MDEV-37835 mysqli silently trims each json_arrayagg result to modulo 64KB

Item_string was always using Type_handler_varchar, even though
varchar only has 2 bytes for the length and Item_string can be longer.

Author: vuvova

mysql-test/main/func_str.result

@@ -5628,4 +5628,10 @@ SET sql_mode=DEFAULT;
 select locate(1,2,NULL);
 locate(1,2,NULL)
 NULL
+#
+# MDEV-37835 mysqli silently trims each json_arrayagg result to modulo 64KB
+#
+select group_concat(v) from ( select '$a' as v union all select '$b' as v) t;
+group_concat(v)
+a...aaaaa,b...bbbbb
 # End of 10.11 tests

mysql-test/main/func_str.test

@@ -2577,4 +2577,12 @@ SET sql_mode=DEFAULT;
 --echo #
 select locate(1,2,NULL);
 
+--echo #
+--echo # MDEV-37835 mysqli silently trims each json_arrayagg result to modulo 64KB
+--echo #
+let $a=`select repeat('a', 65540)`;
+let $b=`select repeat('b', 65540)`;
+replace_regex /a{65535}/a.../ /b{65535}/b.../;
+evalp select group_concat(v) from ( select '$a' as v union all select '$b' as v) t;
+
 --echo # End of 10.11 tests

sql/item.h

@@ -4896,7 +4896,7 @@ class Item_string :public Item_literal
   }
   int save_in_field(Field *field, bool no_conversions) override;
   const Type_handler *type_handler() const override
-  { return &type_handler_varchar; }
+  { return  Type_handler::string_type_handler(max_length); }
   Item *clone_item(THD *thd) const override;
   Item *safe_charset_converter(THD *thd, CHARSET_INFO *tocs) override
   {