Fix loading plugin and engine route sets

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -212,11 +212,14 @@ module ActionDispatch
         self.routes = []
         self.named_routes = NamedRouteCollection.new
 
+        @clear_before_draw = true
+        @finalize_set_on_draw = true
+
         clear!
       end
 
       def draw(&block)
-        clear!
+        clear! if @clear_before_draw
 
         mapper = Mapper.new(self)
         if block.arity == 1
@@ -225,9 +228,13 @@ module ActionDispatch
           mapper.instance_exec(&block)
         end
 
-        @set.add_route(NotFound)
-        install_helpers
-        @set.freeze
+        if @finalize_set_on_draw
+          @set.add_route(NotFound)
+          install_helpers
+          @set.freeze
+        end
+
+        nil
       end
 
       def clear!
@@ -283,7 +290,15 @@ module ActionDispatch
 
       def load_routes!
         if configuration_files.any?
-          configuration_files.each { |config| load(config) }
+          @finalize_set_on_draw = false
+          configuration_files.each_with_index do |config, index|
+            @finalize_set_on_draw = true if index == (configuration_files.length - 1)
+            load(config)
+            @clear_before_draw = false if index == 0
+          end
+          @clear_before_draw = true
+          @finalize_set_on_draw = true
+
           @routes_last_modified = routes_changed_at
         else
           draw do |map|

railties/test/application/routing_test.rb

@@ -81,5 +81,43 @@ module ApplicationTests
       get '/admin/foo'
       assert_equal 'admin::foo', last_response.body
     end
+
+    test "merges with plugin routes" do
+      controller 'foo', <<-RUBY
+        class FooController < ActionController::Base
+          def index
+            render :text => "foo"
+          end
+        end
+      RUBY
+
+      app_file 'config/routes.rb', <<-RUBY
+        ActionController::Routing::Routes.draw do |map|
+          match 'foo', :to => 'foo#index'
+        end
+      RUBY
+
+      plugin 'bar', 'require File.dirname(__FILE__) + "/app/controllers/bar"' do |plugin|
+        plugin.write 'app/controllers/bar.rb', <<-RUBY
+          class BarController < ActionController::Base
+            def index
+              render :text => "bar"
+            end
+          end
+        RUBY
+
+        plugin.write 'config/routes.rb', <<-RUBY
+          ActionController::Routing::Routes.draw do |map|
+            match 'bar', :to => 'bar#index'
+          end
+        RUBY
+      end
+
+      get '/foo'
+      assert_equal 'foo', last_response.body
+
+      get '/bar'
+      assert_equal 'bar', last_response.body
+    end
   end
 end