类似这种的，隔一段时间就会出现一次，导致task失败。后边是改了MySQL的wait_timeout参数，才正常导出了
[2022/10/17 14:56:59.918 +08:00] [INFO] [conn.go:70] ["cannot execute query"] [task=clear_task] [unit=dump] [retryTime=1] [sql="SHOW COLUMNS FROM `ying99_fundtxn`.`campain_discount`"] [args=null] [error="driver: bad connection"]
[2022/10/17 14:57:00.603 +08:00] [ERROR] [dumpling.go:152] ["dump data exits with error"] [task=clear_task] [unit=dump] ["cost time"=16m55.166129053s] [error="ErrCode:32001 ErrClass:\"dump-unit\" ErrScope:\"internal\" ErrLevel:\"high\" Message:\"mydumper/dumpling runs with error, with output (may empty): \" RawCause:\"sql: SHOW COLUMNS FROM `ying99_fundtxn`.`campain_discount`: driver: bad connection\" "]