[ruby-core:115944] [Ruby master Bug#20081] Transfered Fiber doesn't return to Fiber that started it

Issue #20081 has been updated by ioquatix (Samuel Williams).


In think clarifying the documentation is a good idea.

> 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:

I think that's just intuitively correct based on the semantics.

Actually, one could argue that a fiber that was transferred to, that exits, without an explicit transfer, should be considered exiting the thread completely and in your case terminating the program. In this case, Ruby is being a bit generous with "transferring back to the main fiber" IMHO. Transfer should be seen as a one way operation.

----------------------------------------
Bug #20081: Transfered Fiber doesn't return to Fiber that started it
https://bugs.ruby-lang.org/issues/20081#change-105897

* 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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