[ruby-core:117458] [Ruby master Bug#20414] `Fiber#raise` should recurse to `resumed_fiber` rather than failing.

["ioquatix (Samuel Williams) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20414 has been reported by ioquatix (Samuel Williams).

----------------------------------------
Bug #20414: `Fiber#raise` should recurse to `resumed_fiber` rather than failing.
https://bugs.ruby-lang.org/issues/20414

* Author: ioquatix (Samuel Williams)
* Status: Open
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
The following program will fail with `FiberError`, and is difficult to properly clean up:

```ruby
root_fiber = Fiber.current

f1 = Fiber.new do
	root_fiber.transfer
end

f2 = Fiber.new do
	f1.resume
end

f2.transfer

f2.raise("error") # => `raise': attempt to transfer to a resuming fiber (FiberError)
```

This program deliberately set's up a scenario where `f2` is resuming `f1`. Trying to raise an exception on `f2` is impossible, because the only way control flow can return to it, is when `f1` yields or exits.

We can avoid this problem, by raising the exception on f1, and we can do this automatically using the following logic:

```c
static VALUE
fiber_raise(rb_fiber_t *fiber, VALUE exception)
{
    // Add this recursive step:
    if (fiber->resuming_fiber) {
        return fiber_raise(fiber->resuming_fiber, exception);
    }

    // Existing code ...
    else if (FIBER_SUSPENDED_P(fiber) && !fiber->yielding) {
        return fiber_transfer_kw(fiber, -1, &exception, RB_NO_KEYWORDS);
    }
    else {
        return fiber_resume_kw(fiber, -1, &exception, RB_NO_KEYWORDS);
    }
}
```

This makes `Fiber#raise` much more robust and useful for the purpose of stopping fibers, without knowing exactly what they are doing.



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