builtin func char() has different result between MySQL when value is illegal UTF8 character

[Defined2014]

Bug Report

builtin func char() has different result between MySQL when value is illegal UTF8 character

1. Minimal reproduce step (Required)

select char(123456 using utf8);
show warnings;

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

mysql> select char(123456 using utf8);
+-------------------------+
| char(123456 using utf8) |
+-------------------------+
| NULL                    |
+-------------------------+
1 row in set, 2 warnings (0.00 sec)
mysql> show warnings;
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                     |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 3719 | 'utf8' is currently an alias for the character set UTF8MB3, but will be an alias for UTF8MB4 in a future release. Please consider using UTF8MB4 in order to be unambiguous. |
| Warning | 1300 | Invalid utf8mb3 character string: 'E240'                                                                                                                                    |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

3. What did you see instead (Required)

mysql> select char(123456 using utf8);
+-------------------------+
| char(123456 using utf8) |
+-------------------------+
| �@                      |
+-------------------------+
1 row in set (0.01 sec)
mysql> show warnings;
Empty set (0.00 sec)

4. What is your TiDB version? (Required)

mysql> SELECT tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                                             |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.2.0-alpha-2186-g2bdebedfc
Edition: Community
Git Commit Hash: 2bdebedfc042e340c7826d0ac96f635f3949a8b3
Git Branch: master
UTC Build Time: 2021-11-11 05:47:41
GoVersion: go1.16.6
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

[Defined2014]

We didn't check the param for char() is UTF8 character or not. And we should check it. The convert also has same problem. Seems if builtin functions support using utf8, they will have same problem.

code

drop table if exists t;
create table t (a binary(20));
insert into t values (0x1e240);
select convert(t.a using utf8) from t;

result which should be NULL

mysql> select convert(t.a using utf8) from t;
+-------------------------+
| convert(t.a using utf8) |
+-------------------------+
| �@                      |
+-------------------------+
1 row in set (0.00 sec)

[Defined2014]

/assign

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.