The GC does not work if the configuration of GC is wrong

[Win-Man]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

update mysql.tidb set VARIABLE_VALUE="10min" where VARIABLE_NAME="tikv_gc_life_time";
update mysql.tidb set VARIABLE_VALUE="10min" where VARIABLE_NAME="tikv_gc_run_interval";

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

The GC can work fine.
Another solution is returning an error if the util is wrong, so I suggest to update the config of GC using SET instead of update. The mysql.tidb should be a read-only table.

3. What did you see instead (Required)

The GC did not work.

4. What is your TiDB version? (Required)

MySQL [(none)]> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.7
Edition: Community
Git Commit Hash: ed939f3f11599b5a38352c5c160c917df3ebf3eb
Git Branch: heads/refs/tags/v4.0.7
UTC Build Time: 2020-09-29 06:56:48
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[SunRunAway]

@cfzjywxk PTAL

[ti-srebot]

Please edit this comment or add a new comment to complete the following information

Not a bug

1. Remove the 'type/bug' label
2. Add notes to indicate why it is not a bug

I am changing this to "Not a bug" for the purposes of the bug system. The logic is because the issue is caused by invalid configuration and it is really hard for the server to act correctly with invalid data.

That is not to say it is not an issue, it has been addressed by moving the configuration to sysvars which do validate. The problem is the "bug" element can not truly be closed, since it is still possible for users to modify data using the old table interface.