Bypass the database if limit=0 for server-list requests

Not all database engines handle fetching 0 rows the same and there isn't
really a point in going to the database to return nothing, so bypass the
DB query if limit=0.

The Nova API is explicitly checking if limit >= 0 so we can't make any
changes there since they would be backwards incompatible, i.e. to start
enforcing limit > 0.

Closes-Bug: #1296929

Change-Id: Ib3d0318bec972af5f1f25202101e76066f930f34

nova/db/sqlalchemy/api.py

@@ -1850,6 +1850,10 @@ def instance_get_all_by_filters(context, filters, sort_key, sort_dir,
                          include or exclude instances whose
                          vm_state is SOFT_DELETED.
     """
+    # NOTE(mriedem): If the limit is 0 there is no point in even going
+    # to the database since nothing is going to be returned anyway.
+    if limit == 0:
+        return []
 
     sort_fn = {'desc': desc, 'asc': asc}
 

nova/tests/db/test_db_api.py

@@ -1542,6 +1542,11 @@ class InstanceTestCase(test.TestCase, ModelsObjectComparatorMixin):
         filtered_instances = db.instance_get_all_by_filters(self.ctxt, {})
         self._assertEqualListsOfInstances(instances, filtered_instances)
 
+    def test_instance_get_all_by_filters_zero_limit(self):
+        self.create_instance_with_args()
+        instances = db.instance_get_all_by_filters(self.ctxt, {}, limit=0)
+        self.assertEqual([], instances)
+
     def test_instance_metadata_get_multi(self):
         uuids = [self.create_instance_with_args()['uuid'] for i in range(3)]
         meta = sqlalchemy_api._instance_metadata_get_multi(self.ctxt, uuids)