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.
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) endenddefmodule 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) endenddefmodule 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 endend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
GenServer msg loop中的逻辑, 不管是stop, 还是异常(很可能被捕获), 都会触发terminate.
若没有trap_exit, 收到exit消息时, 进程会直接被杀, 不会触发terminate.
若有trap_exit, 可以保证terminate被调用.
因为结果很合乎直觉, 就不看源码深究了.