Handle :undefined in Exception.format_arity/1

[alco]

We get :undefined when a temporary worker of a simple_one_for_one supervisor crashes.

This PR fixes the following error.

18:19:15.088 [error] GenEvent handler Logger.ErrorHandler installed at :error_logger
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Exception.format_arity/1
        (elixir) lib/exception.ex:431: Exception.format_arity(:undefined)
        (elixir) lib/exception.ex:427: Exception.format_mfa/3
        (logger) lib/logger/translator.ex:165: Logger.Translator.child_info/2
        (logger) lib/logger/translator.ex:90: Logger.Translator.translate_supervisor/2
        (logger) lib/logger/error_handler.ex:116: Logger.ErrorHandler.translate/6
        (logger) lib/logger/error_handler.ex:62: Logger.ErrorHandler.log_event/5
        (logger) lib/logger/error_handler.ex:27: Logger.ErrorHandler.handle_event/2
        (stdlib) gen_event.erl:525: :gen_event.server_update/4

To reproduce the error, create a new application with mix new --sup arity, add the following code to it:

defmodule Worker do
  use GenServer
  def start_link, do: GenServer.start_link(__MODULE__, [], [])
  def init([]), do: 1 / 0
end

defmodule Arity do
  use Application

  def start(_type, _args) do
    import Supervisor.Spec
    children = [worker(Worker, [], restart: :temporary)]
    opts = [strategy: :simple_one_for_one, name: __MODULE__]
    Supervisor.start_link(children, opts)
  end

  def start_child do
    Supervisor.start_child(__MODULE__, [])
    receive do end
  end
end

And configure :logger as follows:

config :logger, [
  handle_sasl_reports: true,
]

Finally, run

$ mix run -e Arity.start_child

[alco]

Thanks to @fishcakez for helping diagnose this.

[fishcakez]

Should fix this in Logger.Translator and not Exception. Can you add a test too?

[alco]

Sure, I'll push fixes tomorrow.

[alco]

@fishcakez what is your argument for catching :undefined in the translator? In this particular case the error stems from this function but I am not sure that :undefined does not also appear here or here.

[fishcakez]

I don't think it makes sense to handle :undefined arity in Exception because it is not a valid arity. lt would be ok for the translator to have its own format_mfa.

[alco]

Sorry, I don't really understand how to best implement a custom format_mfa in Logger.Translator without just copying the implementation from Exception.

[fishcakez]

The custom one can call Exception, it just needs to handle the :undefined args/arity before doing so. Something like:

defp format_mfa(mod, fun, :undefined), do: [inspect(mod), ?. | inspect(fun)]
defp format_mfa(mod, fun, args), do: Exception.format_mfa(mod, fun, args)

[alco]

Pushed updated implementation and added a test case.

[fishcakez]

@alco perfect except you forgot to remove this, otherwise 👍.

[alco]

Thanks for the heads-up.