Issue #13821 has been updated by Eregon (Benoit Daloze).


I think this is first of all a problem for semantics.

If we allow fibers to be resumed on another Thread, we allow multiple fibers originally from the same thread to execute concurrently
(so they no longer see the effects of each other perfectly but are exposed to race conditions like threads).

It also means before and after Fiber.yield, the value of Thread.current can change if the Fiber is resumed on another Thread.
This in turns breaks Fiber-locals with the current Thread.current[key] API.

It's also problematic for locks and other resources which are per-thread (some of them native so they cannot be tricked to use the initial Thread of the Fiber):

    Fiber.new { shared = Object.new; lock.synchronize { shared.a += 1; Fiber.yield; shared.b -= 1 } }

The unlock operation will fail because it's on a different thread than the lock operation if the fiber us resumed on another thread.

----------------------------------------
Feature #13821: Allow fibers to be resumed across threads
https://bugs.ruby-lang.org/issues/13821#change-66615

* Author: cremes (Chuck Remes)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
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)


-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>