class WaitingQueue<Queue
  def initialize(*args)
    super
    @mutex=Mutex.new
    @waitThreads=[]
  end
  
  def push(value)
    @mutex.synchronize {
      super
    }
    @waitThreads.each{|th|th.wakeup}
    @waitThreads.uniq!
  end
  def <<(value)
    push(value)
  end
    
  def pop
    @waitThreads<<Thread.current
    loop do
      Thread.stop if empty?
      @mutex.synchronize {
        unless empty?
          value=super
          @waitThreads.delete(self)
          @waitThreads.uniq!
          return value
        end
        
      }
    end
  end
end