waiter.rb

  class Waiter
    def initialize
      @m, @cv = Mutex.new, ConditionVariable.new
      @signalled = false
    end
    def wait
      @m.synchronize{ @cv.wait(@m) until @signalled; @signalled = false }
    end
    def signal
      @m.synchronize{ @signalled = true; @cv.signal }
    end
  end