Allow using a custom assets configuration manifest

[kennyadsl]

Ref https://github.com/rails/sprockets-rails/issues/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.

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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 https://github.com/rails/sprockets-rails/issues/369 I really support to make this optional!

a few notes... the error message has to fixed https://github.com/rails/sprockets-rails/blob/49bf8022c8d3e1d7348b3fe2e0931b2e448f1f58/lib/sprockets/railtie.rb#L72 and there's a failing test https://github.com/rails/sprockets-rails/blob/e9ca63edb6e658cdfcf8a35670c525b369c2ccca/test/test_railtie.rb#L298

[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?

[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.

[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!