Add assets.resolve_assets_in_css_urls configuration option to allow disabling AssetUrlProcessor (#489)

* Allow users to opt out of `Sprockets::Rails::AssetUrlProcessor`

We're introducing a new configuration option `assets.rewrite_css_urls`
to allow users to opt out of the new `AssetUrlProcessor` (by configuring
that option to `false`).

* Rename `rewrite_css_urls` option to `resolve_assets_in_css_urls`

* Reword comment describing `AssetUrlProcessor`

to use language that mirrors the new `resolve_assets_in_css_urls`
configuration option

* Document the new `resolve_assets_in_css_urls` configuration option

Author: rmacklin

README.md

@@ -108,6 +108,10 @@ 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`.
+
 **`config.assets.resolve_with`**
 
 A list of `:environment` and `:manifest` symbols that defines the order that

lib/sprockets/rails/asset_url_processor.rb

@@ -1,6 +1,6 @@
 module Sprockets
   module Rails
-    # Rewrites urls in CSS files with the digested paths
+    # 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)

lib/sprockets/railtie.rb

@@ -97,12 +97,13 @@ def configure(&block)
     end
 
     config.assets = OrderedOptions.new
-    config.assets._blocks     = []
-    config.assets.paths       = []
-    config.assets.precompile  = []
-    config.assets.prefix      = "/assets"
-    config.assets.manifest    = nil
-    config.assets.quiet       = false
+    config.assets._blocks                    = []
+    config.assets.paths                      = []
+    config.assets.precompile                 = []
+    config.assets.prefix                     = "/assets"
+    config.assets.manifest                   = nil
+    config.assets.quiet                      = false
+    config.assets.resolve_assets_in_css_urls = true
 
     initializer :set_default_precompile do |app|
       if using_sprockets4?
@@ -120,7 +121,9 @@ def configure(&block)
     end
 
     initializer :asset_url_processor do |app|
-      Sprockets.register_postprocessor "text/css", ::Sprockets::Rails::AssetUrlProcessor
+      if app.config.assets.resolve_assets_in_css_urls
+        Sprockets.register_postprocessor "text/css", ::Sprockets::Rails::AssetUrlProcessor
+      end
     end
 
     initializer :asset_sourcemap_url_processor do |app|

test/test_railtie.rb

@@ -423,6 +423,23 @@ def test_quiet_assets_inserts_middleware
     assert middleware.each_cons(2).include?([Sprockets::Rails::QuietAssets, Rails::Rack::Logger])
   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
+
   private
     def action_view
       ActionView::Base.new(ActionView::LookupContext.new([]), {}, nil)