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`).

Author: rmacklin

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.rewrite_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.rewrite_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_rewrite_css_urls_defaults_to_true
+    app.initialize!
+
+    assert_equal true, app.config.assets.rewrite_css_urls
+    assert_includes Sprockets.postprocessors['text/css'], Sprockets::Rails::AssetUrlProcessor
+  end
+
+  def test_rewrite_css_urls_when_false_avoids_registering_postprocessor
+    app.configure do
+      config.assets.rewrite_css_urls = false
+    end
+    app.initialize!
+
+    assert_equal false, app.config.assets.rewrite_css_urls
+    refute_includes Sprockets.postprocessors['text/css'], Sprockets::Rails::AssetUrlProcessor
+  end
+
   private
     def action_view
       ActionView::Base.new(ActionView::LookupContext.new([]), {}, nil)