消息与超时

# 发出请求前，分别准备“观察进程”和“配对回复”的 reference
def call(server, request, timeout \\ 1_000) do
  monitor_ref = Process.monitor(server)
  request_ref = make_ref()
  # 同时告诉服务端回信地址和请求编号
  send(server, {:call, self(), request_ref, request})

  receive do
    # 正常回复要匹配 request reference
    {:reply, ^request_ref, response} ->
      # 不再观察，并清掉可能已到达的 DOWN
      Process.demonitor(monitor_ref, [:flush])
      {:ok, response}

    # monitor 单向报告服务进程退出
    {:DOWN, ^monitor_ref, :process, ^server, reason} ->
      {:error, {:server_down, reason}}
  after
    # timeout 只停止等待，不会撤回 request
    timeout ->
      Process.demonitor(monitor_ref, [:flush])
      {:error, :timeout}
  end
end