Don't make a no-op DB call

drivers_uuid can be empty, but we are still doing a DB call. This is causing
a sqlalchemy SAWarning.

SAWarning: The IN-predicate on "instances.uuid" was invoked with an
empty sequence. This results in a contradiction, which nonetheless can
be expensive to evaluate.  Consider alternative strategies for improved
performance.

Conflicts:
        nova/tests/unit/compute/test_compute_mgr.py

Change-Id: Ib8c9b85e84800c9e2ddcdf204851f1b51101926c
Closes-Bug: #1383617
(cherry picked from commit 150428f76e)

nova/compute/manager.py

@@ -691,6 +691,9 @@ class ComputeManager(manager.Manager):
             filters = {}
         try:
             driver_uuids = self.driver.list_instance_uuids()
+            if len(driver_uuids) == 0:
+                # Short circuit, don't waste a DB call
+                return objects.InstanceList()
             filters['uuid'] = driver_uuids
             local_instances = objects.InstanceList.get_by_filters(
                 context, filters, use_slave=True)

nova/tests/compute/test_compute_mgr.py

@@ -813,6 +813,18 @@ class ComputeManagerUnitTestCase(test.NoDBTestCase):
         self.assertEqual([x['uuid'] for x in driver_instances],
                          [x['uuid'] for x in result])
 
+    @mock.patch('nova.virt.driver.ComputeDriver.list_instance_uuids')
+    @mock.patch('nova.db.api.instance_get_all_by_filters')
+    def test_get_instances_on_driver_empty(self, mock_list, mock_db):
+        fake_context = context.get_admin_context()
+        mock_list.return_value = []
+
+        result = self.compute._get_instances_on_driver(fake_context)
+        # instance_get_all_by_filters should not be called
+        self.assertEqual(0, mock_db.call_count)
+        self.assertEqual([],
+                         [x['uuid'] for x in result])
+
     def test_get_instances_on_driver_fallback(self):
         # Test getting instances when driver doesn't support
         # 'list_instance_uuids'