Add obj_make_compatible()

This adds a method to CyborgObject that allows it to convert itself
to and older version, within a compatibility window. So, if an object
had a revision that added or changed the formatting of an attribute,
the obj_make_compatible() method can fix up a primitive representation
before it is sent to a client expecting the older version.

Partial-Implements: blueprint add-description-field-to-device-profiles

Change-Id: I196629059bc32165f161fe9c071a339d63d71c10

cyborg/objects/base.py

@@ -16,6 +16,7 @@
 """Cyborg common internal object model"""
 
 import netaddr
+from oslo_log import log as logging
 from oslo_utils import versionutils
 from oslo_versionedobjects import base as object_base
 
@@ -23,6 +24,9 @@ from cyborg import objects
 from cyborg.objects import fields as object_fields
 
 
+LOG = logging.getLogger(__name__)
+
+
 class CyborgObjectRegistry(object_base.VersionedObjectRegistry):
     def registration_hook(self, cls, index):
         # NOTE(jroll): blatantly stolen from nova
@@ -95,6 +99,22 @@ class CyborgObject(object_base.VersionedObject):
             objs.append(cls._from_db_object(cls(context), db_obj))
         return objs
 
+    def obj_make_compatible(self, primitive, target_version):
+        """Make an object representation compatible with a target version.
+
+        This is responsible for taking the primitive representation of
+        an object and making it suitable for the given target_version.
+        This may mean converting the format of object attributes, removing
+        attributes that have been added since the target version, etc.
+
+        :param:primitive: The result of self.obj_to_primitive()
+        :param:target_version: The version string requested by the recipient
+                               of the object.
+        """
+        _log_backport(self, target_version)
+        super(CyborgObject, self).obj_make_compatible(primitive,
+                                                      target_version)
+
 
 class CyborgObjectSerializer(object_base.VersionedObjectSerializer):
     # Base class to use for object hydration
@@ -199,3 +219,13 @@ class DriverObjectBase(CyborgObject):
 
         obj.obj_reset_changes()
         return obj
+
+
+def _log_backport(ovo, target_version):
+    """Log backported versioned objects."""
+    if target_version and target_version != ovo.VERSION:
+        LOG.debug('Backporting %(obj_name)s from version %(src_vers)s '
+                  'to version %(dst_vers)s',
+                  {'obj_name': ovo.obj_name(),
+                   'src_vers': ovo.VERSION,
+                   'dst_vers': target_version})