Make it possible to add EE only route

And if it cannot find any routes, raise an error

doc/development/ee_features.md

@@ -334,12 +334,17 @@ full implementation details.
 
 ### Code in `config/routes`
 
-When we add `draw :admin` in `config/routes.rb`, the application will also
-load the file located in `config/routes/admin.rb`, and also
-`ee/config/routes/admin.rb` if the file exists.
+When we add `draw :admin` in `config/routes.rb`, the application will try to
+load the file located in `config/routes/admin.rb`, and also try to load the
+file located in `ee/config/routes/admin.rb`.
 
-So if we want to extend a particular route file, just add the same file
-located in `ee/config/routes`.
+It should at least load one file, at most two files. If it cannot find any
+files, an error will be raised.
+
+This means if we want to extend a particular CE route file, just add the same
+file located in `ee/config/routes`. If we want to add an EE only route, we
+could still use `draw :ee_only` and add `ee/config/routes/ee_only.rb` without
+adding `config/routes/ee_only.rb`.
 
 ### Code in `app/controllers/`
 

lib/gitlab/patch/draw_route.rb

@@ -5,16 +5,28 @@
 module Gitlab
   module Patch
     module DrawRoute
-      def draw(routes_name)
-        instance_eval(File.read(Rails.root.join("config/routes/#{routes_name}.rb")))
+      RoutesNotFound = Class.new(StandardError)
 
-        draw_ee(routes_name)
+      def draw(routes_name)
+        draw_ce(routes_name) | draw_ee(routes_name) ||
+          raise(RoutesNotFound.new("Cannot find #{routes_name}"))
+      end
+
+      def draw_ce(routes_name)
+        draw_route(Rails.root.join("config/routes/#{routes_name}.rb"))
       end
 
       def draw_ee(routes_name)
-        path = Rails.root.join("ee/config/routes/#{routes_name}.rb")
+        draw_route(Rails.root.join("ee/config/routes/#{routes_name}.rb"))
+      end
 
-        instance_eval(File.read(path)) if File.exist?(path)
+      def draw_route(path)
+        if File.exist?(path)
+          instance_eval(File.read(path))
+          true
+        else
+          false
+        end
       end
     end
   end