3 files changed, 17 insertions, 6 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index f0c869fba0..57e4adb457 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -34,7 +34,11 @@ module ActionDispatch
       end
 
       def build(app)
-        InstrumentationProxy.new(klass.new(app, *args, &block), inspect)
+        klass.new(app, *args, &block)
+      end
+
+      def build_instrumented(app)
+        InstrumentationProxy.new(build(app), inspect)
       end
     end
 
@@ -119,7 +123,14 @@ module ActionDispatch
     end
 
     def build(app = nil, &block)
-      middlewares.freeze.reverse.inject(app || block) { |a, e| e.build(a) }
+      instrumenting = ActiveSupport::Notifications.notifier.listening?(InstrumentationProxy::EVENT_NAME)
+      middlewares.freeze.reverse.inject(app || block) do |a, e|
+        if instrumenting
+          e.build_instrumented(a)
+        else
+          e.build(a)
+        end
+      end
     end
 
     private
diff --git a/actionpack/test/controller/show_exceptions_test.rb b/actionpack/test/controller/show_exceptions_test.rb
index 8724f9bcdb..1d68a359dc 100644
--- a/actionpack/test/controller/show_exceptions_test.rb
+++ b/actionpack/test/controller/show_exceptions_test.rb
@@ -99,7 +99,7 @@ module ShowExceptions
   class ShowFailsafeExceptionsTest < ActionDispatch::IntegrationTest
     def test_render_failsafe_exception
       @app = ShowExceptionsOverriddenController.action(:boom)
-      middleware = @app.instance_variable_get(:@middleware)
+      middleware = @app
       @exceptions_app = middleware.instance_variable_get(:@exceptions_app)
       middleware.instance_variable_set(:@exceptions_app, nil)
       $stderr = StringIO.new
diff --git a/actionpack/test/dispatch/middleware_stack_test.rb b/actionpack/test/dispatch/middleware_stack_test.rb
index 90f2eccd19..c534e60c74 100644
--- a/actionpack/test/dispatch/middleware_stack_test.rb
+++ b/actionpack/test/dispatch/middleware_stack_test.rb
@@ -121,9 +121,6 @@ class MiddlewareStackTest < ActiveSupport::TestCase
   end
 
   test "instruments the execution of middlewares" do
-    app = @stack.build(proc { |env| [200, {}, []] })
-    env = {}
-
     events = []
 
     subscriber = proc do |*args|
@@ -131,6 +128,9 @@ class MiddlewareStackTest < ActiveSupport::TestCase
     end
 
     ActiveSupport::Notifications.subscribed(subscriber, "process_middleware.action_dispatch") do
+      app = @stack.build(proc { |env| [200, {}, []] })
+
+      env = {}
       app.call(env)
     end