[ruby-core:100854] [Ruby master Feature#17325] Adds Fiber#cancel, which forces a Fiber to break/return

Issue #17325 has been updated by nevans (Nicholas Evans).

Description updated

updated description and github PR

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Feature #17325: Adds Fiber#cancel, which forces a Fiber to break/return
https://bugs.ruby-lang.org/issues/17325#change-88496

* Author: nevans (Nicholas Evans)
* Status: Open
* Priority: Normal
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This skips rescue and catch blocks but runs all ensure blocks. It can be run on any living fiber, and propagates cancellation to child (resumed) fibers.

Basically, I want it to behave as if you called `break` or `return` to jump all the way to the top frame of the fiber from wherever it is.. Exceptions might be too heavy-weight, can't (and shouldn't) be sent to resuming fibers, and can be caught. This can also be emulated via `catch/throw` with an anonymous `Object.new`, but that adds an extra stack frame, and implementing it reliably requires interception of `Fiber.yield`, e.g. adding a `Fiber.throw` (analogous to `Fiber.raise`) which also shouldn't be sent to resuming fibers.

I tried to match the new _(much improved!)_ transfer/resume switch semantics with cancel propagation. When the cancellation propagates all the way to the bottom of the fiber resume stack, the last fiber in the chain will then be resumed. Resuming fibers will not run until they are yielded back into.

If called multiple times, each `cancel` will propagate cancellation and behave as if `break` or `return` was called.

Implementation:  https://github.com/ruby/ruby/pull/3766



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