Made Engine valid rack app with its own middleware stack

railties/lib/rails/application/configuration.rb

@@ -9,7 +9,7 @@ module Rails
 
       attr_accessor :allow_concurrency, :cache_classes, :cache_store,
                     :encoding, :consider_all_requests_local, :dependency_loading,
-                    :filter_parameters,  :log_level, :logger, :middleware,
+                    :filter_parameters,  :log_level, :logger,
                     :plugins, :preload_frameworks, :reload_plugins,
                     :secret_token, :serve_static_assets, :session_options,
                     :time_zone, :whiny_nils

railties/lib/rails/application/railties.rb

@@ -17,7 +17,7 @@ module Rails
       end
 
       def engines
-        @engines ||= ::Rails::Engine.subclasses.map(&:new)
+        @engines ||= ::Rails::Engine.subclasses.map(&:instance)
       end
 
       def plugins

railties/lib/rails/engine.rb

@@ -140,6 +140,19 @@ module Rails
       end
     end
 
+    def app
+      raise "You can't use Engine as rack application without providing valid rack endpoint" unless endpoint
+      @app ||= config.middleware.build(endpoint)
+    end
+
+    def endpoint
+      self.class.endpoint
+    end
+
+    def call(env)
+      app.call(env)
+    end
+
     # Add configured load paths to ruby load paths and remove duplicates.
     initializer :set_load_path, :before => :bootstrap_hook do
       _all_load_paths.reverse_each do |path|

railties/lib/rails/engine/configurable.rb

@@ -3,10 +3,12 @@ module Rails
     module Configurable
       def self.included(base)
         base.extend ClassMethods
+        base.private_class_method :new
       end
 
       module ClassMethods
         delegate :middleware, :root, :paths, :to => :config
+        delegate :call, :to => :instance
 
         def config
           @config ||= Engine::Configuration.new(find_root_with_flag("lib"))
@@ -15,6 +17,15 @@ module Rails
         def inherited(base)
           raise "You cannot inherit from a Rails::Engine child"
         end
+
+        def instance
+          @instance ||= new
+        end
+
+        def endpoint(endpoint = nil)
+          @endpoint = endpoint if endpoint
+          @endpoint
+        end
       end
 
       def config

railties/lib/rails/engine/configuration.rb

@@ -5,10 +5,12 @@ module Rails
     class Configuration < ::Rails::Railtie::Configuration
       attr_reader :root
       attr_writer :eager_load_paths, :autoload_once_paths, :autoload_paths
+      attr_accessor :middleware
 
       def initialize(root=nil)
         super()
         @root = root
+        @middleware = ActionDispatch::MiddlewareStack.new
       end
 
       def paths

railties/test/railties/engine_test.rb

@@ -50,5 +50,40 @@ module RailtiesTest
 
       assert index < initializers.index { |i| i.name == :build_middleware_stack }
     end
+
+    class Upcaser
+      def initialize(app)
+        @app = app
+      end
+
+      def call(env)
+        response = @app.call(env)
+        response[2].upcase!
+        response
+      end
+    end
+
+    test "engine is a rack app and can have his own middleware stack" do
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        class Bukkits
+          class Engine < ::Rails::Engine
+            endpoint lambda { |env| [200, {'Content-Type' => 'text/html'}, 'Hello World'] }
+
+            config.middleware.use ::RailtiesTest::EngineTest::Upcaser
+          end
+        end
+      RUBY
+
+      boot_rails
+
+      Rails::Application.routes.draw do |map|
+        mount(Bukkits::Engine => "/bukkits")
+      end
+
+      env = Rack::MockRequest.env_for("/bukkits")
+      response = Rails::Application.call(env)
+
+      assert_equal "HELLO WORLD", response[2]
+    end
   end
 end