libvirt: Improve the idempotency of iscsi detach

When detaching an iscsi volume it is possible for the iscsi commands
to run concurrently, causing a target to be deleted by one greenthread
while the other is continuing. When removing the iscsi connection,
we should always ignore exit code 21 because that means that the
target has already been removed.

Fixes bug 1057719

Change-Id: I0c9f2623f85a817e2be506f9a6d523d45c76848a

nova/virt/libvirt/volume.py

@@ -199,8 +199,8 @@ def disconnect_volume(self, connection_info, mount_device):
         devices = [dev for dev in devices if dev.startswith(device_prefix)]
         if not devices:
             self._iscsiadm_update(iscsi_properties, "node.startup", "manual",
-                                  check_exit_code=[0, 255])
+                                  check_exit_code=[0, 21, 255])
             self._run_iscsiadm(iscsi_properties, ("--logout",),
-                               check_exit_code=[0, 255])
+                               check_exit_code=[0, 21, 255])
             self._run_iscsiadm(iscsi_properties, ('--op', 'delete'),
                                check_exit_code=[0, 21, 255])