in v5.0.2, tidb_gc_scan_lock_mode default value is different in scratch installed server and upgrade server

[seiya-annie]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

in scratch install server, the default value for tidb_gc_scan_lock_mode is PHYSICAL
but in the upgraded server(don't change this param before upgrade), the default value is LEGACY

MySQL [(none)]>

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

3. What did you see instead (Required)

MySQL [(none)]> show variables like '%tidb_gc_scan_lock_mode%'
-> ;
+------------------------+----------+
| Variable_name | Value |
+------------------------+----------+
| tidb_gc_scan_lock_mode | PHYSICAL |
+------------------------+----------+
1 row in set (0.38 sec)

[tidb@centos76_vm 501]$ diff 501_systemvar.txt 502_systemvar.txt
548c548
< tidb_gc_scan_lock_mode PHYSICAL

tidb_gc_scan_lock_mode LEGACY
650c650

4. What is your TiDB version? (Required)

| Release Version: v5.0.2
Edition: Community
Git Commit Hash: f610f8b
Git Branch: heads/refs/tags/v5.0.2
UTC Build Time: 2021-06-02 02:00:45
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |

[MyonKeminta]

The problem is, on a new-created cluster that haven't do GC, the entry tikv_gc_scan_lock_mode is no written into the mysql.tidb table, and querying the tidb_gc_scan_lock_mode returns the wrong result "PHYSICAL", but the actual behavior is still "LEGACY".

[ti-srebot]

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

Bug

1. Root Cause Analysis (RCA) (optional)

On a new-created cluster that has never do GC, the entry tikv_gc_scan_lock_mode is no written into the mysql.tidb table, and querying the tidb_gc_scan_lock_mode returns the wrong result "PHYSICAL", but the actual behavior is still "LEGACY". The reason is that the global variable loaded the default value from a different place.

2. Symptom (optional)

Create a new cluster and immediately execute select @@global.tidb_gc_scan_lock_mode, it returns "PHYSICAL", instead of the actual mode "LEGACY"

3. All Trigger Conditions (optional)

On a new created cluster (within 10 minutes since starting cluster)

4. Workaround (optional)

No.

5. Affected versions

[v5.0.0:v5.0.2]

6. Fixed versions

[v5.0.3]

[ti-srebot]

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