fixed MDEV-568: Wrong result for a hash index look-up if the index is unique and the key is NULL

Check ability of index to be NULL as it made in MyISAM. UNIQUE with NULL could have several NULL entries so we have to continue even if ve have found a row.

mysql-test/r/heap_hash.result

@@ -382,3 +382,22 @@ INSERT INTO t1 VALUES('A ', 'A ');
 ERROR 23000: Duplicate entry 'A -A ' for key 'key1'
 DROP TABLE t1;
 End of 5.0 tests
+#
+# MDEV-568 (AKA LP BUG#1007981, AKA MySQL bug#44771)
+# Wrong result for a hash index look-up if the index is unique and
+# the key is NULL
+#
+CREATE TABLE t1 ( pk INT PRIMARY KEY, val INT, UNIQUE KEY USING HASH(val)) ENGINE=MEMORY;
+INSERT INTO t1 VALUES (1, NULL);
+INSERT INTO t1 VALUES (2, NULL);
+INSERT INTO t1 VALUES (3, 1);
+INSERT INTO t1 VALUES (4, NULL);
+EXPLAIN SELECT * FROM t1 WHERE val IS NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ref	val	val	5	const	1	Using where
+SELECT * FROM t1 WHERE val IS NULL;
+pk	val
+4	NULL
+2	NULL
+1	NULL
+drop table t1;

mysql-test/t/heap_hash.test

@@ -284,3 +284,18 @@ INSERT INTO t1 VALUES('A ', 'A ');
 DROP TABLE t1;
 
 --echo End of 5.0 tests
+
+--echo #
+--echo # MDEV-568 (AKA LP BUG#1007981, AKA MySQL bug#44771)
+--echo # Wrong result for a hash index look-up if the index is unique and
+--echo # the key is NULL
+--echo #
+CREATE TABLE t1 ( pk INT PRIMARY KEY, val INT, UNIQUE KEY USING HASH(val)) ENGINE=MEMORY;
+
+INSERT INTO t1 VALUES (1, NULL);
+INSERT INTO t1 VALUES (2, NULL);
+INSERT INTO t1 VALUES (3, 1);
+INSERT INTO t1 VALUES (4, NULL);
+EXPLAIN SELECT * FROM t1 WHERE val IS NULL;
+SELECT * FROM t1 WHERE val IS NULL;
+drop table t1;

storage/heap/hp_rkey.c

@@ -63,7 +63,7 @@ int heap_rkey(HP_INFO *info, uchar *record, int inx, const uchar *key,
       info->update= 0;
       DBUG_RETURN(my_errno);
     }
-    if (!(keyinfo->flag & HA_NOSAME))
+    if ((keyinfo->flag & (HA_NOSAME | HA_NULL_PART_KEY)) != HA_NOSAME)
       memcpy(info->lastkey, key, (size_t) keyinfo->length);
   }
   memcpy(record, pos, (size_t) share->reclength);