Bug #21037: Ractors hang with multiple threads

• Status: Open
• Assignee: -
• Target version: -
• ruby -v: ruby 3.5.0dev (2025-01-14T20:59:39Z master f1e32914eb) +PRISM [x86_64-linux]
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[tenderlovemaking (Aaron Patterson)]

The following program hangs, but I don't expect it to hang:

# frozen_string_literal: true

class Map
  def initialize
    @r = Ractor.new {
      cache = { "int" => "integer", "bool" => "boolean" }
      loop do
        key = receive
        Ractor.yield key
      end
    }
    freeze
  end

  def fetch(key)
    @r.send key
    m = @r.take
    raise unless key == m
    m
  end
end

tm = Map.new
t1 = Thread.new { 10.times { |i| p [i + 1,tm.fetch("int")] } }
t2 = Thread.new { 10.times { |i| p [i + 1, tm.fetch("bool")] } }

t1.join
t2.join

I'm not sure why this program hangs yet, but it does both on macOS and Linux. I've tested with master, Ruby 3.4, and Ruby 3.3. I'm not sure why it's hanging yet, but I wanted to file an issue.

[luke-gru (Luke Gruber)]

Maybe this fixes it? https://github.com/ruby/ruby/pull/12520

Edit: I just tried with this branch but it seems unrelated. If you run with RUBY_DEBUG it fails an assertion having to do with ractor sleep (the receive call). I'll take a look when I get a chance.

[luke-gru (Luke Gruber)]

It seems calling ractor.take from multiple threads is unsupported at the moment. I don't know if this is a limitation or a bug. Of course, you can work around it and call select or take from a single thread, like main, and signal a ready event with a queue or something for the consumer threads.

Edit: There are some TODOs in the codebase having to do with waiting threads for ractors, and they explicitly say "TODO: make multithreaded". This is a known problem, at least for @ko1 (Koichi Sasada).