mutex_io.rb

require "thread"

mutex = Mutex.new

i = 0
while true
  i += 1
  Thread.new(i) do |n|
    mutex.synchronize do
      puts "#{n}. started request"
      Thread.new(n) do |n|
        puts "#{n}. started background"
        sleep 10
        puts "#{n}. finished background"
      end
      sleep 1
      puts "#{n}. finished request"
    end
  end
  sleep 2
end