pass a config to the route set

This way we can get the relative_url_root from the application without
setting another global value

actionpack/lib/action_dispatch/railtie.rb

@@ -40,8 +40,6 @@ class Railtie < Rails::Railtie # :nodoc:
       ActionDispatch::Cookies::CookieJar.always_write_cookie = config.action_dispatch.always_write_cookie
 
       ActionDispatch.test_app = app
-
-      ActionDispatch::Routing::RouteSet.relative_url_root = app.config.relative_url_root
     end
   end
 end

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -20,8 +20,6 @@ class RouteSet
       # alias inspect to to_s.
       alias inspect to_s
 
-      mattr_accessor :relative_url_root
-
       class Dispatcher < Routing::Endpoint
         def initialize(defaults)
           @defaults = defaults
@@ -320,11 +318,25 @@ def self.default_resources_path_names
         { :new => 'new', :edit => 'edit' }
       end
 
-      def initialize
+      def self.new_with_config(config)
+        if config.respond_to? :relative_url_root
+          new Config.new config.relative_url_root
+        else
+          # engines apparently don't have this set
+          new
+        end
+      end
+
+      Config = Struct.new :relative_url_root
+
+      DEFAULT_CONFIG = Config.new(nil)
+
+      def initialize(config = DEFAULT_CONFIG)
         self.named_routes = NamedRouteCollection.new
         self.resources_path_names = self.class.default_resources_path_names
         self.default_url_options = {}
 
+        @config                     = config
         @append                     = []
         @prepend                    = []
         @disable_clear_and_finalize = false
@@ -336,6 +348,10 @@ def initialize
         @formatter = Journey::Formatter.new @set
       end
 
+      def relative_url_root
+        @config.relative_url_root
+      end
+
       def request_class
         ActionDispatch::Request
       end

actionpack/test/controller/url_for_test.rb

@@ -30,8 +30,8 @@ def test_nested_optional
         assert_equal '/foo/zot', path
       end
 
-      def add_host!
-        W.default_url_options[:host] = 'www.basecamphq.com'
+      def add_host!(app = W)
+        app.default_url_options[:host] = 'www.basecamphq.com'
       end
 
       def add_port!
@@ -257,12 +257,16 @@ def test_relative_url_root_is_respected
 
       def test_relative_url_root_is_respected_with_environment_variable
         # `config.relative_url_root` is set by ENV['RAILS_RELATIVE_URL_ROOT']
-        ActionDispatch::Routing::RouteSet.relative_url_root = '/subdir'
-        add_host!
+        w = Class.new {
+          config = ActionDispatch::Routing::RouteSet::Config.new '/subdir'
+          r = ActionDispatch::Routing::RouteSet.new(config)
+          r.draw { get ':controller(/:action(/:id(.:format)))' }
+          include r.url_helpers
+        }
+        add_host!(w)
         assert_equal('https://www.basecamphq.com/subdir/c/a/i',
-          W.new.url_for(:controller => 'c', :action => 'a', :id => 'i', :protocol => 'https')
+          w.new.url_for(:controller => 'c', :action => 'a', :id => 'i', :protocol => 'https')
         )
-        ActionDispatch::Routing::RouteSet.relative_url_root = nil
       end
 
       def test_named_routes

railties/lib/rails/engine.rb

@@ -528,7 +528,7 @@ def env_config
     # Defines the routes for this engine. If a block is given to
     # routes, it is appended to the engine.
     def routes
-      @routes ||= ActionDispatch::Routing::RouteSet.new
+      @routes ||= ActionDispatch::Routing::RouteSet.new_with_config(config)
       @routes.append(&Proc.new) if block_given?
       @routes
     end

railties/test/application/url_generation_test.rb

@@ -42,5 +42,18 @@ def index
       get "/"
       assert_equal "/", last_response.body
     end
+
+    def test_routes_know_the_relative_root
+      boot_rails
+      require "rails"
+      require "action_controller/railtie"
+      require "action_view/railtie"
+
+      relative_url = '/hello'
+      ENV["RAILS_RELATIVE_URL_ROOT"] = relative_url
+      app = Class.new(Rails::Application)
+      assert_equal relative_url, app.routes.relative_url_root
+      ENV["RAILS_RELATIVE_URL_ROOT"] = nil
+    end
   end
 end