1 files changed, 27 insertions, 13 deletions
diff --git a/railties/lib/rails/engine.rb b/railties/lib/rails/engine.rb
index 64c15f8f89..5757d235d2 100644
--- a/railties/lib/rails/engine.rb
+++ b/railties/lib/rails/engine.rb
@@ -2,6 +2,7 @@ require 'rails/railtie'
 require 'rails/engine/railties'
 require 'active_support/core_ext/module/delegation'
 require 'pathname'
+require 'thread'
 
 module Rails
   # <tt>Rails::Engine</tt> allows you to wrap a specific Rails application or subset of
@@ -429,6 +430,7 @@ module Rails
       @env_config          = nil
       @helpers             = nil
       @routes              = nil
+      @app_build_lock      = Mutex.new
       super
     end
 
@@ -499,10 +501,13 @@ module Rails
 
     # Returns the underlying rack application for this engine.
     def app
-      @app ||= begin
-        config.middleware = config.middleware.merge_into(default_middleware_stack)
-        config.middleware.build(endpoint)
-      end
+      @app || @app_build_lock.synchronize {
+        @app ||= begin
+          stack = default_middleware_stack
+          config.middleware = build_middleware.merge_into(stack)
+          config.middleware.build(endpoint)
+        end
+      }
     end
 
     # Returns the endpoint for this engine. If none is registered,
@@ -513,18 +518,13 @@ module Rails
 
     # Define the Rack API for this engine.
     def call(env)
-      env.merge!(env_config)
-      if env['SCRIPT_NAME']
-        env[routes.env_key] = env['SCRIPT_NAME'].dup
-      end
-      app.call(env)
+      req = build_request env
+      app.call req.env
     end
 
     # Defines additional Rack env configuration that is added on each call.
     def env_config
-      @env_config ||= {
-        'action_dispatch.routes' => routes
-      }
+      @env_config ||= {}
     end
 
     # Defines the routes for this engine. If a block is given to
@@ -584,7 +584,7 @@ module Rails
     # I18n load paths are a special case since the ones added
     # later have higher priority.
     initializer :add_locales do
-      config.i18n.railties_load_path.concat(paths["config/locales"].existent)
+      config.i18n.railties_load_path << paths["config/locales"]
     end
 
     initializer :add_view_paths do
@@ -686,5 +686,19 @@ module Rails
     def _all_load_paths #:nodoc:
       @_all_load_paths ||= (config.paths.load_paths + _all_autoload_paths).uniq
     end
+
+    private
+
+    def build_request(env)
+      env.merge!(env_config)
+      req = ActionDispatch::Request.new env
+      req.routes = routes
+      req.engine_script_name = req.script_name
+      req
+    end
+
+    def build_middleware
+      config.middleware
+    end
   end
 end