Document and provide useful error message for volume-backed backup

The createBackup API does not support backing up volume-backed
instances. The error message the user gets is not useful about
why the request was invalid, and the API reference docs do not
mention the limitation. This change addresses both of those issues.

Change-Id: I04fd8ab4f8818d9d0ccccb6f6fcb34965b15b8f3
Partial-Bug: #1679314

api-ref/source/servers-admin-action.inc

@@ -20,6 +20,8 @@ Create Server Back Up (createBackup Action)
 
 Creates a back up of a server.
 
+.. note:: This API is not supported for volume-backed instances.
+
 Specify the ``createBackup`` action in the request body.
 
 Policy defaults enable only users with the administrative role or the

nova/compute/api.py

@@ -2788,7 +2788,8 @@ class API(base.Base):
         if compute_utils.is_volume_backed_instance(context, instance):
             LOG.info(_LI("It's not supported to backup volume backed "
                          "instance."), instance=instance)
-            raise exception.InvalidRequest()
+            raise exception.InvalidRequest(
+                _('Backup is not supported for volume-backed instances.'))
         else:
             image_meta = self._create_image(context, instance,
                                             name, 'backup',

nova/tests/unit/api/openstack/compute/test_create_backup.py

@@ -14,12 +14,15 @@
 #    under the License.
 
 import mock
+from oslo_utils import timeutils
+import six
 import webob
 
 from nova.api.openstack import common
 from nova.api.openstack.compute import create_backup \
         as create_backup_v21
 from nova.compute import api
+from nova.compute import utils as compute_utils
 from nova import exception
 from nova import test
 from nova.tests.unit.api.openstack.compute import admin_only_action_common
@@ -334,8 +337,9 @@ class CreateBackupTestsV21(admin_only_action_common.CommonMixin,
                           self.req, fakes.FAKE_UUID, body=body)
 
     @mock.patch.object(common, 'check_img_metadata_properties_quota')
-    @mock.patch.object(api.API, 'backup')
-    def test_backup_volume_backed_instance(self, mock_backup,
+    @mock.patch.object(compute_utils, 'is_volume_backed_instance',
+                       return_value=True)
+    def test_backup_volume_backed_instance(self, mock_is_volume_backed,
                                            mock_check_image):
         body = {
             'createBackup': {
@@ -345,18 +349,20 @@ class CreateBackupTestsV21(admin_only_action_common.CommonMixin,
             },
         }
 
-        instance = fake_instance.fake_instance_obj(self.context)
+        updates = {'vm_state': 'active',
+                   'task_state': None,
+                   'launched_at': timeutils.utcnow()}
+        instance = fake_instance.fake_instance_obj(self.context, **updates)
         instance.image_ref = None
         self.mock_get.return_value = instance
-        mock_backup.side_effect = exception.InvalidRequest()
 
-        self.assertRaises(webob.exc.HTTPBadRequest,
-                          self.controller._create_backup,
-                          self.req, instance['uuid'], body=body)
+        ex = self.assertRaises(webob.exc.HTTPBadRequest,
+                               self.controller._create_backup,
+                               self.req, instance['uuid'], body=body)
         mock_check_image.assert_called_once_with(self.context, {})
-        mock_backup.assert_called_once_with(self.context, instance, 'BackupMe',
-                                              'daily', 3,
-                                              extra_properties={})
+        mock_is_volume_backed.assert_called_once_with(self.context, instance)
+        self.assertIn('Backup is not supported for volume-backed instances',
+                      six.text_type(ex))
 
 
 class CreateBackupPolicyEnforcementv21(test.NoDBTestCase):