Break out the meat of the object hydration process

Since we call the NovaObject base class to hydrate an object, the
subclass implementation does not get a chance to run specific code
during or after hydration. Things like Instance that keep local data for
change tracking need a chance to update themselves after hydration
is complete. This breaks out the meat into a separate method which
can be easily overridden in a subclass.

This also makes Instance use this to ensure metadata tracking is
up-to-date after hydration.

Related to blueprint compute-manager-objects

Change-Id: Ie070c17495f7a4cc6210d256a3170ac9c06e198f

nova/objects/base.py

@@ -223,6 +223,20 @@ class NovaObject(object):
                                                   objver=objver,
                                                   supported=latest_ver)
 
+    @classmethod
+    def _obj_from_primitive(cls, context, objver, primitive):
+        self = cls()
+        self._context = context
+        self.VERSION = objver
+        objdata = primitive['nova_object.data']
+        changes = primitive.get('nova_object.changes', [])
+        for name, field in self.fields.items():
+            if name in objdata:
+                setattr(self, name, field.from_primitive(self, name,
+                                                         objdata[name]))
+        self._changed_fields = set([x for x in changes if x in self.fields])
+        return self
+
     @classmethod
     def obj_from_primitive(cls, primitive, context=None):
         """Object field-by-field hydration."""
@@ -234,18 +248,8 @@ class NovaObject(object):
                                    primitive['nova_object.name']))
         objname = primitive['nova_object.name']
         objver = primitive['nova_object.version']
-        objdata = primitive['nova_object.data']
         objclass = cls.obj_class_from_name(objname, objver)
-        self = objclass()
-        self._context = context
-        self.VERSION = objver
-        for name, field in self.fields.items():
-            if name in objdata:
-                setattr(self, name, field.from_primitive(self, name,
-                                                         objdata[name]))
-        changes = primitive.get('nova_object.changes', [])
-        self._changed_fields = set([x for x in changes if x in self.fields])
-        return self
+        return objclass._obj_from_primitive(context, objver, primitive)
 
     def __deepcopy__(self, memo):
         """Efficiently make a deep copy of this object."""

nova/objects/instance.py

@@ -188,6 +188,13 @@ class Instance(base.NovaPersistentObject, base.NovaObject):
             changes.add('system_metadata')
         return changes
 
+    @classmethod
+    def _obj_from_primitive(cls, context, objver, primitive):
+        self = super(Instance, cls)._obj_from_primitive(context, objver,
+                                                        primitive)
+        self._reset_metadata_tracking()
+        return self
+
     def obj_make_compatible(self, primitive, target_version):
         target_version = (int(target_version.split('.')[0]),
                           int(target_version.split('.')[1]))

nova/tests/objects/test_objects.py

@@ -426,7 +426,15 @@ class _TestObject(object):
                      'nova_object.namespace': 'nova',
                      'nova_object.version': '1.5',
                      'nova_object.data': {'foo': 1}}
-        obj = MyObj.obj_from_primitive(primitive)
+        real_method = MyObj._obj_from_primitive
+
+        def _obj_from_primitive(*args):
+            return real_method(*args)
+
+        with mock.patch.object(MyObj, '_obj_from_primitive') as ofp:
+            ofp.side_effect = _obj_from_primitive
+            obj = MyObj.obj_from_primitive(primitive)
+            ofp.assert_called_once_with(None, '1.5', primitive)
         self.assertEqual(obj.foo, 1)
 
     def test_hydration_version_different(self):