Fix logic error in sqlarchemy api

Change-Id: Ic3074969b8a75f7b38aa3ba9a37b23dcf133aa8e

heat/db/sqlalchemy/api.py

@@ -1316,12 +1316,14 @@ def service_get_all_by_args(context, host, binary, hostname):
 def purge_deleted(age, granularity='days', project_id=None, batch_size=20):
     def _validate_positive_integer(val, argname):
         try:
-            return int(val)
+            val = int(val)
         except ValueError:
             raise exception.Error(_("%s should be an integer") % argname)
+
         if val < 0:
             raise exception.Error(_("%s should be a positive integer")
                                   % argname)
+        return val
 
     age = _validate_positive_integer(age, 'age')
     batch_size = _validate_positive_integer(batch_size, 'batch_size')

heat/tests/db/test_sqlalchemy_api.py

@@ -2018,6 +2018,9 @@ class DBAPIStackTest(common.HeatTestCase):
         self._deleted_stack_existance(admin_ctx, stacks, resources,
                                       events, tmpl_files, (), (0, 1, 2, 3, 4))
 
+        # test wrong age
+        self.assertRaises(exception.Error, db_api.purge_deleted, -1, 'seconds')
+
     def test_purge_project_deleted(self):
         now = timeutils.utcnow()
         delta = datetime.timedelta(seconds=3600 * 7)