From: "luke-gru (Luke Gruber) via ruby-core" <ruby-core@...>
Date: 2026-07-28T18:19:55+00:00
Subject: [ruby-core:126186] [Ruby Bug#22211] Fiber::Scheduler stalls on Ractor::Port.receive

Issue #22211 has been updated by luke-gru (Luke Gruber).


The tricky part in integrating the fiber scheduler with `Ractor::Port` `send/receive` is what to do when you want to wakeup (unblock) a fiber on another Ractor. Normally unblocking a fiber is done on the same Ractor and it's done in a context where you can call directly into `rb_fiber_scheduler_unblock`. When the Fiber Scheduler is on a different Ractor, you can't call its unblock method directly (and a Ruby thread may not even be running on that Ractor. It could be in a syscall, etc.). What we could do is have a special high priority Ruby thread on each Ractor that only calls Fiber Scheduler unblock methods. When you send on a port, you queue up the scheduler unblock operation on that thread and then set the thread to `ready`. The thread scheduler always checks if this thread is ready before dequeuing a real thread, and if so it runs the unblock method. Even that's not ideal because it would go through a thread switch and a fiber switch every time you send to a Ractor that has a Fib
 er Scheduler, but it's the best I can come up with right now.

I created a branch [here](https://github.com/luke-gruber/ruby/tree/port_send_receive_fiber_scheduler) just for experimentation. Right now it creates a new thread on each `Port#send` (not ideal). I'll work on caching the thread soon.

Scheduler work is not our priority at the moment as we're looking into making GC faster with Ractors (ractor-local GC). However I might work on this a bit if I have some time.

----------------------------------------
Bug #22211: Fiber::Scheduler stalls on Ractor::Port.receive
https://bugs.ruby-lang.org/issues/22211#change-118262

* Author: rkh (Konstantin Haase)
* Status: Open
* Assignee: ractor
* ruby -v: ruby 4.1.0dev (2026-07-24T12:18:38Z master 180b664bd9) +PRISM [arm64-darwin25]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
Quickly upfront: This might be a duplicate of #20276, and let me know if the official solution is still to launch a thread for each port so it can block (or add some more complex thread orchestration).

Anyway, I think `Ractor::Port#receive` should call `Fiber::Scheduler#block`/`Fiber::Scheduler#unblock` if a scheduler is set (or some other hook), the way that `Thread::Queue` does. Instead, it completely blocks the scheduler.

Example to reproduce:

``` ruby
Warning[:experimental] = false

class Scheduler
  def initialize
    @fiber   = Fiber.current
    @blocked = Set.new
    @ready   = Queue.new
  end

  def scheduler_close
    @ready.pop.transfer while @blocked.any?
  end

  def block(blocker, timeout = nil)
    raise NotImplementedError, "block: timeout not supported" if timeout
    @blocked << Fiber.current
    @fiber.transfer
  ensure
    @blocked.delete(Fiber.current)
  end

  def unblock(blocker, fiber)              = @ready << fiber
  def io_write(io, buffer, offset, length) = Thread.new { buffer.write(io, offset, length) }.value
  def fiber(&)                             = Fiber.new(blocking: false, &).tap(&:transfer)
  def respond_to_missing?(...)             = true
  def method_missing(method, ...)          = raise NotImplementedError, "#{method}: not implemented"
end

Fiber.set_scheduler(Scheduler.new)

# This would work:
#
#   queue = Queue.new
#   Fiber.schedule { puts queue.pop }
#   queue.push "Hello from Queue!"

port = Ractor::Port.new
Fiber.schedule { puts port.receive }
port.send("Hello from Ractor::Port!") # never gets here
```






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