Update image_base_image_ref during rebuild.

In different location we assume system_metadata.image_base_image_ref
exists, because it is set during instance creation in method
_populate_instance_for_create

But once instance is rebuild, all system_metadata image property a dropped
and replace by new image property and without setting back
image_base_image_ref.

This change propose to set image_base_image_ref during rebuild.

In specific case of shelve/unshelve in Qcow2 backend, image_base_image_ref is
used to rebase disk image, so we ensure this property is set as instance may
have been rebuild before the fix was apply.

Related-Bug: #1732428
Closes-Bug: #1893618
Change-Id: Ia3031ea1f7db8b398f02d2080ca603ded8970200

nova/compute/api.py

@@ -3534,6 +3534,8 @@ class API(base.Base):
             new_sys_metadata = utils.get_system_metadata_from_image(
                 image, flavor)
 
+            new_sys_metadata.update({'image_base_image_ref': image_id})
+
             instance.system_metadata.update(new_sys_metadata)
             instance.save()
             return orig_sys_metadata
@@ -4170,6 +4172,14 @@ class API(base.Base):
         hypervisor.
         """
         instance.task_state = task_states.SHELVING
+
+        # NOTE(aarents): Ensure image_base_image_ref is present as it will be
+        # needed during unshelve and instance rebuild done before Bug/1893618
+        # Fix dropped it.
+        instance.system_metadata.update(
+                {'image_base_image_ref': instance.image_ref}
+        )
+
         instance.save(expected_task_state=[None])
 
         self._record_action_start(context, instance, instance_actions.SHELVE)

nova/tests/unit/compute/test_compute.py

@@ -9045,6 +9045,7 @@ class ComputeAPITestCase(BaseTestCase):
                      'image_ramdisk_id': uuids.ramdisk_id,
                      'image_something_else': 'meow',
                      'preserved': 'preserve this!',
+                     'image_base_image_ref': image_ref,
                      'boot_roles': ''},
                     sys_meta)