Wrong result of select @@session.tx_read_only

[shenli]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

mysql> select @@session.tx_read_only;
+------------------------+
| @@session.tx_read_only |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.00 sec)

mysql> set session transaction read only;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@session.tx_read_only;
+------------------------+
| @@session.tx_read_only |
+------------------------+
|                      1 |
+------------------------+
1 row in set (0.00 sec)

mysql> 

2. What did you expect to see?
   @@session.tx_read_only should be 1 after executing set session transaction read only;

3. What did you see instead?
   @@session.tx_read_only still be 0

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   All tidb-server versions could reproduce this issue.