Revert "Don't use '_TransactionContextManager._async'"

This reverts commit bd7d991309 and bumps
the minimum version of oslo.db to 4.40.0, as that is the first version
of the library to include the renamed attribute.

Change-Id: Ic9e7864be3af7ef362cad5648dfc7bdecd104465
Related-Bug: #1788833

lower-constraints.txt

@@ -79,7 +79,7 @@ oslo.cache==1.26.0
 oslo.concurrency==3.26.0
 oslo.config==6.1.0
 oslo.context==2.19.2
-oslo.db==4.27.0
+oslo.db==4.40.0
 oslo.i18n==3.15.3
 oslo.log==3.36.0
 oslo.messaging==6.3.0

nova/db/sqlalchemy/api.py

@@ -202,7 +202,7 @@ def select_db_reader_mode(f):
         use_slave = keyed_args.get('use_slave', False)
 
         if use_slave:
-            reader_mode = get_context_manager(context).async
+            reader_mode = get_context_manager(context).async_
         else:
             reader_mode = get_context_manager(context).reader
 

requirements.txt

@@ -43,7 +43,7 @@ oslo.log>=3.36.0 # Apache-2.0
 oslo.reports>=1.18.0 # Apache-2.0
 oslo.serialization!=2.19.1,>=2.18.0 # Apache-2.0
 oslo.utils>=3.33.0 # Apache-2.0
-oslo.db>=4.27.0 # Apache-2.0
+oslo.db>=4.40.0 # Apache-2.0
 oslo.rootwrap>=5.8.0 # Apache-2.0
 oslo.messaging>=6.3.0 # Apache-2.0
 oslo.policy>=1.35.0 # Apache-2.0