Fix generating route from engine to other engine

A regression was introduced in 5b3bb6, generating route from within an
engine to an another engine resulted in prefixing a path with the
SCRIPT_NAME value.

The regression was caused by the fact that SCRIPT_NAME should be
appended only if it's the SCRIPT_NAME for the application, not if it's
SCRIPT_NAME from the current engine.

closes #10409

actionpack/lib/action_controller/metal/url_for.rb

@@ -32,7 +32,8 @@ module ActionController
 
       if (same_origin = _routes.equal?(env["action_dispatch.routes"])) ||
          (script_name = env["ROUTES_#{_routes.object_id}_SCRIPT_NAME"]) ||
-         (original_script_name = env['SCRIPT_NAME'])
+         (original_script_name = env['ORIGINAL_SCRIPT_NAME'])
+
         @_url_options.dup.tap do |options|
           if original_script_name
             options[:original_script_name] = original_script_name

railties/lib/rails/application.rb

@@ -93,6 +93,7 @@ module Rails
     # dispatches the request to the underlying middleware stack.
     def call(env)
       env["ORIGINAL_FULLPATH"] = build_original_fullpath(env)
+      env["ORIGINAL_SCRIPT_NAME"] = env["SCRIPT_NAME"]
       super(env)
     end
 

railties/test/railties/mounted_engine_test.rb

@@ -13,6 +13,7 @@ module ApplicationTests
 
       @simple_plugin = engine "weblog"
       @plugin = engine "blog"
+      @metrics_plugin = engine "metrics"
 
       app_file 'config/routes.rb', <<-RUBY
         AppTemplate::Application.routes.draw do
@@ -28,6 +29,7 @@ module ApplicationTests
           scope "/:user", :user => "anonymous" do
             mount Blog::Engine => "/blog"
           end
+          mount Metrics::Engine => "/metrics"
           root :to => 'main#index'
         end
       RUBY
@@ -54,6 +56,34 @@ module ApplicationTests
         end
       RUBY
 
+      @metrics_plugin.write "lib/metrics.rb", <<-RUBY
+        module Metrics
+          class Engine < ::Rails::Engine
+            isolate_namespace(Metrics)
+          end
+        end
+      RUBY
+
+      @metrics_plugin.write "config/routes.rb", <<-RUBY
+        Metrics::Engine.routes.draw do
+          get '/generate_blog_route', to: 'generating#generate_blog_route'
+          get '/generate_blog_route_in_view', to: 'generating#generate_blog_route_in_view'
+        end
+      RUBY
+
+      @metrics_plugin.write "app/controllers/metrics/generating_controller.rb", <<-RUBY
+        module Metrics
+          class GeneratingController < ActionController::Base
+            def generate_blog_route
+              render text: blog.post_path(1)
+            end
+
+            def generate_blog_route_in_view
+              render inline: "<%= blog.post_path(1) -%>"
+            end
+          end
+        end
+      RUBY
 
       @plugin.write "app/models/blog/post.rb", <<-RUBY
         module Blog
@@ -201,6 +231,21 @@ module ApplicationTests
       get "/somone/blog/application_route_in_view"
       assert_equal "/", last_response.body
 
+      # test generating engine's route from other engine
+      get "/metrics/generate_blog_route"
+      assert_equal '/anonymous/blog/posts/1', last_response.body
+
+      get "/metrics/generate_blog_route_in_view"
+      assert_equal '/anonymous/blog/posts/1', last_response.body
+
+      # test generating engine's route from other engine with default_url_options
+      get "/metrics/generate_blog_route", {}, 'SCRIPT_NAME' => '/foo'
+      assert_equal '/foo/anonymous/blog/posts/1', last_response.body
+
+      get "/metrics/generate_blog_route_in_view", {}, 'SCRIPT_NAME' => '/foo'
+      assert_equal '/foo/anonymous/blog/posts/1', last_response.body
+
+
       # test generating application's route from engine with default_url_options
       get "/someone/blog/generate_application_route", {}, 'SCRIPT_NAME' => '/foo'
       assert_equal "/foo/", last_response.body