Fix interference in db unit test

We discovered that two unit test cases added in
I0647bb8545c1464b521a1d866cf5ee674aea2eae cause errors like
  oslo_db.sqlalchemy.enginefacade.AlreadyStartedError:
  this TransactionFactory is already started

when the db tests run selectively with tox -e py38 nova.tests.unit.db
but does not cause errors if the whole unit test suit is run.

This error happened because our db code uses two global transaction
factory, one for the api DB and one for the main DB. There was a global
flag SESSION_CONFIGURED in our Database fixture that guarded against
double initialization of the factory. But the faulty test cases in
question do not use our Database fixture but use the
OpportunisticDBTestMixin from oslo_db. Obviously that fixture does not
know about our SESSION_CONFIGURED global. So if one of the offending
test case ran first in an executor then that initialized the
transaction factory globally and a later test that uses our Database
fixture tried to configure it again leading to the error. For some
unknown reason if these tests were run in the opposite order the faulty
re-initialization did not happen. Probably the OpportunisticDBTestMixin
was able to prevent that.

A previous patch already removed the global SESSION_CONFIGURED flag
from our fixture and replaced it with a per DB specific patch_factory
calls that allow resetting the state of the factory at the end of each
test case. This would already solve the current test case issue as only
our offending test cases would initialize the global factory without
cleanup and we have one test case per DB. So there would be no
interference. However if in the future we add similar test cases then
those can still interfere through the global factory.

So this patch fixes the two offending test case. Also it extends the
DatabasePoisonFixture used for the NoDbTestCase tesst. The poison now
detects if the test case starts any transaction factory.

This poison caught another offender test case,
test_db_sync_with_special_symbols_in_connection_string, that was marked
NoDb but actually using the database. It is now changed to declare
itself as a test that sets up the DB manually and also it is changed to
use the Database fixture instead of touching the global factory
directly.

Closes-Bug: #1948963

Change-Id: Id96f1795034490c13125ebbab49b029fb96af1c7

1 files changed, 7 insertions, 4 deletions

diff --git a/nova/tests/unit/db/api/test_migrations.py b/nova/tests/unit/db/api/test_migrations.py
index 10c0c8feb9..1b14d569db 100644
--- a/nova/tests/unit/db/api/test_migrations.py
+++ b/nova/tests/unit/db/api/test_migrations.py
@@ -275,11 +275,14 @@ class NovaMigrationsWalk(
                 self._migrate_up(connection, revision)
 
     def test_db_version_alembic(self):
-        migration.db_sync(database='api')
+        engine = enginefacade.writer.get_engine()
 
-        script = alembic_script.ScriptDirectory.from_config(self.config)
-        head = script.get_current_head()
-        self.assertEqual(head, migration.db_version(database='api'))
+        with mock.patch.object(migration, '_get_engine', return_value=engine):
+            migration.db_sync(database='api')
+
+            script = alembic_script.ScriptDirectory.from_config(self.config)
+            head = script.get_current_head()
+            self.assertEqual(head, migration.db_version(database='api'))
 
 
 class TestMigrationsWalkSQLite(