1 files changed, 13 insertions, 3 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index f0c869fba0..775110d95e 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
 
@@ -119,11 +123,17 @@ 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
-
       def assert_index(index, where)
         i = index.is_a?(Integer) ? index : middlewares.index { |m| m.klass == index }
         raise "No such middleware to insert #{where}: #{index.inspect}" unless i