Handle :undefined arity in Logger.Translator

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

Thanks to @fishcakez for feedback

Author: alco

lib/logger/lib/logger/translator.ex

@@ -162,12 +162,12 @@ defmodule Logger.Translator do
   defp sup_context(:shutdown_error), do: "shutdown abnormally"
 
   defp child_info(min_level, [{:mfargs, {mod, fun, args}} | debug]) do
-    ["Start Call: ", Exception.format_mfa(mod, fun, args) |
+    ["Start Call: ", format_mfa(mod, fun, args) |
       child_debug(min_level, debug)]
   end
 
   defp child_info(min_level, [{:mfa, {mod, fun, args}} | debug]) do
-    ["Start Call: ", Exception.format_mfa(mod, fun, args) |
+    ["Start Call: ", format_mfa(mod, fun, args) |
       child_debug(min_level, debug)]
   end
 
@@ -248,11 +248,11 @@ defmodule Logger.Translator do
   end
 
   defp crash_call(mod, fun, arity) when is_integer(arity) do
-    Exception.format_mfa(mod, fun, arity)
+    format_mfa(mod, fun, arity)
   end
 
   defp crash_call(mod, fun, args) do
-    Exception.format_mfa(mod, fun, length(args))
+    format_mfa(mod, fun, length(args))
   end
 
   defp crash_debug(:debug,
@@ -295,4 +295,6 @@ defmodule Logger.Translator do
       crash_info(min_level, info, [prefix | prefix])]
   end
 
+  defp format_mfa(mod, fun, :undefined), do: [inspect(mod), ?., to_string(fun) | "/?"]
+  defp format_mfa(mod, fun, args), do: Exception.format_mfa(mod, fun, args)
 end

lib/logger/test/logger/translator_test.exs

@@ -31,6 +31,12 @@ defmodule Logger.TranslatorTest do
     end
   end
 
+  defmodule MyTemporaryWorker do
+    use GenServer
+    def start_link, do: GenServer.start_link(__MODULE__, [], [])
+    def init([]), do: 1 / 0
+  end
+
   setup_all do
     sasl_reports? = Application.get_env(:logger, :handle_sasl_reports, false)
     Application.put_env(:logger, :handle_sasl_reports, true)
@@ -476,6 +482,15 @@ defmodule Logger.TranslatorTest do
     """
   end
 
+  test "handles :undefined MFA properly" do
+    children = [Supervisor.Spec.worker(MyTemporaryWorker, [], restart: :temporary)]
+    opts = [strategy: :simple_one_for_one]
+    {:ok, sup} = Supervisor.start_link(children, opts)
+    assert capture_log(:info, fn ->
+      {:error, {:badarith, _}} = Supervisor.start_child(sup, [])
+    end) =~ "Start Call: Logger.TranslatorTest.MyTemporaryWorker.start_link/?"
+  end
+
   def task(parent, fun \\ (fn() -> raise "oops" end)) do
     Process.unlink(parent)
     receive do: (:go -> fun.())