Sprockets can't use cache, because SassCompressor#cache_key changes on every run

[ledestin]

Problem

Running a single system test takes about 50 seconds on a project with lots of SASS files. I've investigated Sprockets and found out that SASS files are processed each time a test runs, and Sprockets cache isn't used. And it is not used, because of a random cache key in SassCompressor.

SassCompressor is monkey patched by sassc-rails and uses a random #cache_key:

sassc-rails/lib/sassc/rails/compressor.rb

Line 14 in bf66ead

@cache_key = SecureRandom.uuid

Possible solution

Sprockets uses the following cache key:
@cache_key = "#{self.class.name}:#{Autoload::Sass::VERSION}:#{VERSION}:#{DigestUtils.digest(options)}".freeze
https://github.com/rails/sprockets/blob/386ab2ba215f828602b6e6bfee9bfc95dc23d574/lib/sprockets/sass_compressor.rb#L46

By using the Sprockets @cache_key code, I was able to make the test 2x faster (50 -> 25 seconds).

Would it be possible to use a cache key that doesn't change on each run, when no other things have changed?

[ylecuyer]

I spent most of my afternoon hunting this bug and I had the same conclusion as yours. Our assets pipeline went from almost 2 minutes to 30 seconds with this fix.

[Aesthetikx]

Indeed, I just spent most of my evening on this one :)