Add what's new entry

Doc/whatsnew/3.13.rst

@@ -295,6 +295,28 @@ asyncio
  never awaited).
  (Contributed by Arthur Tacca and Jason Zhang in :gh:`115957`.)
 
+* Improved behavior of :class:`asyncio.TaskGroup` when an outside cancellation
+ collides with an internal cancellation. For example, when two task groups
+ are nested and both experience an exception in a child task simultaneously,
+ it was possible that the outer task group would hang, because its internal
+ cancellation was swallowed by the inner task group.
+
+ In the case where a task group is cancelled from the outside and also must
+ raise an :exc:`ExceptionGroup`, it will now call the parent task's
+ :meth:`~asyncio.Task.cancel` method. This allows a :keyword:`try` /
+ :keyword:`except* <except_star>` surrounding the task group to handle
+ the exceptions in the ``ExceptionGroup`` without losing the cancellation.
+ The :exc:`asyncio.CancelledError` will be raised at the next
+ :keyword:`await` (which may be implied at the end of a surrounding
+ :keyword:`async with` block).
+
+ An added benefit of these changes is that task groups now preserve the
+ cancellation count (:meth:`asyncio.Task.cancelling`).
+
+ In order to handle some corner cases, :meth:`asyncio.Task.uncancel` may now
+ reset the undocumented ``_must_cancel`` flag when the cancellation count
+ reaches zero.
+
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