[issue46771] Add some form of cancel scopes

[Sam Bull]

Sam Bull <m6f6g7 at sambull.org> added the comment:

> If the task is already pending a cancellation targeted at a cancel scope, the task itself cannot be cancelled anymore since calling cancel() again on the task is a no-op. This would be solved by updating the cancel message on the second call.

> I think Andrew missed one case: in his second diagram, what if the explicit cancel() happened *after* the timeout (but still in the same iteration)? That's the case that makes me want to delete those two lines from Task.cancel() (see my earlier message).

To expand on this point, I've been looking at solving the race conditions in async-timeout. To see how such a race condition can end up with a task never exiting, take a look at this example: https://github.com/aio-libs/async-timeout/issues/229#issuecomment-908502523

In the condition Guido describes, the user's cancellation is suppressed and the code runs forever.

I also wrote tests that seem to reliably reproduce the race condition (the 2nd one still seems unfixable with the current solutions, the 1st was fixed with the nonce/sentinel trick): https://github.com/aio-libs/async-timeout/commit/ab04eb53dcf49388b6e6eacf0a50bafe19c5c74b#diff-60a009a48129ae41018d588c32a6d94c54d1d2948cbc3b831fc27a9c8fdbac68L364-L421

You can see the flow of execution from the call_order assert at the end.

I think most of the solutions proposed here will still not solve this race condition. I initially proposed a solution at: https://bugs.python.org/issue45098

In short, I think that every time we call .cancel(), we need to raise another CancelledError. So, in this race condition you would get 2 CancelledErrors (via an ExceptionGroup). Then our code can catch the error with our nonce/sentinel and handle that, but also reraise any other errors which are unrelated.

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nosy: +dreamsorcerer

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