Make Serializer/Conductor able to backlevel objects

This improves handling the situation where nova-api sends newer
objects to an older compute node. In this case, the serializer detects
the IncompatibleObjectVersion error and asks conductor to backlevel it.
This means we call into a new method in conductor with the primitive
object that we can't handle, and basically ask it to hydrate it and
call obj_make_compatible() on our behalf and pass it back to us.

This eliminates the need for API to know whether it's talking to
an older compute and allows a newer conductor to assist older
compute nodes during upgrades to a significant degree.

Related-Bug: #1258256
Change-Id: I4b17c8382619e4f73b83c026bf68549ac67a68a2

nova/conductor/api.py

@@ -304,6 +304,9 @@ class LocalAPI(object):
     def compute_unrescue(self, context, instance):
         return self._manager.compute_unrescue(context, instance)
 
+    def object_backport(self, context, objinst, target_version):
+        return self._manager.object_backport(context, objinst, target_version)
+
 
 class LocalComputeTaskAPI(object):
     def __init__(self):

nova/conductor/manager.py

@@ -76,7 +76,7 @@ class ConductorManager(manager.Manager):
     namespace.  See the ComputeTaskManager class for details.
     """
 
-    RPC_API_VERSION = '1.61'
+    RPC_API_VERSION = '1.62'
 
     def __init__(self, *args, **kwargs):
         super(ConductorManager, self).__init__(service_name='conductor',
@@ -616,6 +616,9 @@ class ConductorManager(manager.Manager):
     def compute_reboot(self, context, instance, reboot_type):
         self.compute_api.reboot(context, instance, reboot_type)
 
+    def object_backport(self, context, objinst, target_version):
+        return objinst.obj_to_primitive(target_version=target_version)
+
 
 class ComputeTaskManager(base.Base):
     """Namespace for compute methods.

nova/conductor/rpcapi.py

@@ -120,6 +120,7 @@ class ConductorAPI(rpcclient.RpcProxy):
     ...  - Remove security_group_get_by_instance() and
            security_group_rule_get_by_security_group()
     1.61 - Return deleted instance from instance_destroy()
+    1.62 - Added object_backport()
     """
 
     BASE_RPC_API_VERSION = '1.0'
@@ -473,6 +474,11 @@ class ConductorAPI(rpcclient.RpcProxy):
         return cctxt.call(context, 'object_action', objinst=objinst,
                           objmethod=objmethod, args=args, kwargs=kwargs)
 
+    def object_backport(self, context, objinst, target_version):
+        cctxt = self.client.prepare(version='1.62')
+        return cctxt.call(context, 'object_backport', objinst=objinst,
+                          target_version=target_version)
+
 
 class ComputeTaskAPI(rpcclient.RpcProxy):
     """Client side of the conductor 'compute' namespaced RPC API