[issue46771] Add some form of cancel scopes

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

To make this cleanly interact with timeout, TaskGroup etc., the CancelOnEvent class should have a "did-I-cancel" flag which is set in the _cancel_on_event() callback. Then if that flag is set it should call .uncancel(), and if that returns a value > 0, it should bubble the CancelledError out; otherwise it can raise EventRaised (if the condition is set).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46771>
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