Only build middleware proxy when instrumentating

The instrumentation proxy adds three stack frames per-middleware, even
when nothing is listening.

This commit, when the middleware stack is built, only adds
instrumentation when the `process_middleware.action_dispatch` event has
already been subscribed to.

The advantage to this is that we don't have any extra stack frames in
apps which don't need middleware instrumentation.

The disadvantage is that the subscriptions need to be in place when the
middleware stack is built (during app boot). I think this is likely okay
because temporary AS::Notifications subscriptions are strongly
discouraged.

1 files changed, 13 insertions, 2 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