role (RBAC) related code does not handle username or host with `@` correctly

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> create user u1;
Query OK, 0 rows affected (0.01 sec)

mysql> grant 'aa@bb' to u1;
Query OK, 0 rows affected (0.01 sec)

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

mysql> show grants for u1;
+--------------------------------+
| Grants for u1@%                |
+--------------------------------+
| GRANT USAGE ON *.* TO `u1`@`%` |
| GRANT `aa@bb`@`%` TO `u1`@`%`  |
+--------------------------------+
2 rows in set (0.00 sec)

3. What did you see instead (Required)

mysql> show grants for u1;
+--------------------------------+
| Grants for u1@%                |
+--------------------------------+
| GRANT USAGE ON *.* TO 'u1'@'%' |
| GRANT 'aa'@'bb' TO 'u1'@'%'    |
+--------------------------------+
2 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

master 27f8ff3
This should affect all the versions since role feature was introduced.