Fix mounted engine named routes regression

When generating the url for a mounted engine through its proxy, the path should be the sum of three parts:

1. Any `SCRIPT_NAME` request header or the value of `ActionDispatch::Routing::RouteSet#relative_url_root`.
2. A prefix (the engine's mounted path).
3. The path of the named route inside the engine.

Since commit https://github.com/rails/rails/commit/44ff0313c121f528a68b3bd21d6c7a96f313e3d3, this has been broken. Step 2 has been changed to:

2. A prefix (the value of `ActionDispatch::Routing::RouteSet#relative_url_root` + the engine's mounted path).

The value of `ActionDispatch::Routing::RouteSet#relative_url_root` is taken into account in step 1 of the route generation and should be ignored when generating the mounted engine's prefix in step 2.

This commit fixes the regression by having `ActionDispatch::Routing::RouteSet#url_for` check `options[:relative_url_root]` before falling back to `ActionDispatch::Routing::RouteSet#relative_url_root`. The prefix generating code then sets `options[:relative_url_root]` to an empty string. This empty string is used instead of `ActionDispatch::Routing::RouteSet#relative_url_root` and avoids the duplicate `relative_url_root` value in the final result.

This resolves #20920 and resolves #21459

1 files changed, 49 insertions, 0 deletions

diff --git a/railties/test/railties/engine_test.rb b/railties/test/railties/engine_test.rb
index 79bd7a8241..2c82f728ee 100644
--- a/railties/test/railties/engine_test.rb
+++ b/railties/test/railties/engine_test.rb
@@ -1304,6 +1304,55 @@ YAML
       assert_equal '/foo/bukkits/bukkit', last_response.body
     end
 
+    test "paths are properly generated when application is mounted at sub-path and relative_url_root is set" do
+      add_to_config "config.relative_url_root = '/foo'"
+
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        module Bukkits
+          class Engine < ::Rails::Engine
+            isolate_namespace Bukkits
+          end
+        end
+      RUBY
+
+      app_file "app/controllers/bar_controller.rb", <<-RUBY
+        class BarController < ApplicationController
+          def index
+            render text: bukkits.bukkit_path
+          end
+        end
+      RUBY
+
+      app_file "config/routes.rb", <<-RUBY
+        Rails.application.routes.draw do
+          get '/bar' => 'bar#index', :as => 'bar'
+          mount Bukkits::Engine => "/bukkits", :as => "bukkits"
+        end
+      RUBY
+
+      @plugin.write "config/routes.rb", <<-RUBY
+        Bukkits::Engine.routes.draw do
+          get '/bukkit' => 'bukkit#index'
+        end
+      RUBY
+
+      @plugin.write "app/controllers/bukkits/bukkit_controller.rb", <<-RUBY
+        class Bukkits::BukkitController < ActionController::Base
+          def index
+            render text: main_app.bar_path
+          end
+        end
+      RUBY
+
+      boot_rails
+
+      get("/bukkits/bukkit", {}, {'SCRIPT_NAME' => '/foo'})
+      assert_equal '/foo/bar', last_response.body
+
+      get("/bar", {}, {'SCRIPT_NAME' => '/foo'})
+      assert_equal '/foo/bukkits/bukkit', last_response.body
+    end
+
   private
     def app
       Rails.application