[issue46771] Add some form of cancel scopes

[Alex Grönholm]

Alex Grönholm <alex.gronholm at nextday.fi> added the comment:

I'm not trying to argue that asyncio should be changed to have level cancellation or even cancel scopes as built-in (at this point), but expanding the low level API to make implementing these features possible in third party libraries without the awkward hacks we have now.

As for async-timeout, it suffers from the same problem as AnyIO and Quattro: that cancellations of the entire task can be inadvertently swallowed by the async context manager in edge cases. I hadn't even thought of the possibility of this happening until one of AnyIO's users reported just such a problem: https://github.com/agronholm/anyio/issues/374

I just couldn't think of any way to correctly support such things without at least _some_ changes to the task cancellation behavior, and allowing .cancel() to update the cancel message seemed like the least invasive option. I'm all ears if someone has a better solution.

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nosy:  -ajoino

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