MDEV-13097 Online alter of a partitioned MyISAM table with auto_increment

MyISAM only allows online alter if autoincrement didn't change.
MyISAM detects that by comparing new autoinc value from create_info,
with the old one, stored in MYI. But in partitioned tables,
create_info->auto_increment_value is for the whole table, max of
autoinc values of individual MYI partitions. So *some* MYI partitions
will inevitably think that alter table changes auto_increment value
and will deny online alter.

Fix: only compare autoinc values, if the user has used AUTO_INCREMENT
in the ALTER TABLE statement.

mysql-test/r/partition_alter.result

@@ -92,3 +92,23 @@ t1	CREATE TABLE `t1` (
  PARTITION p2 VALUES LESS THAN ('2020-10-19') ENGINE = MyISAM) */
 insert t1 values (2, '2020-01-03', 20);
 drop table t1;
+create table t1 (id_1 int auto_increment, id_2 int, id_3 int, d1 date, dt1 datetime default current_timestamp, dt2 datetime default current_timestamp on update current_timestamp, primary key (id_2, id_3), key(id_1)) partition by hash(id_2) partitions 3 (partition p01, partition p02, partition p03);
+insert into t1 values(0, 1, 1, NULL, now(), now());
+alter online table t1 delay_key_write=1;
+show create table t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `id_1` int(11) NOT NULL AUTO_INCREMENT,
+  `id_2` int(11) NOT NULL,
+  `id_3` int(11) NOT NULL,
+  `d1` date DEFAULT NULL,
+  `dt1` datetime DEFAULT CURRENT_TIMESTAMP,
+  `dt2` datetime DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  PRIMARY KEY (`id_2`,`id_3`),
+  KEY `id_1` (`id_1`)
+) ENGINE=MyISAM AUTO_INCREMENT=2 DEFAULT CHARSET=latin1 DELAY_KEY_WRITE=1
+/*!50100 PARTITION BY HASH (id_2)
+(PARTITION p01 ENGINE = MyISAM,
+ PARTITION p02 ENGINE = MyISAM,
+ PARTITION p03 ENGINE = MyISAM) */
+drop table t1;

mysql-test/t/partition_alter.test

@@ -103,3 +103,12 @@ show create table t1;
 insert t1 values (2, '2020-01-03', 20);
 drop table t1;
 --list_files $datadir/test
+
+#
+# MDEV-13097 Online alter of a partitioned MyISAM table with auto_increment
+#
+create table t1 (id_1 int auto_increment, id_2 int, id_3 int, d1 date, dt1 datetime default current_timestamp, dt2 datetime default current_timestamp on update current_timestamp, primary key (id_2, id_3), key(id_1)) partition by hash(id_2) partitions 3 (partition p01, partition p02, partition p03);
+insert into t1 values(0, 1, 1, NULL, now(), now());
+alter online table t1 delay_key_write=1;
+show create table t1;
+drop table t1;

storage/myisam/ha_myisam.cc

@@ -2266,7 +2266,8 @@ bool ha_myisam::check_if_incompatible_data(HA_CREATE_INFO *create_info,
 {
   uint options= table->s->db_options_in_use;
 
-  if (create_info->auto_increment_value != stats.auto_increment_value ||
+  if ((create_info->used_fields & HA_CREATE_USED_AUTO &&
+       create_info->auto_increment_value != stats.auto_increment_value) ||
       create_info->data_file_name != data_file_name ||
       create_info->index_file_name != index_file_name ||
       table_changes == IS_EQUAL_NO ||