Add internal test only application when doing mix test

Question

I am trying to figure out how to test my project and I was going to try to make Fakes for the edges of my system. In my case that is the network, a udp socket. To do that, I was going to start up a GenServer to manage the "faked" open connections. I can't seem to make a test only application and add it without mix getting mad. Given this application I defined in test/support

defmodule MyProjectTest.Application do
  use Application

  def start(_type, _args) do
    children = [
      GenUdpFake,
    ]

    opts = [strategy: :one_for_one, name: MyProjectTest.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

I tried to use the following in my mix.exs

  def application do
    [
      mod: {MyProject.Application, []},
      extra_applications: [:logger, :runtime_tools] ++ extra_applications(Mix.env())
    ]
  end

  defp extra_applications(:test) do
    [MyProjectTest.Application]
  end

  defp extra_applications(_) do
    []
  end

I always get the following error running mix test.

** (Mix) Could not start application Elixir.MyProjectTest.Application: could not find application file: Elixir.MyProjectTest.Application.app

However, I even went into the build folder and found _build/test/lib/my_project/ebin/Elixir.MyProjectTest.Application.beam. I even found it in my_project.app under the modules, but not the applications. So, I am supper confused what I am doing wrong. I can't seem to find any info about adding an application that isn't from dependencies to the extra_application: property. The mix documentation doesn't really tell me much more. The application documentation doesn't really say anything. I am pretty sure all I want is 'Elixir.MyProjectTest.Application' to be included in the applications section of my_project.app. Is this something I am not allowed to do? If not, what am I missing?

Answer 1

I think the issue is that although the module is called Application, it's not a beam application.

You probably want to list it in the children of your test supervision tree, rather than the extra_applications. Or skip the extra application module completely, and put GenUdpFake in the children.

Another option is to start the fake in your test setup, with

{:ok, _} = start_supervised(GenUdpFake)