Ractor incompatibility: DefaultFactory not shareable, C extension methods marked ractor-unsafe

[mensfeld]

The msgpack gem (1.8.0) cannot be used inside Ractors on Ruby 4.0. Every operation - MessagePack.pack, .unpack, creating Packer/Unpacker/Factory instances - fails. This makes it impossible to use Ractors for parallel MessagePack serialization/deserialization.

#!/usr/bin/env ruby
# frozen_string_literal: true

# POC: msgpack gem 1.8.x is incompatible with Ruby 4.0 Ractors

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  gem "msgpack", "~> 1.8"
end

require "msgpack"

puts "Ruby #{RUBY_VERSION}, msgpack #{MessagePack::VERSION}"
puts "-" * 50

tests = {
  "MessagePack.pack"       => -> { Ractor.new { MessagePack.pack({"a" => 1}) }.value },
  "MessagePack.unpack"     => -> { Ractor.new(MessagePack.pack("hi")) { |d| MessagePack.unpack(d) }.value },
  "Packer.new in Ractor"   => -> { Ractor.new { MessagePack::Packer.new.write("x").to_s }.value },
  "Unpacker.new in Ractor" => -> { Ractor.new(MessagePack.pack(1)) { |d| u = MessagePack::Unpacker.new; u.feed(d); u.read }.value },
  "Factory.new in Ractor"  => -> { Ractor.new { MessagePack::Factory.new }.value },
  "Send Packer to Ractor"  => -> { Ractor.new(MessagePack::Packer.new) { |p| p.to_s }.value },
  "Access DefaultFactory"  => -> { Ractor.new { MessagePack::DefaultFactory }.value },
}

tests.each do |name, test|
  print "#{name}: "
  begin
    test.call
    puts "OK"
  rescue Ractor::RemoteError => e
    puts "FAILED — #{e.cause.class}: #{e.cause.message}"
  rescue => e
    puts "FAILED — #{e.class}: #{e.message}"
  end
end