Module dependency not detected a compile time

[kipcole9]

I'm not certain if this is a bug or working as expected. If the latter then my apologies for the report.

Environment

• Elixir 1.10.3 and master, erlang 22.3.3
• Operating system: macOS Catalina

Current behavior

A module referenced in a Plug configuration is not compiled at compile time in :test. In this test repo, the following error occurs

Code example

defmodule DemoWeb.Router do
  use DemoWeb, :router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_flash
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
  end

  pipeline :localized do
    plug Cldr.Plug.SetLocale,
      apps: [:cldr, :gettext],
      cldr: Demo.Cldr,  # <-- Error occurs because this module is not compiled when the plug definition is compiled in prod and test
      from: [:query, :accept_language]
  end
  ...

Reproduce the error

% MIX_ENV=test mix compile
...
==> ex_cldr_units
Compiling 8 files (.ex)
Generated ex_cldr_units app
==> demo
Compiling 16 files (.ex)

== Compilation error in file lib/demo_web/router.ex ==
** (Cldr.UnknownBackendError) The backend Demo.Cldr is not known or not a backend module.
    (ex_cldr 2.14.0) lib/cldr/plug/plug_set_locale.ex:426: Cldr.Plug.SetLocale.validate_cldr/2
    (ex_cldr 2.14.0) lib/cldr/plug/plug_set_locale.ex:119: Cldr.Plug.SetLocale.init/1
...

The error does not occur in :dev mode, presumably because Plug.init/1 is called at runtime where as in :prod and :test its called at compile time.

Workaround

requireing the module forces compilation order.

defmodule DemoWeb.Router do
  use DemoWeb, :router
  require Demo.Cldr  # <---- requiring the module forces compilation order
  ...
end

% MIX_ENV=test mix compile
Compiling 16 files (.ex)

16:56:00.929 [info]  Downloaded locale "de"
Generating Demo.Cldr for 4 locales named ["de", "en", "en-001", "root"] with a default locale named "de"
Generated demo app

Expected behavior

The compiler detects the module dependency and compiles it.

[wojtekmach]

I wonder if it's because config :phoenix, :plug_init_mode, :runtime (https://github.com/syfgkjasdkn/cldr-demo/blob/master/config/dev.exs#L59)

[kipcole9]

I just updated the notes to reflect it works fine in :dev, its in :test that this occurs (and probably in :prod) due to the different runtime versus compile time execution of Plug.init/1 as you note.

[josevalim]

Kip, how are you checking if the module exists? It seems you are checking it in a way that does not guarantee it was compiled, especially within Plug that works at compile time. It feels your CLDR plug should call Code.ensure_compiled on the module during its init. Alternatively, you can change your loaded check to actually invoke a function on the module: backend.__cldr__, and that will guarantee it is loaded at runtime or guarantee it is compiled at compile time.

[kipcole9]

José, you are right, I'm only calling Code.ensure_loaded/1. No problem at all to change to Code.ensure_compiled/2 if you think thats appropriate (I try to avoid it given that suggestion in the docs). But given the compile-time behaviour it makes sense.

FWIW here's the current code:

    defp validate_app_and_scope!(app, module) when not is_nil(app) and is_atom(module) do
      cond do
        app in @app_options && Code.ensure_loaded?(module) ->
          :ok

        app in @app_options ->
          raise ArgumentError, "Backend module #{inspect(module)} is unavailable"

        true ->
          raise ArgumentError, "App #{inspect(app)} is unknown"
      end
    end

[josevalim]

You do want to avoid it but given you are trying to verify it at compile-time, then at least on this case you shouldn't. Closing this for now but feel free to follow up if you have questions.