Process css files so that they get digested paths for asset files

This is required so that we can use cssbundling-rails and reference images that will receive digested paths

lib/sprockets/rails/asset_url_processor.rb

@@ -0,0 +1,16 @@
+module Sprockets
+  module Rails
+    # Rewrites urls in CSS files with the digested paths
+    class AssetUrlProcessor
+      REGEX = /url\(\s*["']?(?!(?:\#|data|http))([^"'\s)]+)\s*["']?\)/
+
+      def self.call(input)
+        context = input[:environment].context_class.new(input)
+        data = input[:data].gsub(REGEX) do |_match|
+          "url(#{context.asset_path($1)})"
+        end
+        { data: data }
+      end
+    end
+  end
+end

lib/sprockets/railtie.rb

@@ -4,6 +4,8 @@
 require 'active_support/core_ext/module/remove_method'
 require 'active_support/core_ext/numeric/bytes'
 require 'sprockets'
+
+require 'sprockets/rails/asset_url_processor'
 require 'sprockets/rails/context'
 require 'sprockets/rails/helper'
 require 'sprockets/rails/quiet_assets'
@@ -116,6 +118,10 @@ def configure(&block)
       end
     end
 
+    initializer :asset_url_processor do |app|
+      Sprockets.register_postprocessor "text/css", ::Sprockets::Rails::AssetUrlProcessor
+    end
+
     config.assets.version     = ""
     config.assets.debug       = false
     config.assets.compile     = true

test/test_asset_url_processor.rb

@@ -0,0 +1,18 @@
+require 'minitest/autorun'
+require 'sprockets/railtie'
+
+
+Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)
+class TestAssetUrlProcessor < Minitest::Test
+  def test_silences_with_default_prefix
+    env = Sprockets::Environment.new
+    env.context_class.class_eval do
+      def asset_path(path, options = {})
+        'image-hexcodegoeshere.png'
+      end
+    end
+    input = { environment: env, data: "url('image.png')", filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal({ data: "url(image-hexcodegoeshere.png)" }, output)
+  end
+end