thread_safe_queue.rb

require 'thread'

q = Queue.new
eq = Enumerator.new(-> { q.size }) do |y|
  loop do
    y << q.pop
  end
end

doubler = Thread.new do
  eq.each do |n|
    break if n == :stop
    p n * n
  end
end

q << 1
q << 2
q << 4
p eq.size
q << :stop
doubler.join