CREF for sclass shared between threads

cref.rb

num_threads = (ARGV[0] || 2).to_i
num_iterations = (ARGV[1] || num_threads).to_i

def url_helpers
  Module.new do
    class << self
      # This line changes everything. It causes the GIL to switch to another
      # thread. That other thread mutates the CREF that is accidentally shared
      # between these independent sclass scopes, causing the def below to add
      # the method on an entirely different sclass.
      puts "inside"
      attr_reader :before
      def url_options; {}; end
      attr_reader :after
    end

    unless respond_to? :url_options
      raise "method definiton has failed - #{self.methods(false)}"
    end
  end
end

threads = []
num_threads.times do |thread_num|
  threads << Thread.new(thread_num) do |num|
    puts "S - #{num}"
    num_iterations.times do |i|
      url_helpers
    end
    puts "E - #{num}"
  end
end

threads.each do |thr|
  thr.join
end

Hey Evan, which line? #6, class << self? Would this be the case in any codebase where this is used in a threaded app?