[ruby-core:116007] [Ruby master Feature#20102] Introduce `Fiber#resuming?`

Issue #20102 has been updated by Eregon (Benoit Daloze).


ioquatix (Samuel Williams) wrote in #note-4:
> @Eregon I appreciate your input. I don't mind doing that, but isn't that the same for all methods on Fiber?

It is, and many methods on Fiber are already not allowed to be called for Fiber belonging to other threads (e.g. resume/transfer/storage/...).
Since we are adding a new method, we might as well encourage correct usage and discourage incorrect usage (+ it means the state for resuming does not need to become `volatile` which would be bad for performance).

> In other words, the GVL provides a bit of a safety net.

It does not, the GVL could be released e.g. just after `resuming?` is done but before the caller gets the value and handle it.

----------------------------------------
Feature #20102: Introduce `Fiber#resuming?`
https://bugs.ruby-lang.org/issues/20102#change-106001

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
----------------------------------------
There are some tricky edge cases when using `Fibre#raise` and `Fiber#kill`, e.g.

```ruby
fiber = nil

killer = Fiber.new do
  fiber.raise("Stop")
end

fiber = Fiber.new do
  killer.resume
end

fiber.resume
# 4:in `raise': attempt to raise a resuming fiber (FiberError)
# 4:in `block in <main>'
```

Async has to deal with this edge case explicitly by rescuing the exception:

https://github.com/socketry/async/blob/ffd019d9c1d547926a28fe8f36bf7bfe91d8a168/lib/async/task.rb#L226-L233

I'd like to avoid doing that and instead just ask "Can I kill/raise on this fiber right now?" which is determined by whether the fiber itself can be resumed or transferred to.

To address this, I'd like to introduce `Fiber#resuming?`:

```c
/*
 *  call-seq: fiber.resumed? -> true or false
 *
 *  Whether the fiber is currently resumed.
 */
VALUE
rb_fiber_resuming_p(VALUE fiber_value)
{
    struct rb_fiber_struct *fiber = fiber_ptr(fiber_value);

    if (FIBER_TERMINATED_P(fiber)) return RUBY_Qfalse;

    return RBOOL(fiber->resuming_fiber);
}
```

See the PR: https://github.com/ruby/ruby/pull/9382



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