Test supervision of Genservers

[ndrean]

I was just checking the Supervisor and I have some trouble running the simple code below. This returns "1st argument: not a pid"

I checked the same code with IEx. When I loop the code, I need a long Process.sleep. Is there an Erlang limitation, amplified in Livebook?

defmodule Test do
  use GenServer
  def new do: GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  def init(_), do: {:ok, :ok}
end

#<-- start test
Supervisor.start_link( [%{id: Test, start: {Test, :new, []}}],  strategy: :one_for_one, name: MySup)
Supervisor.which_children(MySup) |> IO.inspect()

pid = Process.whereis(Test)
Process.alive?(pid) |> IO.inspect()
Process.exit(pid, :shutdown)
Process.alive?(pid) |> IO.inspect()
Process.whereis(Test)
|> Process.alive?()
#-->

[jonatanklosko]

The last call to Process.whereis(Test) may return nil if the supervisor doesn't restart the child fast enough. This may fail even if you run it for a first time.

Sidenote: in Livebook you usually want to use Kino.start_child to make sure the process is shutdown before next evaluation (particularly relevant for named processes, because the second evaluation would fail to start those).

[ndrean]

Ok, thks again. I added Kino like this plus a delay. Seems to work!

child_spec = %{id: Test, start: {Test, :new, []}}
Kino.start_child(child_spec)
Supervisor.start_link([child_spec], strategy: :one_for_one, name: MySup)

Process.whereis(Test)  |> Kino.inspect()
[{Test, pid, :worker, [Test]}] = Supervisor.which_children(MySup) |> Kino.inspect()
Process.exit(pid, :true)
Process.alive?(pid) |> Kino.inspect()
Process.sleep(1000)
[{Test, pid, :worker, [Test]}] = Supervisor.which_children(MySup) |> Kino.inspect()
Process.alive?(pid)

[jonatanklosko]

Kino.start_child already starts the process (in this case Test) under a kino supervisor, so then you don't need the other supervisor. Alternatively, you could start the supervisor itself under Kino.start_child.

[ndrean]

yes, I forgot the #