@@hostname always shows localhost

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> select @@hostname;
+------------+
| @@hostname |
+------------+
| localhost  |
+------------+
1 row in set (0.00 sec)

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

mysql> select @@hostname;
+---------------+
| @@hostname    |
+---------------+
| 192.168.0.101 |
+---------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

mysql> select @@hostname;
+------------+
| @@hostname |
+------------+
| localhost  |
+------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

master

The reason: in #24412, there is a little mistake:

if hostname, err := os.Hostname(); err != nil {        // should be `err == nil`
	variable.SetSysVar(variable.Hostname, hostname)
}