MDEV-34417 Wrong result set with utf8mb4_danish_ci and BNLH join

There were erroneous calls for charpos() in key_hashnr() and key_buf_cmp().
These functions are never called with prefix segments.

The charpos() calls were wrong. Before the change BNHL joins
- could return wrong result sets, as reported in MDEV-34417
- were extremely slow for multi-byte character sets, because
  the hash was calculated on string prefixes, which increased
  the amount of collisions drastically.

This patch fixes the wrong result set as reported in MDEV-34417,
as well as (partially) the performance problem reported in MDEV-34352.

mysql-test/main/ctype_uca.result

@@ -15370,3 +15370,39 @@ DROP TABLE t1;
 #
 # End of MariaDB-10.2 tests
 #
+#
+# Start of 10.5 tests
+#
+#
+# MDEV-34417 Wrong result set with utf8mb4_danish_ci and BNLH join
+#
+CREATE TABLE t1 (a VARCHAR(500) CHARACTER SET utf8mb4 COLLATE utf8mb4_danish_ci);
+INSERT INTO t1 VALUES ('aaaa'),('åå');
+SELECT * FROM t1 WHERE a='aaaa';
+a
+aaaa
+åå
+SET join_cache_level=1;
+SELECT * FROM t1 NATURAL JOIN t1 t2;
+a
+aaaa
+åå
+aaaa
+åå
+# Expect a BNHL join
+SET join_cache_level=3;
+EXPLAIN SELECT * FROM t1 NATURAL JOIN t1 t2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+1	SIMPLE	t2	hash_ALL	NULL	#hash#$hj	2003	test.t1.a	2	Using where; Using join buffer (flat, BNLH join)
+SELECT * FROM t1 NATURAL JOIN t1 t2;
+a
+aaaa
+åå
+aaaa
+åå
+DROP TABLE t1;
+SET join_cache_level=DEFAULT;
+#
+# End of 10.5 tests
+#

mysql-test/main/ctype_uca.test

@@ -696,3 +696,32 @@ DROP TABLE t1;
 --echo #
 --echo # End of MariaDB-10.2 tests
 --echo #
+
+
+--echo #
+--echo # Start of 10.5 tests
+--echo #
+
+--echo #
+--echo # MDEV-34417 Wrong result set with utf8mb4_danish_ci and BNLH join
+--echo #
+
+CREATE TABLE t1 (a VARCHAR(500) CHARACTER SET utf8mb4 COLLATE utf8mb4_danish_ci);
+INSERT INTO t1 VALUES ('aaaa'),('åå');
+SELECT * FROM t1 WHERE a='aaaa';
+
+SET join_cache_level=1;
+SELECT * FROM t1 NATURAL JOIN t1 t2;
+
+--echo # Expect a BNHL join
+SET join_cache_level=3;
+EXPLAIN SELECT * FROM t1 NATURAL JOIN t1 t2;
+SELECT * FROM t1 NATURAL JOIN t1 t2;
+
+DROP TABLE t1;
+SET join_cache_level=DEFAULT;
+
+
+--echo #
+--echo # End of 10.5 tests
+--echo #

sql/key.cc

@@ -754,13 +754,11 @@ ulong key_hashnr(KEY *key_info, uint used_key_parts, const uchar *key)
 
     if (is_string)
     {
-      if (cs->mbmaxlen > 1)
-      {
-        size_t char_length= cs->charpos(pos + pack_length,
-                                        pos + pack_length + length,
-                                        length / cs->mbmaxlen);
-        set_if_smaller(length, char_length);
-      }
+      /*
+        Prefix keys are not possible in BNLH joins.
+        Use the whole string to calculate the hash.
+      */
+      DBUG_ASSERT((key_part->key_part_flag & HA_PART_KEY_SEG) == 0);
       cs->hash_sort(pos+pack_length, length, &nr, &nr2);
       key+= pack_length;
     }
@@ -864,25 +862,13 @@ bool key_buf_cmp(KEY *key_info, uint used_key_parts,
     if (is_string)
     {
       /*
-        Compare the strings taking into account length in characters
-        and collation
+        Prefix keys are not possible in BNLH joins.
+        Compare whole strings.
       */
-      size_t byte_len1= length1, byte_len2= length2;
-      if (cs->mbmaxlen > 1)
-      {
-        size_t char_length1= cs->charpos(pos1 + pack_length,
-                                         pos1 + pack_length + length1,
-                                         length1 / cs->mbmaxlen);
-        size_t char_length2= cs->charpos(pos2 + pack_length,
-                                         pos2 + pack_length + length2,
-                                         length2 / cs->mbmaxlen);
-        set_if_smaller(length1, char_length1);
-        set_if_smaller(length2, char_length2);
-      }
-      if (length1 != length2 ||
-          cs->strnncollsp(pos1 + pack_length, byte_len1,
-                          pos2 + pack_length, byte_len2))
-        return TRUE;
+      DBUG_ASSERT((key_part->key_part_flag & HA_PART_KEY_SEG) == 0);
+      if (cs->strnncollsp(pos1 + pack_length, length1,
+                          pos2 + pack_length, length2))
+        return true;
       key1+= pack_length; key2+= pack_length;
     }
     else