def start_task(self, row, scan=True):
if row[STATE_COL] in RUNNING_STATES :<EOL><INDENT>self.scan_tasks()<EOL>return<EOL><DEDENT>row[STATE_COL] = State.WAITING<EOL>row[STATENAME_COL] = StateNames[State.WAITING]<EOL>self.update_task_db(row)<EOL>if scan:<EOL><INDENT>self.scan_tasks()<EOL><DEDENT>
启动上传任务.

        将任务状态设定为Uploading, 如果没有超过最大任务数的话;
        否则将它设定为Waiting.