Bug #30912972 ASSERTION `KEYLEN == M_START_KEY.LENGTH' FAILED

- A bug in RefOrNullIterator::Read where we called
ha_index_next_same() with a key_part_map value instead of
using keylen triggered an assertion on partitioned tables
containing a null value.
- Fixed the issue by passing m_ref->key_length to the
index_next_same method.
- Added MTR test case that triggers this assertion without
the fix.

RB#24008 approved by Steinar H. Gunderson <steinar.gunderson@oracle.com>

mysql-test/r/subquery_bugs.result

@@ -1485,3 +1485,17 @@ RTRIM(table1.col_tinyint_key) IS NOT NULL;
 field1
 0
 DROP TABLE t1, t2, t3;
+#
+# Bug #30912972 ASSERTION KEYLEN == M_START_KEY.LENGTH FAILED|PARTITION_HANDLER.CC
+#
+CREATE TABLE t1 (
+col_int_key bigint DEFAULT NULL,
+KEY(col_int_key)
+) PARTITION BY KEY(col_int_key) PARTITIONS 10;
+INSERT INTO t1 VALUES
+(NULL);
+SELECT 42
+WHERE 11 NOT IN
+(SELECT col_int_key FROM t1);
+42
+DROP TABLE t1;

mysql-test/t/subquery_bugs.test

@@ -1156,3 +1156,21 @@ eval explain $query;
 eval $query;
 
 DROP TABLE t1, t2, t3;
+
+--echo #
+--echo # Bug #30912972 ASSERTION KEYLEN == M_START_KEY.LENGTH FAILED|PARTITION_HANDLER.CC
+--echo #
+
+CREATE TABLE t1 (
+  col_int_key bigint DEFAULT NULL,
+  KEY(col_int_key)
+) PARTITION BY KEY(col_int_key) PARTITIONS 10;
+
+INSERT INTO t1 VALUES
+(NULL);
+
+SELECT 42
+WHERE 11 NOT IN
+(SELECT col_int_key FROM t1);
+
+DROP TABLE t1;

sql/sql_executor.cc

@@ -4165,7 +4165,7 @@ int RefOrNullIterator::Read() {
         HA_READ_KEY_EXACT);
   } else {
     error = table()->file->ha_index_next_same(
-        table()->record[0], key_buff_and_map.first, key_buff_and_map.second);
+        table()->record[0], key_buff_and_map.first, m_ref->key_length);
   }
 
   if (error == 0) {