談談GenServer的terminate回調.

概述

GenServer的terminate callback在進程退出時會被調用.
但若沒有:erlang.process_flag(:trap_exit, true), 進程可能被悄無聲息地kill掉, 而不走terminate回調.

示例

App.Supervisor下有一個App.Worker, 代碼如下.

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

沒有trap_exit

若沒有:erlang.process_flag(:trap_exit, true).

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

均不會觸發terminate回調.

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

會觸發terminate回調.

有trap_exit

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

會觸發terminate回調.

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

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

會收到{:EXIT, #PID<0.149.0>, :normal}消息, 若處理了消息, 不會觸發terminate回調.

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)

會觸發terminate回調.

總結

GenServer msg loop中的邏輯, 不管是stop, 還是異常(很可能被捕獲), 都會觸發terminate.
若沒有trap_exit, 收到exit消息時, 進程會直接被殺, 不會觸發terminate.
若有trap_exit, 可以保證terminate被調用.