Comment 7 for bug 1597596

Looking at nova/compute/manager.py in _allocate_network_async() there is code and comment

                instance.system_metadata['network_allocated'] = 'True'
                # NOTE(JoshNang) do not save the instance here, as it can cause
                # races. The caller shares a reference to instance and waits
                # for this async greenthread to finish before calling
                # instance.save().

But that doesn't seem to be true, the corresponding code in

                    # NOTE(JoshNang) This also saves the changes to the
                    # instance from _allocate_network_async, as they aren't
                    # saved in that function to prevent races.
                    instance.save(expected_task_state=
                            task_states.BLOCK_DEVICE_MAPPING)

gets executed earlier, as in the log I can see the self.driver.spawn() call below this code being executed before _allocate_network_async logs the assigned network info.