[#MDEV-25428] String unexpectedly truncated by JSON

This is a query from json_no_table.test. The test runs it with utf8, and it works as it says. However, with latin1 it fails to avoid truncation:

SET NAMES 'latin1';

SELECT * FROM (SELECT JSON_UNQUOTE("This is a string that should not be truncated") AS field1) AS DERIVED_TABLE;

10.2 c6d0531c
MariaDB [test]> SET NAMES 'latin1';
Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> SELECT * FROM (SELECT JSON_UNQUOTE("This is a string that should not be truncated") AS field1) AS DERIVED_TABLE;
+-----------------+
\| field1 \|
+-----------------+
\| This is a strin \|
+-----------------+
1 row in set (0.000 sec)

MariaDB [test]> SET NAMES utf8;
Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> SELECT * FROM (SELECT JSON_UNQUOTE("This is a string that should not be truncated") AS field1) AS DERIVED_TABLE;
+-----------------------------------------------+
\| field1 \|
+-----------------------------------------------+
\| This is a string that should not be truncated \|
+-----------------------------------------------+
1 row in set (0.001 sec)

Reproducible on 10.2-10.6.

MySQL returns the full string regardless the character set:

MySQL 8.0.23
MySQL [test]> SET NAMES 'latin1';
Query OK, 0 rows affected (0.000 sec)

MySQL [test]> SELECT * FROM (SELECT JSON_UNQUOTE("This is a string that should not be truncated") AS field1) AS DERIVED_TABLE;
+-----------------------------------------------+
\| field1 \|
+-----------------------------------------------+
\| This is a string that should not be truncated \|
+-----------------------------------------------+
1 row in set (0.001 sec)

MySQL [test]> SET NAMES utf8;
Query OK, 0 rows affected, 1 warning (0.000 sec)

MySQL [test]> SELECT * FROM (SELECT JSON_UNQUOTE("This is a string that should not be truncated") AS field1) AS DERIVED_TABLE;
+-----------------------------------------------+
\| field1 \|
+-----------------------------------------------+
\| This is a string that should not be truncated \|
+-----------------------------------------------+
1 row in set (0.001 sec)

[MDEV-25428] String unexpectedly truncated by JSON_UNQUOTE depending on charset Created: 2021-04-15 Updated: 2023-04-27
Status:	Open
Project:	MariaDB Server
Component/s:	Character Sets, JSON
Affects Version/s:	10.2, 10.3, 10.4, 10.5, 10.6
Fix Version/s:	10.4, 10.5