ask the request object for the session

The flash middleware shouldn't know how to look up the session object.
Just ask the request for that information.

actionpack/lib/action_dispatch/middleware/flash.rb

@@ -276,7 +276,7 @@ def call(env)
       req = ActionDispatch::Request.new env
       @app.call(env)
     ensure
-      session    = Request::Session.find(req) || {}
+      session    = req.session || {}
       flash_hash = req.flash_hash
 
       if flash_hash && (flash_hash.present? || session.key?('flash'))