Raise InstanceNotFound when save FK constraint fails

Currently, in the Instance.save method, if an ObjectField is dirty,
it will be updated in the db before the instance record is updated
(separate transaction). If the instance does not have a row, the
foreign key constraint fails and DBReferenceError is raised from
oslo.db. This scenario can happen in cells if an instance delete
request comes in before it's scheduled, during the broadcast to
delete in all cells. This is expected and InstanceNotFound is
caught and handled.

This change catches DBReferenceError and raises InstanceNotFound.

Conflicts:
        nova/tests/unit/objects/test_instance.py

NOTE(mriedem): The test conflict is because we don't have commit
6d6fdeeff8 in stable/kilo and that's
not needed for this bug fix.

Closes-Bug: #1462128

Change-Id: I821f8c121760e0fb74d7ae5c680bd0c361f92128
(cherry picked from commit 5f869d0c04)

nova/objects/instance.py

@@ -15,6 +15,7 @@
 import copy
 
 from oslo_config import cfg
+from oslo_db import exception as db_exc
 from oslo_log import log as logging
 from oslo_serialization import jsonutils
 from oslo_utils import timeutils
@@ -817,6 +818,13 @@ class Instance(base.NovaPersistentObject, base.NovaObject,
                 except AttributeError:
                     LOG.exception(_LE('No save handler for %s'), field,
                                   instance=self)
+                except db_exc.DBReferenceError:
+                    # NOTE(melwitt): This will happen if we instance.save()
+                    # before an instance.create() and FK constraint fails.
+                    # In practice, this occurs in cells during a delete of
+                    # an unscheduled instance. Otherwise, it could happen
+                    # as a result of bug.
+                    raise exception.InstanceNotFound(instance_id=self.uuid)
             elif field in changes:
                 updates[field] = self[field]
 

nova/tests/unit/objects/test_instance.py

@@ -18,6 +18,7 @@ import iso8601
 import mock
 from mox3 import mox
 import netaddr
+from oslo_db import exception as db_exc
 from oslo_serialization import jsonutils
 from oslo_utils import timeutils
 
@@ -487,6 +488,21 @@ class _TestInstanceObject(object):
         mock_update.assert_called_once_with(
             self.context, inst.uuid, {'vcpu_model': None})
 
+    def test_save_objectfield_missing_instance_row(self):
+        error = db_exc.DBReferenceError('table', 'constraint', 'key',
+                                        'key_table')
+        instance = fake_instance.fake_instance_obj(self.context)
+        fields_with_save_methods = [field for field in instance.fields
+                                    if hasattr(instance, '_save_%s' % field)]
+        for field in fields_with_save_methods:
+            @mock.patch.object(instance, '_save_%s' % field)
+            def _test(mock_save_field):
+                mock_save_field.side_effect = error
+                instance._changed_fields.add(field)
+                self.assertRaises(exception.InstanceNotFound,
+                                  instance.save)
+            _test()
+
     def test_get_deleted(self):
         fake_inst = dict(self.fake_instance, id=123, deleted=123)
         fake_uuid = fake_inst['uuid']