Propagate digested paths for asset files from v3 to v2

README.md

@@ -117,6 +117,9 @@ config.assets.configure do |env|
 end
 ```
 
+**`config.assets.resolve_assets_in_css_urls`**
+
+ When this option is enabled, sprockets-rails will register a CSS postprocessor to resolve assets referenced in [`url()`](https://developer.mozilla.org/en-US/docs/Web/CSS/url()) function calls and replace them with the digested paths. Defaults to `true`.
 
 ## Complementary plugins
 

lib/sprockets/rails/asset_url_processor.rb

@@ -0,0 +1,22 @@
+module Sprockets
+  module Rails
+    # Resolve assets referenced in CSS `url()` calls and replace them with the digested paths
+    class AssetUrlProcessor
+      REGEX = /url\(\s*["']?(?!(?:\#|data|http))(?<relativeToCurrentDir>\.\/)?(?<path>[^"'\s)]+)\s*["']?\)/
+      def self.call(input)
+        context = input[:environment].context_class.new(input)
+        data    = input[:data].gsub(REGEX) do |_match|
+          path = Regexp.last_match[:path]
+          begin
+            "url(#{context.asset_path(path)})"
+          rescue => e
+            puts "AssetUrlProcessor: Error processing asset |#{path}|: #{e.class.name}: #{e.message}"
+            "url(#{path})"
+          end
+        end
+
+        context.metadata.merge(data: data)
+      end
+    end
+  end
+end

lib/sprockets/railtie.rb

@@ -5,6 +5,7 @@
 require 'sprockets'
 require 'sprockets/rails/helper'
 require 'sprockets/rails/version'
+require 'sprockets/rails/asset_url_processor'
 
 module Rails
   class Application
@@ -77,6 +78,13 @@ def configure(&block)
     config.assets.debug      = false
     config.assets.compile    = true
     config.assets.digest     = false
+    config.assets.resolve_assets_in_css_urls = true
+
+    initializer :asset_url_processor do |app|
+      if app.config.assets.resolve_assets_in_css_urls
+        Sprockets.register_postprocessor "text/css", ::Sprockets::Rails::AssetUrlProcessor
+      end
+    end
 
     rake_tasks do |app|
       require 'sprockets/rails/task'

test/test_asset_url_processor.rb

@@ -0,0 +1,70 @@
+require 'minitest/autorun'
+require 'sprockets/railtie'
+
+
+Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)
+class TestAssetUrlProcessor < Minitest::Test
+  FIXTURES_PATH = File.expand_path("../fixtures", __FILE__)
+
+  def setup
+    @env = Sprockets::Environment.new
+    @env.append_path FIXTURES_PATH
+    @env.context_class.class_eval do
+      include ::Sprockets::Rails::Context
+    end
+    @env.context_class.digest_assets = true
+
+    @logo_digest = @env["logo.png"].etag
+    @logo_uri    = @env["logo.png"].uri
+  end
+
+  def test_basic
+    input = { environment: @env, data: 'background: url(logo.png);', filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal("background: url(/logo-#{@logo_digest}.png);", output[:data])
+  end
+
+  def test_spaces
+    input = { environment: @env, data: 'background: url( logo.png );', filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal("background: url(/logo-#{@logo_digest}.png);", output[:data])
+  end
+
+  def test_single_quote
+    input = { environment: @env, data: "background: url('logo.png');", filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal("background: url(/logo-#{@logo_digest}.png);", output[:data])
+  end
+
+  def test_double_quote
+    input = { environment: @env, data: 'background: url("logo.png");', filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal("background: url(/logo-#{@logo_digest}.png);", output[:data])
+  end
+
+  def test_dependencies_are_tracked
+    input = { environment: @env, data: 'background: url(logo.png);', filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal(1, output[:links].size)
+    assert_equal(@logo_uri, output[:links].first)
+  end
+
+  def test_relative
+    input = { environment: @env, data: 'background: url(./logo.png);', filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal("background: url(/logo-#{@logo_digest}.png);", output[:data])
+  end
+
+  def test_subdirectory
+    input = { environment: @env, data: "background: url('jquery/jquery.js');", filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    jquery_digest = 'c6910e1db4a5ed4905be728ab786471e81565f4a9d544734b199f3790de9f9a3'
+    assert_equal("background: url(/jquery/jquery-#{jquery_digest}.js);", output[:data])
+  end
+
+  def test_protocol_relative_paths
+    input = { environment: @env, data: "background: url(//assets.example.com/assets/fontawesome-webfont-82ff0fe46a6f60e0ab3c4a9891a0ae0a1f7b7e84c625f55358379177a2dcb202.eot);", filename: 'url2.css', metadata: {} }
+    output = Sprockets::Rails::AssetUrlProcessor.call(input)
+    assert_equal("background: url(//assets.example.com/assets/fontawesome-webfont-82ff0fe46a6f60e0ab3c4a9891a0ae0a1f7b7e84c625f55358379177a2dcb202.eot);", output[:data])
+  end
+end

test/test_railtie.rb

@@ -256,4 +256,21 @@ def test_manifest_path_respects_rails_public_path
       assert_match %r{test_public/assets$}, manifest.dir
     end
   end
+
+  def test_resolve_assets_in_css_urls_defaults_to_true
+    app.initialize!
+
+    assert_equal true, app.config.assets.resolve_assets_in_css_urls
+    assert_includes Sprockets.postprocessors['text/css'], Sprockets::Rails::AssetUrlProcessor
+  end
+
+  def test_resolve_assets_in_css_urls_when_false_avoids_registering_postprocessor
+    app.configure do
+      config.assets.resolve_assets_in_css_urls = false
+    end
+    app.initialize!
+
+    assert_equal false, app.config.assets.resolve_assets_in_css_urls
+    refute_includes Sprockets.postprocessors['text/css'], Sprockets::Rails::AssetUrlProcessor
+  end
 end