class Queue

def close
  @closed = true
  2.times do
    Thread.pass
    num_waiting.times do
      push_without_close CLOSE_MESSAGE
    end
  end
  self
end

def closed?
  !!defined?(@closed)
end

def pop_with_close(non_block = false)
  begin
    r = pop_without_close(non_block || closed?)
    r unless CLOSE_MESSAGE == r
  rescue ThreadError
    raise if non_block || !closed?
  end
end

def push_with_close(arg)
  raise ClosedQueueError, 'queue closed' if closed?
  push_without_close(arg)
end