code: def _begin_write(session: UpdateSession,
                   loop: asyncio.AbstractEventLoop,
                   rootfs_file_path: str):
      
      session.set_progress(0)
      session.set_stage(Stages.WRITING)
      write_future = asyncio.ensure_future(loop.run_in_executor(
          None, file_actions.write_update, rootfs_file_path,
          session.set_progress))
  
      def write_done(fut):
          exc = fut.exception()
          if exc:
              session.set_error(getattr(exc, 'short', str(type(exc))),
                                str(exc))
          else:
              session.set_stage(Stages.DONE)
  
      write_future.add_done_callback(write_done)
docs: Start the write process.