Avoid circular reference during serialization

When an instance with numa topology is re-scheduled the conductor
migrate task blows with circular reference during request spec
serialization. It happens because there are ovos in the request spec
that jsonutils.dumps only serialize if requested explicitly.

This patch makes the explicit request.

This is a stable only bug fix as the borken code was removed in Stein by
the feature patch I4244f7dd8fe74565180f73684678027067b4506e

Closes-Bug: #1864665

Change-Id: I1942bfa9bd1baf8738d34c287216db7b59000a36

nova/conductor/tasks/migrate.py

@@ -9,6 +9,7 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+import functools
 
 from oslo_log import log as logging
 from oslo_serialization import jsonutils
@@ -292,7 +293,17 @@ class MigrationTask(base.TaskBase):
         # oslo.messaging #1529084 to transform datetime values into strings.
         # tl;dr: datetimes in dicts are not accepted as correct values by the
         # rpc fake driver.
-        legacy_spec = jsonutils.loads(jsonutils.dumps(legacy_spec))
+        # NOTE(gibi): convert_instances=True is needed as the legacy_spec might
+        # contain ovo instances in the numa_topology field and those are
+        # causing circular reference during serialization otherwise.
+        legacy_spec = jsonutils.loads(
+            jsonutils.dumps(
+                legacy_spec,
+                default=functools.partial(
+                    jsonutils.to_primitive, convert_instances=True
+                ),
+            )
+        )
 
         LOG.debug("Calling prep_resize with selected host: %s; "
                   "Selected node: %s; Alternates: %s", host, node,

nova/tests/unit/conductor/tasks/test_migrate.py

@@ -10,8 +10,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import functools
 import mock
 
+from oslo_serialization import jsonutils
+
 from nova.compute import rpcapi as compute_rpcapi
 from nova.conductor.tasks import migrate
 from nova import context
@@ -123,7 +126,14 @@ class MigrationTaskTestCase(test.NoDBTestCase):
                 self.request_spec.requested_destination)
 
         task = self._generate_task()
-        legacy_request_spec = self.request_spec.to_legacy_request_spec_dict()
+        legacy_request_spec = jsonutils.loads(
+            jsonutils.dumps(
+                self.request_spec.to_legacy_request_spec_dict(),
+                default=functools.partial(
+                    jsonutils.to_primitive, convert_instances=True
+                ),
+            )
+        )
         gmv_mock.return_value = 23
 
         # We just need this hook point to set a uuid on the
@@ -230,6 +240,18 @@ class MigrationTaskTestCase(test.NoDBTestCase):
                                         task.instance, task._migration,
                                         task._held_allocations)
 
+    def test_execute_with_cpu_topoloy(self):
+        self.request_spec = objects.RequestSpec(
+            numa_topology=objects.InstanceNUMATopology(
+                cells=[
+                    objects.InstanceNUMACell(
+                        cpu_topology=objects.VirtCPUTopology()
+                    )
+                ]
+            )
+        )
+        self._test_execute()
+
 
 class MigrationTaskAllocationUtils(test.NoDBTestCase):
     @mock.patch('nova.objects.ComputeNode.get_by_host_and_nodename')