[ruby-core:115943] [Ruby master Bug#20081] Transfered Fiber doesn't return to Fiber that started it

Issue #20081 has been updated by rmosolgo (Robert Mosolgo).


Thanks for taking a look! I was going from the example in the docs (https://docs.ruby-lang.org/en/master/Fiber.html#method-i-transfer), where the return value of `.transfer` is used by the caller. 

And `.transfer` _does_ return to the caller, as long as the caller is the main Fiber, for example, this prints out a sequence of messages:  

```ruby 
parent = Fiber.current
worker = Fiber.new do |initial_arg|
  puts initial_arg # => 2. Begin Worker
  puts parent.transfer("3. Resume Parent") # => 4. Resume Worker
  "5. Exit Worker"
end

puts "1. Start"
puts worker.transfer("2. Begin Worker")
# => "3. Resume Parent"
puts worker.transfer("4. Resume Worker")
# => "5. Exit Worker"
puts "6. Exit Script"
```


But it behaves differently when inside a surrounding, non-main Fiber: 

```ruby 
manager = Fiber.new do |manager_init|
  puts manager_init # => 1. Start
  parent = Fiber.current
  worker = Fiber.new do |initial_arg|
    puts initial_arg # => 2. Begin Worker
    puts parent.transfer("3. Resume Parent")
    "5. Exit Worker"
  end

  puts worker.transfer("2. Begin Worker")
  # => "3. Resume Parent"
  puts worker.transfer("4. Resume Worker")
  # => "5. Exit Worker"

  "6. Exit Script" # This isn't printed
end

puts manager.transfer("1. Start")

# 1. Start
# 2. Begin Worker
# 3. Resume Parent
# 4. Resume Worker
# 5. Exit Worker
# 
# ... (6 doesn't print here)
```

Maybe the first example is a special case, since it's the main Fiber. I'd be happy to add a comment about that to the Fiber docs if that'd be helpful!



----------------------------------------
Bug #20081: Transfered Fiber doesn't return to Fiber that started it
https://bugs.ruby-lang.org/issues/20081#change-105896

* Author: rmosolgo (Robert Mosolgo)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-darwin22]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
Hi! I'm trying to figure out how to make sure that Fibers started with `.transfer` end up _terminated_, not just suspended. (If they're suspended, Rails thinks they're still alive, and they continue to hold onto database connections, see: https://github.com/rmosolgo/graphql-ruby/issues/4739#issuecomment-1866930914.)

So, I'm looking for way to make sure that any Fiber I start with `.transfer` will be properly terminated. But what I noticed is that when a transfer-based Fiber terminates, it gives control back to the top-most Fiber, not the Fiber which transfered to it. Is this intended? Here's a script to replicate the issue: 


```ruby
manager = Fiber.new do
  parent = Fiber.current
  worker = Fiber.new do
    puts "2. Begin Worker"
    parent.transfer
    puts "4. End Worker"
  end

  puts "1. Transfer 1"
  worker.transfer
  puts "3. Transfer 2"
  worker.transfer
  puts "5. Finished manager"
end

manager.transfer
puts "6. Finished script"
```

I expect the steps to print in order: 

```
1. Transfer 1
2. Begin Worker
3. Transfer 2
4. End Worker
5. Finished manager
6. Finished script
```

But instead, `5. ...` is skipped: 

```
1. Transfer 1
2. Begin Worker
3. Transfer 2
4. End Worker
6. Finished script
```

I think that's because my `worker` fiber terminates and passes control back to the top-level Fiber. 

Should it have passed control back to the `manager`? Or is there another way to make sure `worker` is terminated, and `manager` gets control?



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