Running code when a script exits

fxn

I have a script executed with mix run that has an infinite loop that collects some data in an agent. I’d like to print that data when I interrupt the script.

Tried System.at_exit/1 without success, maybe because the only way I know to interrupt the execution is C-c C-c.

Is there a way to accomplish that?

19 comments

#mix-run

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2020-06-20 07:59:40 UTC

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NobbZ

Ctrl+C, Ctrl+C kills the Virtual Machine. Ctrl+G, Q should exit in a way that the exit handler gets called, alternatively you can try kill $BEAM_PID from another terminal.

Post #1

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josevalim

Creator of Elixir

There are two questions here:

How to shut the system down cleanly
How to handle certain signals (interrupt)

To answer 1, System.stop/1 is the answer. If terminate/2 callbacks are not invoked, make sure those processes are trapping exits.

To answer 2, since Erlang/OTP 20, there is OS signal handling: http://erlang.org/doc/man/kernel_app.html#os-signal-event-handler

It is a bit of a long answer. In a nutshell, you need to implement a gen_event handler and react to the callbacks you are interested. There is a more detailed guide here: https://medium.com/@ellispritchard/graceful-shutdown-on-kubernetes-with-signals-erlang-otp-20-a22325e8ae98

Post #13

josevalim

Creator of Elixir

Another trick you could do is System.at_exit(fn int -> your_code(); System.stop(int) end).

Post #17

fxn

I have investigated a bit the solution with the signal handler.

Erlang executes :init.stop() on SIGTERM by default. That means that a simple kill PID shuts the thing down orderly.

However, you need to figure out the PID of the process, and open a different terminal to execute the command. Besides, custom code passed to System.at_exit/1 is not invoked by the event handler.

Ctrl-C sends SIGINT. The only shortcut that I’ve seen sends a different signal is Ctrl-\, which sends SIGQUIT. So, I am using this event handler:

defmodule QuitScript do
  @behaviour :gen_event

  def init(_), do: {:ok, nil}
  def handle_event(:sigquit, _state), do: :remove_handler
  def handle_event(signal, state), do: :erl_signal_handler.handle_event(signal, state)
  def handle_call(_, state), do: {:ok, :ok, state}

  def terminate(_reason, _state) do
    IO.puts("My exit code")
    System.stop()
  end
end

:gen_event.swap_handler(:erl_signal_server, {:erl_signal_handler, []}, {QuitScript, []})

Notice that the terminate/2 callback allows you to run custom code on exit.

If you throw that to a script and execute it for example like this

mix run --no-halt foo.exs

Ctrl-C still behaves as always, and additionally Ctrl-\ shuts the thing down orderly (you only see ^\ echoed in the terminal, not too bad).

Post #18

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fxn

Alternatively, we could move System.stop() to the callback:

defmodule QuitScript do
  @behaviour :gen_event

  def init(_), do: {:ok, nil}
  def handle_event(:sigquit, _state), do: System.stop()
  def handle_event(signal, state), do: :erl_signal_handler.handle_event(signal, state)
  def handle_call(_, state), do: {:ok, :ok, state}

  def terminate(_reason, _state) do
    IO.puts("My exit code")
  end
end

:gen_event.swap_handler(:erl_signal_server, {:erl_signal_handler, []}, {QuitScript, []})

Looks more natural to me this way, though in the previous example the order in which the custom code runs is more obvious.

Post #19