Fix incomplete instance data returned after build failure

This change fixes a race in _cleanup_build_artifacts. We were updating
the instance mapping to point at the cell in which the instance was
created before the instance record was complete, i.e. before the related
BDMs and tags were created in the cell DB. Updating the instance mapping
exposes the cell's version of the instance to the API. If the API happened
to fetch it before it was finished being created it would return
incomplete data.

Co-Authored-By: Matt Riedemann <mriedem.os@gmail.com>

Conflicts:
      nova/tests/unit/conductor/test_conductor.py

NOTE(mriedem): The conflict is due to not having change
I0c764e441993e32aafef0b18049a425c3c832a50 in Rocky.

Closes-Bug: #1820337
Change-Id: If966eb1161c842ff49aa530e4482dbca87b61a3e
(cherry picked from commit 64f6cbc912)
(cherry picked from commit eec39762cf)

nova/conductor/manager.py

@@ -1411,13 +1411,6 @@ class ComputeTaskManager(base.Base):
                                               'build_instances', updates, exc,
                                               request_spec)
 
-            # TODO(mnaser): The cell mapping should already be populated by
-            #               this point to avoid setting it below here.
-            inst_mapping = objects.InstanceMapping.get_by_instance_uuid(
-                context, instance.uuid)
-            inst_mapping.cell_mapping = cell
-            inst_mapping.save()
-
             # In order to properly clean-up volumes when deleting a server in
             # ERROR status with no host, we need to store BDMs in the same
             # cell.
@@ -1433,6 +1426,18 @@ class ComputeTaskManager(base.Base):
                 with nova_context.target_cell(context, cell) as cctxt:
                     self._create_tags(cctxt, instance.uuid, tags)
 
+            # NOTE(mdbooth): To avoid an incomplete instance record being
+            #                returned by the API, the instance mapping must be
+            #                created after the instance record is complete in
+            #                the cell, and before the build request is
+            #                destroyed.
+            # TODO(mnaser): The cell mapping should already be populated by
+            #               this point to avoid setting it below here.
+            inst_mapping = objects.InstanceMapping.get_by_instance_uuid(
+                context, instance.uuid)
+            inst_mapping.cell_mapping = cell
+            inst_mapping.save()
+
             # Be paranoid about artifacts being deleted underneath us.
             try:
                 build_request.destroy()

nova/tests/unit/conductor/test_conductor.py

@@ -2056,6 +2056,60 @@ class ConductorTaskTestCase(_BaseTaskTestCase, test_compute.BaseTestCase):
 
         self.assertTrue(mock_build.called)
 
+    @mock.patch('nova.objects.InstanceMapping.get_by_instance_uuid')
+    def test_cleanup_build_artifacts(self, inst_map_get):
+        """Simple test to ensure the order of operations in the cleanup method
+        is enforced.
+        """
+        req_spec = fake_request_spec.fake_spec_obj()
+        build_req = fake_build_request.fake_req_obj(self.context)
+        instance = build_req.instance
+        bdms = objects.BlockDeviceMappingList(objects=[
+            objects.BlockDeviceMapping(instance_uuid=instance.uuid)])
+        tags = objects.TagList(objects=[objects.Tag(tag='test')])
+        cell1 = self.cell_mappings['cell1']
+        cell_mapping_cache = {instance.uuid: cell1}
+        err = exc.TooManyInstances('test')
+
+        # We need to assert that BDMs and tags are created in the cell DB
+        # before the instance mapping is updated.
+        def fake_create_block_device_mapping(*args, **kwargs):
+            inst_map_get.return_value.save.assert_not_called()
+
+        def fake_create_tags(*args, **kwargs):
+            inst_map_get.return_value.save.assert_not_called()
+
+        with test.nested(
+            mock.patch.object(self.conductor_manager,
+                              '_set_vm_state_and_notify'),
+            mock.patch.object(self.conductor_manager,
+                              '_create_block_device_mapping',
+                              side_effect=fake_create_block_device_mapping),
+            mock.patch.object(self.conductor_manager, '_create_tags',
+                              side_effect=fake_create_tags),
+            mock.patch.object(build_req, 'destroy'),
+            mock.patch.object(req_spec, 'destroy'),
+        ) as (
+            _set_vm_state_and_notify, _create_block_device_mapping,
+            _create_tags, build_req_destroy, req_spec_destroy,
+        ):
+            self.conductor_manager._cleanup_build_artifacts(
+                self.context, err, [instance], [build_req], [req_spec], bdms,
+                tags, cell_mapping_cache)
+        # Assert the various mock calls.
+        _set_vm_state_and_notify.assert_called_once_with(
+            test.MatchType(context.RequestContext), instance.uuid,
+            'build_instances',
+            {'vm_state': vm_states.ERROR, 'task_state': None}, err, req_spec)
+        _create_block_device_mapping.assert_called_once_with(
+            cell1, instance.flavor, instance.uuid, bdms)
+        _create_tags.assert_called_once_with(
+            test.MatchType(context.RequestContext), instance.uuid, tags)
+        inst_map_get.return_value.save.assert_called_once_with()
+        self.assertEqual(cell1, inst_map_get.return_value.cell_mapping)
+        build_req_destroy.assert_called_once_with()
+        req_spec_destroy.assert_called_once_with()
+
     @mock.patch('nova.objects.CellMapping.get_by_uuid')
     def test_bury_in_cell0_no_cell0(self, mock_cm_get):
         mock_cm_get.side_effect = exc.CellMappingNotFound(uuid='0')