autotests do a lot of calls to update_engine_client. But the update_engine_client also calls into the update_engine daemon through dbus. But since update_engine is single threaded and some operations inside the update_engine (specially while updating) takes too long, there is a chance that the call to update_engine fails even though there was no problem with the update_engine itself. It is a good idea to retry dbus calls in the update_engine_client based on the errors to ensure we are not failing on some valid states.