GET_LOCK(str,timeout)

Tries to obtain a lock with a name given by the string str, using a timeout of timeout seconds. A negative timeout value means infinite timeout. The lock is exclusive. While held by one session, other sessions cannot obtain a lock of the same name.

• 尝试使用超时秒数的超时获得一个由字符串str给出的名称的锁。
• 负超时值意味着无限超时。
• 锁是独占的。
• 只能由一个会话持有，其他会话无法获得同名的锁。

参考：https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_get-lock
参考：https://blog.csdn.net/tangtong1/article/details/51792617