Fix(SprocketsRenderer) support Sprockets::Manifest if assets were precompiled

[rmosolgo]

Previously, SprocketsRenderer would live-compile assets, even in production. However, this is impossible in Rails 5 (not just improper) because there is no Sprockets::Environment at Rails.application.assets.

Instead, there is a Sprockets::Manifest at Rails.application.assets_manifest which tells where each file's compiled counterpart is.

So, if the Sprockets::Environment isn't available, use the manifest.

TODO

• Prefer the Manifest over the Environment (to avoid double-compiling assets)
• Somehow, handle cases where assets are pushed to a CDN, not the local Rails server
  • Check for asset_host config?, no because that doesn't necessarily mean the files are remote
  • Extract get_asset_content to classes
  • Accept a custom AssetContainer

[losvedir]

Somehow, handle cases where assets are pushed to a CDN, not the local Rails server

I think in most cases, doesn't the CDN just serve essentially as a caching layer, with the underlying Rails server still serving the assets (the first time)?

At least in our setup with heroku/cloudfront, which I think is fairly common, we precompile assets so they're served via the /public/assets/ folder, and we use, e.g. <%= stylesheet_link_tag 'app' %>. We have our asset_host set to our cloudfront CDN, so the generated link ends up looking like https://abc123.cloudfront.net/assets/app-def345.css.

Then when the first request comes in to cloudfront after a new deploy, the filename hash will have changed, so cloudfront will proxy through to our site, and then cache and serve the asset from then on.

• Check for asset_host config?

This wouldn't exactly work for us, I don't think. We do have it set, but we still have the assets precompiled on the server as well, so maybe accessible to Sprockets? (I'm not 100% clear how sprockets works...).

Just FYI.

[rmosolgo]

Yeah, I think I was off the mark on asset_host. It sounds like your case would be handled by the default Sprockets::Manifest.

I was just poking around our own production environment, though, and I noticed that our assets aren't in public/assets, so I'll have to figure out how to handle that!

[rmosolgo]

Ug, looks like Manifest has changed a lot in the different sprockets versions