MDEV-24784 JSON_ARRAYAGG charset issue

set the proper charset for a temporary result

mysql-test/main/func_json.result

@@ -1646,5 +1646,16 @@ SELECT JSON_OBJECTAGG('\\', 1);
 JSON_OBJECTAGG('\\', 1)
 {"\\":1}
 #
+# MDEV-24784 JSON_ARRAYAGG charset issue
+#
+set names utf8;
+select json_arrayagg('ä'), json_objectagg(1, 'ä');
+json_arrayagg('ä')	json_objectagg(1, 'ä')
+["ä"]	{"1":"ä"}
+set names latin1;
+select json_arrayagg('ä'), json_objectagg(1, 'ä');
+json_arrayagg('ä')	json_objectagg(1, 'ä')
+["ä"]	{"1":"ä"}
+#
 # End of 10.5 tests
 #

mysql-test/main/func_json.test

@@ -1109,6 +1109,15 @@ SELECT JSON_OBJECTAGG('\"', 1);
 SELECT JSON_OBJECTAGG('\\', 1);
 
 --echo #
---echo # End of 10.5 tests
+--echo # MDEV-24784 JSON_ARRAYAGG charset issue
 --echo #
+--disable_service_connection
+set names utf8;
+select json_arrayagg('ä'), json_objectagg(1, 'ä');
+set names latin1;
+select json_arrayagg('ä'), json_objectagg(1, 'ä');
+--enable_service_connection
 
+--echo #
+--echo # End of 10.5 tests
+--echo #

sql/item_jsonfunc.cc

@@ -3907,6 +3907,13 @@ int Arg_comparator::compare_e_json_str_basic(Item *j, Item *s)
   return MY_TEST(sortcmp(res1, res2, compare_collation()) == 0);
 }
 
+bool Item_func_json_arrayagg::fix_fields(THD *thd, Item **ref)
+{
+  bool res= Item_func_group_concat::fix_fields(thd, ref);
+  m_tmp_json.set_charset(collation.collation);
+  return res;
+}
+
 
 String *Item_func_json_arrayagg::get_str_from_item(Item *i, String *tmp)
 {

sql/item_jsonfunc.h

@@ -570,6 +570,7 @@ class Item_func_json_arrayagg : public Item_func_group_concat
   }
 
   const char *func_name() const override { return "json_arrayagg("; }
+  bool fix_fields(THD *thd, Item **ref) override;
   enum Sumfunctype sum_func() const override { return JSON_ARRAYAGG_FUNC; }
 
   String* val_str(String *str) override;