DBMS: Fix db_sync between N and N+1 releases

If we start cinderlib from the N release with the DBMS persistence
plugin on a DB where we have already run it with release N+1 then we'll
get an `oslo_db.exception.DBMigrationError` exception.

This is because cinderlib automatically does a `db_sync` on each
initialization, but on N this will fail because there will be unknown
(as in newer) migrations applied to the DB.

This patch takes this possibility and checks the exception raised by
`db_sync` and ignores it if the `migrate` package complains with a
VersionNotFoundError.

Closes-Bug: #1868145
Change-Id: I539919e01f603d19cde8750ad92e3d2b4ac2fbc7

cinderlib/tests/unit/persistence/test_dbms.py

@@ -14,6 +14,7 @@
 #    under the License.
 
 import tempfile
+from unittest import mock
 
 from cinder.db.sqlalchemy import api as sqla_api
 from cinder import objects as cinder_ovos
@@ -121,5 +122,31 @@ class TestDBPersistence(base.BasePersistenceTest):
         self.assertEqual('__DEFAULT__', self.persistence.DEFAULT_TYPE.name)
 
 
+class TestDBPersistenceNewerSchema(base.helper.TestHelper):
+    """Test DBMS plugin can start when the DB has a newer schema."""
+    CONNECTION = 'sqlite:///' + tempfile.NamedTemporaryFile().name
+    PERSISTENCE_CFG = {'storage': 'db',
+                       'connection': CONNECTION}
+
+    @classmethod
+    def setUpClass(cls):
+        pass
+
+    def _raise_exc(self):
+        inner_exc = dbms.migrate.exceptions.VersionNotFoundError()
+        exc = dbms.exception.DBMigrationError(inner_exc)
+        self.original_db_sync()
+        raise(exc)
+
+    def test_newer_db_schema(self):
+        self.original_db_sync = dbms.migration.db_sync
+        with mock.patch.object(dbms.migration, 'db_sync',
+                               side_effect=self._raise_exc) as db_sync_mock:
+            super(TestDBPersistenceNewerSchema, self).setUpClass()
+            db_sync_mock.assert_called_once()
+            self.assertIsInstance(cinderlib.Backend.persistence,
+                                  dbms.DBPersistence)
+
+
 class TestMemoryDBPersistence(TestDBPersistence):
     PERSISTENCE_CFG = {'storage': 'memory_db'}