[ruby-core:115883] [Ruby master Bug#20082] Killing fibers across threads: unexpected exception

Issue #20082 has been updated by rubyFeedback (robert heiler).


The error message is indeed somewhat peculiar:

"attempt to resume **a resumed fiber** ( **double resume** )"

I am not sure what "double resume" means here. But, even aside
from this, I could not even try to want to explain what "to
resume a resumed fiber" really means. Perhaps something else
was meant here? Or it was meant to convey that the code tried
to resume an already-resumed fiber. Either way it is indeed
peculiar.

I'd even be radical and suggest to get rid of fibers and instead
put everything under Thread, but I guess that is not possible
for various reasons. To me Threads seem easier to understand 
than Fibers though.

----------------------------------------
Bug #20082: Killing fibers across threads: unexpected exception
https://bugs.ruby-lang.org/issues/20082#change-105840

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
For providing the example in a changelog, I tried to imitate killing fibers belonging to other threads.
Documentation [claims](https://docs.ruby-lang.org/en/master/Fiber.html#method-i-kill)

> Raises FiberError if called on a fiber belonging to another thread.

So, I created this artificial example to check how it works:
```ruby
fibers = []

Thread.new {
  f = Fiber.new { (1..).each { sleep(0.1) } }
  fibers << f
  f.resume
}
sleep(0.1) # to make sure the thread has started

fibers.last.kill
```
The example indeed fails with `FiberError`, but the error message is confusing:
```
in `kill': attempt to resume a resumed fiber (double resume) (FiberError)
```

Is this an expected message for such case? Or am I misunderstanding something?



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