Remove SESSION_CONFIGURED global from DB fixture

The SESSION_CONFIGURED global flag is used in the Database fixture to
guard against the reconfiguration of the DB context in each test as the
global transaction factory does not allow such reconfiguration. However
this global is error prone for multiple reasons:

 * there are tests that actually configure the factory outside of the
   fixture causing tests to interfere

 * we use one single global flag but we always have two separate
   Database fixture one for the api DB and one for the main DB. Still
   the fixture instantiated first will do the configuration of both
   DB factory.

This patch replaces the global with two individual oslo_db enginefacade
patch_factory() calls that allows patching and resetting the global
factory per test case.

Change-Id: Ifc070d19a18a2d66f1a7bd5898428b12901dfe9e

nova/tests/fixtures/nova.py

@@ -31,6 +31,7 @@ from openstack import service_description
 from oslo_concurrency import lockutils
 from oslo_config import cfg
 from oslo_db import exception as db_exc
+from oslo_db.sqlalchemy import enginefacade
 from oslo_log import log as logging
 import oslo_messaging as messaging
 from oslo_messaging import conffixture as messaging_conffixture
@@ -64,7 +65,6 @@ CONF = cfg.CONF
 LOG = logging.getLogger(__name__)
 
 DB_SCHEMA = collections.defaultdict(str)
-SESSION_CONFIGURED = False
 PROJECT_ID = '6f70656e737461636b20342065766572'
 
 
@@ -611,20 +611,21 @@ class Database(fixtures.Fixture):
         """
         super().__init__()
 
-        # NOTE(pkholkin): oslo_db.enginefacade is configured in tests the
-        # same way as it is done for any other service that uses DB
-        global SESSION_CONFIGURED
-        if not SESSION_CONFIGURED:
-            main_db_api.configure(CONF)
-            api_db_api.configure(CONF)
-            SESSION_CONFIGURED = True
-
         assert database in {'main', 'api'}, f'Unrecognized database {database}'
 
         self.database = database
         self.version = version
 
         if database == 'main':
+            # NOTE(gibi): this inject a new factory for each test and
+            # self.factory_reset is used to clean up the factory at the end
+            # of the test case. This way we can let each test configure the
+            # factory so we can avoid having a global flag guarding against
+            # factory re-configuration
+            self.factory_reset = main_db_api.context_manager.patch_factory(
+                enginefacade._TransactionFactory())
+            main_db_api.configure(CONF)
+
             if connection is not None:
                 ctxt_mgr = main_db_api.create_context_manager(
                     connection=connection)
@@ -634,6 +635,12 @@ class Database(fixtures.Fixture):
         elif database == 'api':
             assert connection is None, 'Not supported for the API database'
 
+            # NOTE(gibi): similar note applies here as for the main_db_api
+            # above
+            self.factory_reset = api_db_api.context_manager.patch_factory(
+                enginefacade._TransactionFactory())
+            api_db_api.configure(CONF)
+
             self.get_engine = api_db_api.get_engine
 
     def setUp(self):
@@ -656,6 +663,7 @@ class Database(fixtures.Fixture):
         engine.dispose()
 
     def reset(self):
+        self.factory_reset()
         engine = self.get_engine()
         engine.dispose()
         self._cache_schema()