Fix Engine#routes to not call draw_paths multiple times

railties/lib/rails/engine.rb

@@ -486,8 +486,10 @@ def env_config
     end
 
     def routes
-      @routes ||= ActionDispatch::Routing::RouteSet.new
-      @routes.draw_paths.concat paths["config/routes"].paths
+      @routes ||= ActionDispatch::Routing::RouteSet.new.tap do |routes|
+        routes.draw_paths.concat paths["config/routes"].paths
+      end
+
       @routes.append(&Proc.new) if block_given?
       @routes
     end

railties/test/unit/engine_test.rb

@@ -12,5 +12,15 @@ def initialize(*args)
 
       assert !engine.routes?
     end
+
+    it "does not add more paths to routes on each call" do
+      engine = Class.new(Rails::Engine)
+
+      engine.routes
+      length = engine.routes.draw_paths.length
+
+      engine.routes
+      assert_equal length, engine.routes.draw_paths.length
+    end
   end
 end