when erlang gen-server's terminate is call

概述

GenServer的terminate callback在进程退出时会被调用.
但若没有:erlang.process_flag(:trap_exit, true), 进程可能被悄无声息地kill掉, 而不走terminate回调.

terminate 被调用的几种情况

gen_server定义了6个callback接口：

init/1
handle_call/3
handle_cast/2
handle_info/2
terminate/2
code_change/3

对于callback的实现者来说，理解callback函数的触发点是最重要的，本文只讨论terminate的调用。

terminate的被调用有如下几种情况：

• handle_call返回 {stop, Reason, Reply, StateN} -> terminate(Reason, StateN)
• handle_info handle_cast返回 {stop, Reason, StateN} -> terminate(Reason, StateN)
• handle_call handle_info handle_cast 抛出异常(在gen_server.erl中被捕获为{‘EXIT’, What}) -> terminate(What, State)
• handle_call handle_info handle_cast 返回值非法 -> terminate({bad_return_value, Reply}, State)
• 收到来自Parent的{‘EXIT’, Parent, Reason}消息：

设置了一个timeout或者无限等待的情况下，supervisor是通过exit(Pid, shutdown)来通知子进程退出的，所以，若supervisor下的gen_server worker进程没有设为系统进程，worker进程不会收到来自Parent的Exit消息，故terminate不会被调用。

示例

App.Supervisor下有一个App.Worker, 代码如下.

defmodule App do
  use Application
  require Logger

  def start(_type, _args) do
    children = [
      %{id: App.Supervisor, start: {App.Supervisor, :start_link, []}, type: :supervisor}
    ]

    opts = [strategy: :one_for_one, name: __MODULE__]
    Supervisor.start_link(children, opts)
  end
end

defmodule App.Supervisor do
  use Supervisor
  require Logger

  def start_link() do
    Logger.info("app.sup start")
    Supervisor.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init([]) do
    children = [
      %{id: App.Worker, start: {App.Worker, :start_link, []}, type: :worker}
    ]

    opts = [strategy: :one_for_one, name: __MODULE__]
    Supervisor.init(children, opts)
  end
end

defmodule App.Worker do
  use GenServer
  require Logger
  require Record

  Record.defrecordp(:state, [])

  def start_link() do
    Logger.info("app.worker start")
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  def init(_) do
    # :erlang.process_flag(:trap_exit, true)
    {:ok, nil}
  end

  def handle_call(:stop, _from, state) do
    {:stop, nil, state}
  end

  def handle_call(:raise, _from, state) do
    raise RuntimeError
    {:reply, nil, state}
  end

  def terminate(reason, state) do
    Logger.warn("app.worker terminate #{inspect(reason)} #{inspect(state)}")
    :ok
  end
end

没有trap_exit

若没有:erlang.process_flag(:trap_exit, true).

• Supervisor.stop(App.Supervisor)
• Supervisor.terminate_child(App.Supervisor, App.Worker)
• Process.exit(:erlang.whereis(App.Worker), :normal)

均不会触发terminate回调.

• GenServer.call(App.Worker, :stop)
• GenServer.call(App.Worker, :raise)

会触发terminate回调.

有trap_exit

• Supervisor.stop(App.Supervisor)
• Supervisor.terminate_child(App.Supervisor, App.Worker)

会触发terminate回调.

20:25:00.568 [warn]  app.worker terminate :shutdown nil

• Process.exit(:erlang.whereis(App.Worker), :normal)

会收到{:EXIT, #PID<0.149.0>, :normal}消息, 若处理了消息, 不会触发terminate回调.

20:25:59.849 [warn]  unknown_info:{:EXIT, #PID<0.134.0>, :normal}

• Process.exit(:erlang.whereis(App.Worker), :shutdown)

20:27:55.170 [warn]  unknown_info:{:EXIT, #PID<0.134.0>, :shutdown}

• GenServer.call(App.Worker, :stop)
• GenServer.call(App.Worker, :raise)

会触发terminate回调.

总结

terminate被调用的方式有如下几种：

• 返回值触发
• 代码错误抛出异常被捕获
• 收到来自系统的Exit消息

如果希望gen_server进程崩溃时terminate一定被调用到(exit(Pid, kill)除外)，设为system进程即可。
本篇文章部分内容来自于多年前写的blog, 当时未记录源码内容.