door_lock_mutex.rb

class DoorLock
  def initialize(locked)
    @locked = locked
    @mutex = Mutex.new
  end
  
  def open?
    @mutex.synchronize { !@locked }
  end

  def unlock!
    unless open?
      @mutex.synchronize do
        puts "Opening the door!"
        @locked = false 
      end
    end
  end
end