lib/action_dispatch/middleware/stack.rb in actionpack-6.0.0.rc1 vs lib/action_dispatch/middleware/stack.rb in actionpack-6.0.0.rc2

@@ -32,12 +32,16 @@
           klass.class.to_s
         end
       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
 
     # This class is used to instrument the execution of a single middleware.
     # It proxies the `call` method transparently and instruments the method
     # call.
@@ -117,10 +121,17 @@
     def use(klass, *args, &block)
       middlewares.push(build_middleware(klass, args, block))
     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)