Include application's helpers and router helpers by default, but include engine's ones for controllers inside isolated namespace

actionmailer/lib/action_mailer/railtie.rb

@@ -1,5 +1,6 @@
 require "action_mailer"
 require "rails"
+require "action_mailer/railties/routes_helpers"
 
 module ActionMailer
   class Railtie < Rails::Railtie
@@ -20,6 +21,7 @@ module ActionMailer
       ActiveSupport.on_load(:action_mailer) do
         include app.routes.url_helpers
         include app.routes.mounted_helpers(:app)
+        extend ::ActionMailer::Railties::RoutesHelpers
         options.each { |k,v| send("#{k}=", v) }
       end
     end

actionmailer/lib/action_mailer/railties/routes_helpers.rb

@@ -0,0 +1,12 @@
+module ActionMailer
+  module Railties
+    module RoutesHelpers
+      def inherited(klass)
+        super(klass)
+        if namespace = klass.parents.detect {|m| m.respond_to?(:_railtie) }
+          klass.send(:include, namespace._railtie.routes.url_helpers)
+        end
+      end
+    end
+  end
+end

actionpack/lib/action_controller/base.rb

@@ -223,8 +223,12 @@ module ActionController
 
     def self.inherited(klass)
       super
+      if namespace = klass.parents.detect {|m| m.respond_to?(:_railtie) }
+        klass.helper(all_helpers_from_path(namespace._railtie.config.paths.app.helpers.to_a))
+      else
         klass.helper :all if klass.superclass == ActionController::Base
       end
+    end
 
     ActiveSupport.run_load_hooks(:action_controller, self)
   end

actionpack/lib/action_controller/metal/helpers.rb

@@ -101,8 +101,12 @@ module ActionController
 
         # Extract helper names from files in <tt>app/helpers/**/*_helper.rb</tt>
         def all_application_helpers
+          all_helpers_from_path(helpers_path)
+        end
+
+        def all_helpers_from_path(path)
           helpers = []
-          Array.wrap(helpers_path).each do |path|
+          Array.wrap(path).each do |path|
             extract  = /^#{Regexp.quote(path.to_s)}\/?(.*)_helper.rb$/
             helpers += Dir["#{path}/**/*_helper.rb"].map { |file| file.sub(extract, '\1') }
           end

actionpack/lib/action_controller/railtie.rb

@@ -4,6 +4,7 @@ require "action_dispatch/railtie"
 require "action_view/railtie"
 require "active_support/deprecation/proxy_wrappers"
 require "active_support/deprecation"
+require "action_controller/railties/routes_helpers"
 
 module ActionController
   class Railtie < Rails::Railtie
@@ -50,7 +51,7 @@ module ActionController
       options.helpers_path         ||= paths.app.helpers.to_a
 
       ActiveSupport.on_load(:action_controller) do
-        include app.routes.url_helpers
+        extend ::ActionController::Railties::RoutesHelpers.with(app.routes)
         include app.routes.mounted_helpers(:app)
         options.each { |k,v| send("#{k}=", v) }
       end

actionpack/lib/action_controller/railties/routes_helpers.rb

@@ -0,0 +1,17 @@
+module ActionController
+  module Railties
+    module RoutesHelpers
+      def self.with(routes)
+        Module.new do
+          define_method(:inherited) do |klass|
+            super(klass)
+            if namespace = klass.parents.detect {|m| m.respond_to?(:_railtie) }
+              routes = namespace._railtie.routes
+            end
+            klass.send(:include, routes.url_helpers)
+          end
+        end
+      end
+    end
+  end
+end

railties/lib/rails/engine.rb

@@ -199,6 +199,15 @@ module Rails
         @endpoint = endpoint if endpoint
         @endpoint
       end
+
+      def isolated_engine_for(mod)
+        _engine = self
+        mod.singleton_class.instance_eval do
+          define_method(:_railtie) do
+            _engine
+          end
+        end
+      end
     end
 
     delegate :middleware, :root, :paths, :to => :config

railties/test/railties/engine_test.rb

@@ -298,5 +298,134 @@ module RailtiesTest
       response = Rails.application.call(env)
       assert_equal response[2].path, File.join(@plugin.path, "public/bukkits.html")
     end
+
+    test "shared engine should include application's helpers" do
+      app_file "config/routes.rb", <<-RUBY
+        AppTemplate::Application.routes.draw do
+          match "/foo" => "bukkits/foo#index", :as => "foo"
+          match "/foo/show" => "bukkits/foo#show"
+        end
+      RUBY
+
+      app_file "app/helpers/some_helper.rb", <<-RUBY
+        module SomeHelper
+          def something
+            "Something... Something... Something..."
+          end
+        end
+      RUBY
+
+      @plugin.write "app/controllers/bukkits/foo_controller.rb", <<-RUBY
+        class Bukkits::FooController < ActionController::Base
+          def index
+            render :inline => "<%= something %>"
+          end
+
+          def show
+            render :text => foo_path
+          end
+        end
+      RUBY
+
+      boot_rails
+
+      env = Rack::MockRequest.env_for("/foo")
+      response = Rails.application.call(env)
+      assert_equal "Something... Something... Something...", response[2].body
+
+      env = Rack::MockRequest.env_for("/foo/show")
+      response = Rails.application.call(env)
+      assert_equal "/foo", response[2].body
+    end
+
+    test "isolated engine should include only its own routes and helpers" do
+      @plugin.write "lib/bukkits.rb", <<-RUBY
+        module Bukkits
+          class Engine < ::Rails::Engine
+            isolated_engine_for Bukkits
+          end
+        end
+      RUBY
+
+      app_file "config/routes.rb", <<-RUBY
+        AppTemplate::Application.routes.draw do
+          match "/bar" => "bar#index", :as => "bar"
+          mount Bukkits::Engine => "/bukkits", :as => "bukkits"
+        end
+      RUBY
+
+      @plugin.write "config/routes.rb", <<-RUBY
+        Bukkits::Engine.routes.draw do
+          match "/foo" => "bukkits/foo#index", :as => "foo"
+          match "/foo/show" => "bukkits/foo#show"
+          match "/from_app" => "bukkits/foo#from_app"
+          match "/routes_helpers_in_view" => "bukkits/foo#routes_helpers_in_view"
+        end
+      RUBY
+
+      app_file "app/helpers/some_helper.rb", <<-RUBY
+        module SomeHelper
+          def something
+            "Something... Something... Something..."
+          end
+        end
+      RUBY
+
+      @plugin.write "app/helpers/engine_helper.rb", <<-RUBY
+        module EngineHelper
+          def help_the_engine
+            "Helped."
+          end
+        end
+      RUBY
+
+      @plugin.write "app/controllers/bukkits/foo_controller.rb", <<-RUBY
+        class Bukkits::FooController < ActionController::Base
+          def index
+            render :inline => "<%= help_the_engine %>"
+          end
+
+          def show
+            render :text => foo_path
+          end
+
+          def from_app
+            render :inline => "<%= (self.respond_to?(:bar_path) || self.respond_to?(:something)) %>"
+          end
+
+          def routes_helpers_in_view
+            render :inline => "<%= foo_path %>, <%= app.bar_path %>"
+          end
+        end
+      RUBY
+
+      @plugin.write "app/mailers/bukkits/my_mailer.rb", <<-RUBY
+        module Bukkits
+          class MyMailer < ActionMailer::Base
+          end
+        end
+      RUBY
+
+      boot_rails
+
+      assert_equal Bukkits._railtie, Bukkits::Engine
+      assert ::Bukkits::MyMailer.method_defined?(:foo_path)
+
+      env = Rack::MockRequest.env_for("/bukkits/from_app")
+      response = AppTemplate::Application.call(env)
+      assert_equal "false", response[2].body
+
+      env = Rack::MockRequest.env_for("/bukkits/foo/show")
+      response = AppTemplate::Application.call(env)
+      assert_equal "/bukkits/foo", response[2].body
+
+      env = Rack::MockRequest.env_for("/bukkits/foo")
+      response = AppTemplate::Application.call(env)
+      assert_equal "Helped.", response[2].body
+
+      env = Rack::MockRequest.env_for("/bukkits/routes_helpers_in_view")
+      response = AppTemplate::Application.call(env)
+      assert_equal "/bukkits/foo, /bar", response[2].body
+    end
   end
 end