MDEV-16869 String functions don't respect character set of JSON_VALUE.

Item_func_json_value::val_str() produced string of wrong charset.

mysql-test/r/func_json.result

@@ -766,3 +766,15 @@ DROP TABLE t1;
 select json_array(1,uuid(),compress(5.140264e+307));
 json_array(1,uuid(),compress(5.140264e+307))
 NULL
+#
+# MDEV-16869 String functions don't respect character set of JSON_VALUE.
+#
+create table t1(json_col TEXT) DEFAULT CHARSET=latin1;
+insert into t1 values (_latin1 X'7B226B657931223A2253EC227D');
+select JSON_VALUE(json_col, '$.key1')= _latin1 X'53EC' from t1;
+JSON_VALUE(json_col, '$.key1')= _latin1 X'53EC'
+1
+select REPLACE(JSON_VALUE(json_col, '$.key1'), 'null', '') = _latin1 X'53EC' from t1;
+REPLACE(JSON_VALUE(json_col, '$.key1'), 'null', '') = _latin1 X'53EC'
+1
+drop table t1;

mysql-test/t/func_json.test

@@ -429,3 +429,14 @@ DROP TABLE t1;
 
 select json_array(1,uuid(),compress(5.140264e+307));
 
+
+--echo #
+--echo # MDEV-16869 String functions don't respect character set of JSON_VALUE.
+--echo #
+
+create table t1(json_col TEXT) DEFAULT CHARSET=latin1;
+insert into t1 values (_latin1 X'7B226B657931223A2253EC227D');
+select JSON_VALUE(json_col, '$.key1')= _latin1 X'53EC' from t1;
+select REPLACE(JSON_VALUE(json_col, '$.key1'), 'null', '') = _latin1 X'53EC' from t1;
+drop table t1;
+

sql/item_jsonfunc.cc

@@ -479,7 +479,7 @@ String *Item_func_json_value::val_str(String *str)
                   (const uchar *) js->ptr() + js->length());
 
   str->length(0);
-  str->set_charset(&my_charset_utf8mb4_bin);
+  str->set_charset(collation.collation);
 
   path.cur_step= path.p.steps;
 continue_search: