Respect explicitly configured manifest path for modern Rails/Sprockets

[krasnoukhov]

In our Rails 6/7 apps that use Sprockets 3/4 we configure manifest path explicitly like so:

config.assets.manifest = Rails.root.join("config/manifest.json")

Because of that asset_sync does not work properly because it does not pick up proper manifest, since it's located out of assets directory. Here is relevant code in sprockets source:

https://github.com/rails/sprockets/blob/3.x/lib/sprockets/manifest.rb#L54-L63

So for us the solution was to explicitly path manifest path (which is nil unless defined explicitly) as a 3rd argument to Sprockets::Manifest.new which actually makes it work. This works for both Sprockets 3 and 4.

We've been running this patch for years now. Hopefully this makes sense and this contribution gets accepted. There are no specs for manifest-related code so I was only able to add a spec for manifest path.

Also, looks like manifest.yml is not a thing from Rails 4: https://github.com/rails/sprockets-rails#changes-from-rails-3x
So maybe that is something to be cleaned up.

[PikachuEXE]

Looks fine merging
But will wait for #435 before release (unless it's stuck

[krasnoukhov]

Thanks @PikachuEXE, sounds good

[krasnoukhov]

Looks like #435 is a bit stuck after all... Do you mind cutting a release @PikachuEXE? Thanks

[PikachuEXE]

Released in https://rubygems.org/gems/asset_sync/versions/2.18.1