Ensure migration 209 works with NULL fkey values

Modify the sub-query on migration 209's backup to not select NULL
items which cause the NOT IN to always be false and consequently
the backup not to run resulting in a violation of the foreign keys.

To test, add an instance with NULL uuid to check the backups still
work correctly and avoid violating the foreign key constraint.
If there are NULL values in a NOT IN () set the result is always
false. Therefore having this instance inserted causes migration 209
to fail unless the IN set sub-query is modified appropriately.

Change-Id: Id0f8c8cb082df8861e85f925b979ddd934a95bfc
Closes-Bug: 1240325

nova/db/sqlalchemy/migrate_repo/versions/209_add_missing_foreign_keys.py

@@ -47,7 +47,8 @@ def upgrade(migrate_engine):
         table_dump.create()
         for column, ref_table_name, ref_column_name in indexes:
             ref_table = load_tables[ref_table_name]
-            subq = select([getattr(ref_table.c, ref_column_name)])
+            subq = select([getattr(ref_table.c, ref_column_name)]).where(
+                           getattr(ref_table.c, ref_column_name) != None)
             sql = utils.InsertFromSelect(table_dump, table.select().where(
                 ~ getattr(table.c, column).in_(subq)))
             sql_del = table.delete().where(

nova/tests/db/test_migrations.py

@@ -2785,6 +2785,17 @@ def _pre_upgrade_209(self, engine):
             change_tables[i].delete().execute()
             change_tables[i].insert().values(data[i]).execute()
 
+        # NOTE(jhesketh): Add instance with NULL uuid to check the backups
+        #                 still work correctly and avoid violating the foreign
+        #                 key constraint. If there are NULL values in a NOT IN
+        #                 () set the result is always false. Therefore having
+        #                 this instance inserted here causes migration 209 to
+        #                 fail unless the IN set sub-query is modified
+        #                 appropriately. See bug/1240325
+        db_utils.get_table(engine, 'instances').insert(
+            {'uuid': None}
+        ).execute()
+
     def _check_209(self, engine, data):
         if engine.name == 'sqlite':
             return