[ruby-core:115956] [Ruby master Feature#13821] Allow fibers to be resumed across threads

28 Dec 2023

      Issue #13821 has been updated by ioquatix (Samuel Williams).

One issue was the use of the copy-coroutine. As we've removed this and replaced it with a pthread fallback, we can now potentially allow fibers to be resumed across threads. I'll put together a PR so we can experiment.

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Feature #13821: Allow fibers to be resumed across threads
https://bugs.ruby-lang.org/issues/13821#change-105913

* Author: cremes (Chuck Remes)
* Status: Assigned
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
----------------------------------------
Given a Fiber created in ThreadA, Ruby 2.4.1 (and earlier releases) raise a FiberError if the fiber is resumed in ThreadB or any other thread other than the one that created the original Fiber.

Sample code attached to demonstrate problem.

If Fibers are truly encapsulating all of the data for the continuation, we should be allowed to move them between Threads and resume their operation.

Why?

One use-case is to support the async-await asynchronous programming model. In that model, a method marked async runs *synchronously* until the #await method is encountered. At that point the method is suspended and control is returned to the caller. When the #await method completes (asynchronously) then it may resume the suspended method and continue. The only way to capture this program state, suspend and resume, is via a Fiber.

example:

```
class Wait
  include AsyncAwait

  def dofirst
    async do
      puts 'Synchronously print dofirst.'
      result = await { dosecond }
      puts 'dosecond is complete'
      result
    end
  end

  def dosecond
    async do
      puts 'Synchronously print dosecond from async task.'
      slept = await { sleep 3 }
      puts 'Sleep complete'
      slept
    end
  end

  def run
    task = dofirst
    puts 'Received task'
    p AsyncAwait::Task.await(task)
  end
end

Wait.new.run
```
```
# Expected output:
# Synchronous print dofirst.
# Received task
# Synchronously print dosecond from async task.
# Sleep complete
# dosecond is complete
# 3
```
Right now the best way to accomplish suspension of the #dofirst and #dosecond commands and allow them to run asynchronously is by passing those blocks to *another thread* (other than the callers thread) so they can be encapsulated in a new Fiber and then yielded. When it's time to resume after #await completes, that other thread must lookup the fiber and resume it. This is lots of extra code and logic to make sure that fibers are only resumed on the threads that created them. Allowing Fibers to migrate between threads would eliminate this problem.

---Files--------------------------------
fiber_across_threads.rb (377 Bytes)
wait.rb (728 Bytes)

-- 
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