[ruby-core:115942] [Ruby master Bug#20089] Fiber#kill transfers to root fiber

Issue #20089 has been updated by ioquatix (Samuel Williams).


Transfer is uni-directional and keeps no state about the caller. It's up to the caller to implement its own control flow if preferred. It's a low level operation which must be used more carefully.

```ruby
manager = Fiber.new do
  worker = Fiber.new do
    puts "2. Begin Worker"
    manager.transfer
    Fiber.current.kill
    puts "This should never print -- killed"
  ensure
    manager.transfer
  end

  puts "1. Transfer to Worker"
  worker.transfer
  puts "3. Killing Worker"
  worker.transfer
  puts "4. Finished manager"
end

manager.transfer
puts "5. Finished script"
```

By explicitly adding the flow control, we can achieve your desired output. Hope this helps.

----------------------------------------
Bug #20089: Fiber#kill transfers to root fiber
https://bugs.ruby-lang.org/issues/20089#change-105895

* Author: rmosolgo (Robert Mosolgo)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-darwin22]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
I was hoping to use `Fiber#kill` to clean up formerly `.transfer`-d Fibers and work around https://bugs.ruby-lang.org/issues/20081, but I found that `Fiber#kill` has a similar control flow jump behavior. Is this on purpose, or a bug? 

Here's a script to test the behavior: 

```ruby
manager = Fiber.new do
  worker = Fiber.new do
    puts "2. Begin Worker"
    manager.transfer
    puts "This should never print -- killed"
  end

  puts "1. Transfer to Worker"
  worker.transfer
  puts "3. Killing Worker"
  worker.kill
  puts "4. Finished manager"
end

manager.transfer
puts "5. Finished script"
```

I expected items `1` through `5` to be printed in order, but in fact, `4` is never printed: 

```
$ ruby fiber_transfer_test.rb
1. Transfer to Worker
2. Begin Worker
3. Killing Worker
5. Finished script
```

It seems like `worker.kill` is transferring control to the top-level fiber instead of giving it back to `manager`.



I also tried having the thread kill _itself_, hoping it would return to the fiber that originally `.transfer`ed to it, but it also seems to jump out: 

```ruby
manager = Fiber.new do
  worker = Fiber.new do
    puts "2. Begin Worker"
    manager.transfer
    Fiber.current.kill
    puts "This should never print -- killed"
  end

  puts "1. Transfer to Worker"
  worker.transfer
  puts "3. Killing Worker"
  worker.transfer
  puts "4. Finished manager"
end

manager.transfer
puts "5. Finished script"
```

Prints: 

```
1. Transfer to Worker
2. Begin Worker
3. Killing Worker
5. Finished script
```



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