Make environment version affect asset digest

[lime]

Based on discussion in rails/sprockets-rails#240, this is an attempt to bring back the behaviour of Sprockets 2.

The intended use case is to force every asset to be regenerated with a new filename by bumping the environment version. This is useful for example when serving assets from a CDN with long expiration times.

The old behaviour was removed in 07e5e29. The entire digest setup has changed a lot since then. It's not entirely clear to me where the best spot now would be to reintroduce the version – should it affect all digest calculation throughout the library, or just the digest which ends up as part of the filename?

I've started by making a very minimal change that would achieve the intended behaviour. I assume that this will not be the version that gets merged, but maybe it can serve as a starting point. :) Let me know your thoughts.

[matthewd]

At a glance, this sounds pretty reasonable to me. I believe we want to do two things:

1. Change the filenames on the final compiled assets
2. Run the original asset source through all the processors again

And this sounds like it will do both.

(We should add a test of the latter, though)

[schneems]

I'm fine with this, kind of baffled why it was removed. Why have a user set-able version if it does nothing?

[schneems]

Thanks for the PR, going to go ahead and merge. Can you send me another PR with a test for ensuring that the asset gets re-run through other processors?

[lime]

I'd be happy to try, though I'm not really sure how to test it. Maybe you can give me some pointers in the right direction?

It may even be that the run-through-all-processors behaviour has been intact all along. That's the way I interpreted the discussion in rails/sprockets-rails#240 (comment). Definitely agree that it should be tested if it isn't already.

[matthewd]

My first thought is an erb-processed asset of the form:

var version = <%= something?.env.version %>;

That would just prove that as well as changing the filename, we've successfully rebuilt the file from scratch -- no intermediate cache keeping anything old alive.

[jacobbednarz]

@schneems Just checking, is the master branch here for the 4.x release? Or the 3.x? I see this has landed in master but interested in using on 3.x and it's not 100% clear which "next version" this one will land in.

[jacobbednarz]

straight after updating this issue, I seen it is tagged for the v4.0.0.beta4 release 🙄 Is this something you'd be interested in someone backporting to 3.x?

[jacobbednarz]

Seeing how we needed this functionality for 3.x, I've gone ahead and opened #478 in case you want to merge this.

[rafbm]

This change only covered assets with a preprocessor, which notably excludes images. I attempted a fix in #680.