2 files changed, 26 insertions, 4 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb b/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb
index 02ccfbc81a..7c43c781c7 100644
--- a/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb
+++ b/actionpack/lib/action_dispatch/middleware/session/cookie_store.rb
@@ -24,9 +24,10 @@ module ActionDispatch
     #
     #   Rails.application.config.session_store :cookie_store, key: '_your_app_session'
     #
-    # By default, your secret key base is derived from your application name in
-    # the test and development environments. In all other environments, it is stored
-    # encrypted in the <tt>config/credentials.yml.enc</tt> file.
+    # In the development and test environments your application's secret key base is
+    # generated by Rails and stored in a temporary file in <tt>tmp/development_secret.txt</tt>.
+    # In all other environments, it is stored encrypted in the
+    # <tt>config/credentials.yml.enc</tt> file.
     #
     # If your application was not updated to Rails 5.2 defaults, the secret_key_base
     # will be found in the old <tt>config/secrets.yml</tt> file.
diff --git a/actionpack/lib/action_dispatch/middleware/stack.rb b/actionpack/lib/action_dispatch/middleware/stack.rb
index 7a2fcd6db7..f0c869fba0 100644
--- a/actionpack/lib/action_dispatch/middleware/stack.rb
+++ b/actionpack/lib/action_dispatch/middleware/stack.rb
@@ -34,7 +34,28 @@ module ActionDispatch
       end
 
       def build(app)
-        klass.new(app, *args, &block)
+        InstrumentationProxy.new(klass.new(app, *args, &block), 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