Add ObjectActionFailed exception and make Instance use it

The obj_load_attr() code in Instance slipped through review with
just a "raise Exception()" for the case where the load cannot
complete. This adds a new exception for the case, fixes the
Instance.obj_load_attr() method, and adds a test to verify it.

Related to blueprint compute-api-objects

Change-Id: I73c966deb3c498019370eb83ade64bff2c48aec0

nova/exception.py

@@ -1290,6 +1290,10 @@ class IncompatibleObjectVersion(NovaException):
     msg_fmt = _('Version %(objver)s of %(objname)s is not supported')
 
 
+class ObjectActionError(NovaException):
+    msg_fmt = _('Object action %(action)s failed because: %(reason)s')
+
+
 class CoreAPIMissing(NovaException):
     msg_fmt = _("Core API extensions are missing: %(missing_apis)s")
 

nova/objects/instance.py

@@ -17,6 +17,7 @@ import copy
 from nova.cells import opts as cells_opts
 from nova.cells import rpcapi as cells_rpcapi
 from nova import db
+from nova import exception
 from nova import notifications
 from nova.objects import base
 from nova.objects import instance_fault
@@ -394,7 +395,9 @@ class Instance(base.NovaObject):
             extra.append('fault')
 
         if not extra:
-            raise Exception('Cannot load "%s" from instance' % attrname)
+            raise exception.ObjectActionError(
+                action='obj_load_attr',
+                reason='attribute %s not lazy-loadable' % attrname)
 
         # NOTE(danms): This could be optimized to just load the bits we need
         instance = self.__class__.get_by_uuid(self._context,
@@ -405,7 +408,9 @@ class Instance(base.NovaObject):
         if hasattr(instance, base.get_attrname(attrname)):
             self[attrname] = instance[attrname]
         else:
-            raise Exception('Cannot load "%s" from instance' % attrname)
+            raise exception.ObjectActionError(
+                action='obj_load_attr',
+                reason='loading %s requires recursion' % attrname)
 
 
 def _make_instance_list(context, inst_list, db_inst_list, expected_attrs):

nova/tests/objects/test_instance.py

@@ -21,6 +21,7 @@ import netaddr
 from nova.cells import rpcapi as cells_rpcapi
 from nova import context
 from nova import db
+from nova import exception
 from nova.network import model as network_model
 from nova.objects import base
 from nova.objects import instance
@@ -155,6 +156,11 @@ class _TestInstanceObject(object):
         self.assertEqual(sys_meta2, {'foo': 'bar'})
         self.assertRemotes()
 
+    def test_load_invalid(self):
+        inst = instance.Instance()
+        self.assertRaises(exception.ObjectActionError,
+                          inst.obj_load_attr, 'foo')
+
     def test_get_remote(self):
         # isotime doesn't have microseconds and is always UTC
         ctxt = context.get_admin_context()