in_main_ractor.rb

module InMainRactor
  def self.pipe() = Ractor.new { loop { Ractor.yield Ractor.recv } }
  REQUEST = pipe
  LOCK = pipe
  LOCK << :lock
  RESPONSE = pipe
  MAIN_RACTOR = Ractor.current
  class Delegator < BasicObject
    def initialize(receiver)
      @receiver = receiver
    end
    def method_missing(method, *args, **kwargs)
      lock = ::InMainRactor::LOCK.take
      ::InMainRactor::REQUEST << [@receiver, method, args, kwargs]
      response = ::InMainRactor::RESPONSE.take
      ::InMainRactor::LOCK << lock
      response
    end
  end
  
  def self.run_main_ractor
    loop do
      receiver, method, args, kwargs = REQUEST.take
      RESPONSE << receiver.__send__(method, *args, **kwargs)
    rescue => e
      RESPONSE << e
    end
  end

  Thread.new { run_main_ractor }
  module Delegatable
    def in_main_ractor
      if ::Ractor.current == ::InMainRactor::MAIN_RACTOR
        self
      else
        InMainRactor::Delegator.new self
      end
    end
  end
end

in_main_ractor_sample.rb

require_relative './in_main_ractor'

Module.include InMainRactor::Delegatable

class Foo
  def self.bar
    @@x ||= 0
    @@x += 1
  end
end

10.times do
  Ractor.new do
    100.times { Foo.in_main_ractor.bar }
  end
  p :done
end
sleep 1