ttl job's start time will be override by mistake after failover

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. create a TTL table

 create table t1(t timestamp default current_timestamp) TTL=`t`+interval 1 MINUTE ttl_job_interval='1h';

2. whe job running, check the table status:

> select * from mysql.tidb_ttl_table_status\G
***************************[ 1. row ]***************************
table_id                       | 106
parent_table_id                | 106
table_statistics               | <null>
last_job_id                    | <null>
last_job_start_time            | <null>
last_job_finish_time           | <null>
last_job_ttl_expire            | <null>
last_job_summary               | <null>
current_job_id                 | 1027d8d0-ac77-4ef6-88d1-b3d481457aed
current_job_owner_id           | 78928a9a-c379-4c22-921f-2994ada99057
current_job_owner_addr         | <null>
current_job_owner_hb_time      | 2023-06-19 11:59:25
current_job_start_time         | 2023-06-19 11:58:52
current_job_ttl_expire         | 2023-06-19 11:57:51
current_job_state              | <null>
current_job_status             | running
current_job_status_update_time | 2023-06-19 11:59:25

3. kill the running tidb and restart a new one. Then the new tidb will take over the job, and then check the table status again:

> select * from mysql.tidb_ttl_table_status\G
***************************[ 1. row ]***************************
table_id                       | 106
parent_table_id                | 106
table_statistics               | <null>
last_job_id                    | <null>
last_job_start_time            | <null>
last_job_finish_time           | <null>
last_job_ttl_expire            | <null>
last_job_summary               | <null>
current_job_id                 | 1027d8d0-ac77-4ef6-88d1-b3d481457aed
current_job_owner_id           | 531cb4c2-4b3f-49e6-85d8-fb1ef46e181f
current_job_owner_addr         | <null>
current_job_owner_hb_time      | 2023-06-19 12:01:34
current_job_start_time         | 2023-06-19 12:01:34
current_job_ttl_expire         | 2023-06-19 11:57:51
current_job_state              | <null>
current_job_status             | running
current_job_status_update_time | 2023-06-19 12:01:34
1 row in set
Time: 0.002s

Notice that the current_job_start_time is override with a new value.

2. What did you expect to see? (Required)

current_job_start_time should not be changed for a same job

3. What did you see instead (Required)

current_job_start_time changed

4. What is your TiDB version? (Required)

master