1 files changed, 34 insertions, 3 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index b82f8aa3a3..775110d95e 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -36,6 +36,31 @@ module ActionDispatch
       def build(app)
         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.
+    class InstrumentationProxy
+      EVENT_NAME = "process_middleware.action_dispatch"
+
+      def initialize(middleware, class_name)
+        @middleware = middleware
+
+        @payload = {
+          middleware: class_name,
+        }
+      end
+
+      def call(env)
+        ActiveSupport::Notifications.instrument(EVENT_NAME, @payload) do
+          @middleware.call(env)
+        end
+      end
     end
 
     include Enumerable
@@ -97,12 +122,18 @@ module ActionDispatch
       middlewares.push(build_middleware(klass, args, block))
     end
 
-    def build(app = Proc.new)
-      middlewares.freeze.reverse.inject(app) { |a, e| e.build(a) }
+    def build(app = nil, &block)
+      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