context: Warn on mixed uses of old and new db methods

It ends up with two sessions for a context and can lead
subtle caching issues.

Related-Bug: #1803745
Change-Id: Ic04efbe6915293d2f8d181c9a5348cc8aa58bd62

neutron_lib/context.py

@@ -15,6 +15,7 @@
 import collections
 import copy
 import datetime
+import warnings
 
 from oslo_context import context as oslo_context
 from oslo_db.sqlalchemy import enginefacade
@@ -145,6 +146,11 @@ class Context(ContextBaseWithSession):
         # TODO(akamyshnikova): checking for session attribute won't be needed
         # when reader and writer will be used
         if hasattr(super(Context, self), 'session'):
+            if self._session:
+                warnings.warn('context.session is used with and without '
+                              'new enginefacade. Please update the code to '
+                              'use new enginefacede consistently.',
+                              DeprecationWarning)
             return super(Context, self).session
         if self._session is None:
             self._session = db_api.get_writer_session()