Add debug logging when Instance raises OrphanedObjectError

This logging would be helpful in debugging issues when
OrphanedObjectError is raised by an instance. Currently, there is
not a way to identify which instance is attempting to lazy-load a
field while orphaned. Being able to locate the instance in the
database could also help with recovery/cleanup when a problematic
record is disrupting operation of a deployment.

Change-Id: I093de2839c1bb7c949a0812e07b63de4cc5ed167
(cherry picked from commit e0fbb6f)
(cherry picked from commit f32deaa)

Author: melwitt

nova/objects/instance.py

@@ -1090,6 +1090,11 @@ def clear_numa_topology(self):
     def obj_load_attr(self, attrname):
         # NOTE(danms): We can't lazy-load anything without a context and a uuid
         if not self._context:
+            if 'uuid' in self:
+                LOG.debug(
+                    "Lazy-load of '%s' attempted by orphaned instance",
+                    attrname, instance=self
+                )
             raise exception.OrphanedObjectError(method='obj_load_attr',
                                                 objtype=self.obj_name())
         if 'uuid' not in self:

nova/tests/unit/objects/test_instance.py

@@ -1633,6 +1633,21 @@ def test_save_objectfield_reraises_if_not_instance_related(self):
         self._test_save_objectfield_fk_constraint_fails(
                 'other_foreign_key', db_exc.DBReferenceError)
 
+    @mock.patch('nova.objects.instance.LOG.debug')
+    def test_obj_load_attr_log(self, mock_log_debug):
+        # Instance with no UUID should not log.
+        instance = objects.Instance()
+        self.assertRaises(
+            exception.OrphanedObjectError, instance.obj_load_attr, 'foo')
+        mock_log_debug.assert_not_called()
+        # Instance with UUID should log.
+        instance = objects.Instance(
+            uuid='127a0d59-b88c-422b-b9a1-2dc7cc51fb9a')
+        self.assertRaises(
+            exception.OrphanedObjectError, instance.obj_load_attr, 'foo')
+        msg = "Lazy-load of '%s' attempted by orphaned instance"
+        mock_log_debug.assert_called_once_with(msg, 'foo', instance=instance)
+
 
 class TestRemoteInstanceObject(test_objects._RemoteTest,
                                _TestInstanceObject):