Supervisor.terminate_child doesn't call terminate callback on child

Questions / Help
1

I was trying to stop children of a Supervisor through terminate_child/2 and expected that the GenServer callback terminate/2 is called on the child but it is not.
So now I am curious if this is correct and when to use Supervisor.terminate_child/2 over GenServer.stop/1.
Here is an example:

defmodule Test do
  use GenServer
  
  def start_link(_) do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end
  
  def init(args) do
  {:ok, args}
  end
  
  def handle_call(:test, _from, state) do
    {:reply, :test_reply, state}
  end
  
  def terminate(_reason, _state) do
    IO.inspect("terminate")
  end
end

{:ok, pid} = Supervisor.start_link([Test], strategy: :one_for_one)

GenServer.call(Test, :test)
|> IO.inspect()
# :test_reply

Supervisor.terminate_child(pid, Test)
|> IO.inspect()
# :ok 
# but no 'terminate'

Thanks for your insights!

3

You GenServer is not trapping exits so when the supervisor sends it an exit signal to terminate the GenServer it just immediately dies. You need to implement an init/1 callback in which you do, amongst other things, a Process.flag(:trap_exit, true) which then makes the GenServer process catch that exit signal, call the terminate/2 callback and then die.

3 Likes
4

Thanks! Sorry I should have read the docs more carefully. Its clearly written there: https://hexdocs.pm/elixir/GenServer.html#c:terminate/2

1 Like