Bug#57687 crash when reporting duplicate group_key error and utf8

Fixing DoS regression problem.

Using "key_part->fieldnr - 1" to access the desired field
is only correct in real INSERT queries.
In case of inserting records into a temporary table
when performing GROUP BY queries this expression does not work.

Fix: Instead of accessing field_length and comparing it
to key_part->length, there is an easier way to check if
we're dealing with a prefix key: check key_part_flag against
HA_PART_KEY_SEG flag.

mysql-test/r/ctype_utf8.result

@@ -4885,5 +4885,16 @@ maketime(`a`,`a`,`a`)
 DROP TABLE t1;
 SET sql_mode=default;
 #
+# Bug#57687 crash when reporting duplicate group_key error and utf8
+# Make sure to modify this when Bug#58081 is fixed.
+#
+SET NAMES utf8;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (0), (0), (1), (0), (0);
+SELECT COUNT(*) FROM t1, t1 t2 
+GROUP BY INSERT('', t2.a, t1.a, (@@global.max_binlog_size));
+ERROR 23000: Duplicate entry '107374182410737418241' for key 'group_key'
+DROP TABLE t1;
+#
 # End of 5.5 tests
 #

mysql-test/t/ctype_utf8.test

@@ -1529,6 +1529,18 @@ DROP TABLE t1, t2;
 SET NAMES utf8;
 --source include/ctype_numconv.inc
 
+--echo #
+--echo # Bug#57687 crash when reporting duplicate group_key error and utf8
+--echo # Make sure to modify this when Bug#58081 is fixed.
+--echo #
+SET NAMES utf8;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (0), (0), (1), (0), (0);
+--error ER_DUP_ENTRY
+SELECT COUNT(*) FROM t1, t1 t2 
+GROUP BY INSERT('', t2.a, t1.a, (@@global.max_binlog_size));
+DROP TABLE t1;
+
 
 --echo #
 --echo # End of 5.5 tests

sql/key.cc

@@ -364,9 +364,7 @@ void key_unpack(String *to,TABLE *table,uint idx)
         while (tmp_end > tmp.ptr() && !*--tmp_end) ;
         tmp.length(tmp_end - tmp.ptr() + 1);
       }
-      if (cs->mbmaxlen > 1 &&
-          table->field[key_part->fieldnr - 1]->field_length !=
-          key_part->length)
+      if (cs->mbmaxlen > 1 && (key_part->key_part_flag & HA_PART_KEY_SEG))
       {
         /* 
           Prefix key, multi-byte charset.