Elixir: Testing GenEvent for error reports

Question

I have a GenEvent that has been added as a handler like so :error_logger.add_report_handler(HoloNet.ErrorLogger)

So that errors/exceptions are captured and forwarded to a exception monitoring service.

I have the following code in a event behaviour

defmodule MyApp.ErrorLogger do
  use GenEvent

  @bugsnag_client Application.get_env(:my_app, :bugsnag_client)

  def init(_opts), do: {:ok, self}

  def handle_event({:error_report, _gl, {_pid, _type, [message | _]}}, state) do
    { error, stacktrace } = extract_exception(message[:error_info])
    context = extract_context(stacktrace)

    @bugsnag_client.notify(error,stacktrace, context: context, release_stage: Mix.env |> to_string)

    {:ok, state}
  end

  def handle_event({_level, _gl, _event}, state) do
    {:ok, state}
  end

  defp extract_exception(error_info) do
    { _, exception, _ } = error_info
     exception
  end

  defp extract_context(stacktrace) do
    stacktrace |> List.first |> elem 0
  end

end

The Client that makes the http request is mocked out using the application config.

defmodule Bugsnag.Mock do
  @behaviour Bugsnag

  def notify(error,stacktrace, options \\ []), do: nil
end

It works as it should when in production, but I wanted to have some test coverage.

I was thinking of testing this by crashing a GenServer or causing some exception then see if notify gets called. This doesn't feel very functional/Elixir, but I wanted to test that errors would be captured when an error is caused.

Answer 1

I say go ahead and crash things. :erlang.exit/2 will do the trick.

OTP and supervision trees are not easy. Testing how the application behaves under error conditions is necessary it you really want to achieve elixir's promised fault-tolerance.