve the cancellation count
reported by :meth:`asyncio.Task.cancelling`.
.. versionchanged:: 3.13
Improved handling of simultaneous internal and external cancellations
and correct preservation of cancellation counts.
Sleeping
========
@ -1369,6 +1390,15 @@ Task Object
catching :exc:`CancelledError`, it needs to call this method to remove
the cancellation state.
When this method decrements the cancellation count to zero,
the method checks if a previous :meth:`cancel` call had arranged
for :exc:`CancelledError` to be thrown into the task.
If it hasn't been thrown yet, that arrangement will be
rescinded (by resetting the internal ``_must_cancel`` flag).
.. versionchanged:: 3.13
Changed to rescind pending cancellation requests upon reaching zero.
.. method:: cancelling()
Return the number of pending cancellation requests to this Task, i.e.,