Allow using a custom assets configuration manifest

[kennyadsl]

Ref #369

Using Sprockets 4, at the moment, the only allowed path for the newly required configuration manifest is the default one (app/assets/config/manifest.js) and there is no way to change it.

This PR adds a way to change the default location by adding a new config.assets.config_manifest option, that could be set like this:

config.assets.config_manifest = File.expand_path('my_custom_path/config/manifest.js', __dir__)

Additional context

In Solidus, we use a custom dummy app to perform our specs. We need to configure the path with custom locations for the configurations files we use. This PR allows us to set a custom configuration manifest path, otherwise, we would not be able to be compatible with Sprockets 4.

Here are some relevant parts of our DummyApp, I hope it helps to contextualize our issue.

And this is the failed build output (only first lines are relevant) now that sprockets 4 is used.

---

This is my first contribution to this project so feel free to point out if I missed something important or how can I improve the PR. Thanks!

[ahorek]

here's an old discussion about this #369 I really support to make this optional!

a few notes...
the error message has to fixed

sprockets-rails/lib/sprockets/railtie.rb

Line 72 in 49bf802

msg = "Expected to find a manifest file in `app/assets/config/manifest.js`\n" +

and there's a failing test

sprockets-rails/test/test_railtie.rb

Line 298 in e9ca63e

def test_manifest_path

[kennyadsl]

Thanks, @ahorek! Not sure failure is related now.

[kennyadsl]

Looking twice, I don't think this is a good change since, if I got it correctly, the config.assets.manifest configuration was used as the location of the output manifest (.yml) after the precompile (searching for config.assets.manifest here).

Even if this is a somehow accepted solution, I think we should use another option to avoid messing with the old purpose of this configuration. Maybe something like config.assets.configuration_manifest?

[elia]

👍 FWIW, just one small suggestion for the spec

[elia]

Suggested change

	Dir.chdir(app.root) do
	dir = "app/assets/config/foo"
	FileUtils.mkdir_p(dir)
	File.open("#{ dir }/bar.json", "w") do |f|
	f << ""
	end
	end
	app.root.join("app/assets/config/foo/bar.json").tap { |path| path.dirname.mkpath }.write('')

[kennyadsl]

@elia I preferred to use the same format to create the file as already used on top of this document, but I'm open to change it. Let's see what other maintainers think about this.

[rafaelfranca]

We need a different config for this. The assets.manifest is the location of the output manifest. Maybe config.assets.config_manifest?

[kennyadsl]

@rafaelfranca added a new config named config_manifest for this. Let me know if this works!

[kennyadsl]

Another improvement (maybe with another PR) could be falling back to the Sprocket 3 behavior (app.config.assets.precompile += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]) if the configuration manifest does not exist. Do we really need to raise an exception if the configuration manifest is not present in the host application? Thoughts?

[rafaelfranca]

Yes, we need to raise an exception. LOOSE_APP_ASSETS is a proc and procs are not supported anymore in the precompile option.

[rafaelfranca]

Let's set this default value on line 135.

[kennyadsl]

@rafaelfranca sorry, I'm not sure to get this request. Do you mean the whole set_config_manifest_path initializer or just the default value of the app.config.assets.config_manifest configuration?

Setting the whole initializer at line 135 is not working since when used in the set_default_precompile initializer (before line 135) it is raising an exception since it's waiting for a string but it gets nil:

Error:
TestRailtie#test_task_precompile:
NoMethodError: undefined method `start_with?' for nil:NilClass

[rafaelfranca]

I mean setting ::Rails.root.join("app/assets/config/manifest.js") (the default value when the config is not set) and removing this || from this intializer.

[kennyadsl]

I don't think it works this way. The Rails app has not yet been initialized when the code at that level is evaluated.

I'm doing this:

    # ...
    config.assets.manifest    = nil
    config.assets.quiet       = false
    config.assets.config_manifest = ::Rails.root.join("app/assets/config/manifest.js")

    initializer :set_config_manifest_path do |app|
      config_manifest_path = Pathname.new(app.config.assets.config_manifest)
      raise ManifestNeededError.new(config_manifest_path) unless config_manifest_path.exist?
      app.config.assets.config_manifest = config_manifest_path.to_s
    end

but Rails.root does not seem to be ready yet at that point; I get this exception:

undefined method `join' for nil:NilClass (NoMethodError)

I think that to use ::Rails.root we need to call it from an initializer's context, right?

[aldesantis]

@rafaelfranca any updates on this by any chance? It'd be really great to have it! 🙏

[kennyadsl]

@rafaelfranca hey there, is there any thing I can do to move this forward?

[kennyadsl]

@rafaelfranca friendly ping if you have time to point me in a good direction to move things forward here. Thanks!