overlocking next-record in REPEATABLE-READ with delete from where x <= num

MariaDB [drupal_test]> create table x ( x bigint unsigned not null auto_increment, PRIMARY KEY x(x) );

Query OK, 0 rows affected (0.02 sec)

MariaDB [drupal_test]> select @@tx_isolation;

+-----------------+

| @@tx_isolation  |

+-----------------+

| REPEATABLE-READ |

+-----------------+

1 row in set (0.00 sec)

MariaDB [drupal_test]> insert into x values (1),(2),(3);

Query OK, 3 rows affected (0.00 sec)

Records: 3  Duplicates: 0  Warnings: 0

MariaDB [drupal_test]> begin

    -> ;

Query OK, 0 rows affected (0.00 sec)

MariaDB [drupal_test]> delete from x where x <= 3;

Query OK, 3 rows affected (0.00 sec)

---TRANSACTION 290E30, ACTIVE 12 sec

2 lock struct(s), heap size 376, 4 row lock(s), undo log entries 3

MySQL thread id 7437, OS thread handle 0x7f37307f5700, query id 3298653 localhost debian-sys-maint sleeping

TABLE LOCK table `drupal_test`.`x` trx id 290E30 lock mode IX

RECORD LOCKS space id 82547 page no 3 n bits 72 index `PRIMARY` of table `drupal_test`.`x` trx id 290E30 lock_mode X

MariaDB [drupal_test]> insert into x values (DEFAULT);

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

MariaDB [drupal_test]> set tx_isolation='READ-COMMITTED'; begin; delete from x where x <= 3;

Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

Query OK, 3 rows affected (0.00 sec)

---TRANSACTION 290E33, ACTIVE 44 sec

2 lock struct(s), heap size 376, 3 row lock(s), undo log entries 3

MySQL thread id 7437, OS thread handle 0x7f37307f5700, query id 3298661 localhost debian-sys-maint sleeping

TABLE LOCK table `drupal_test`.`x` trx id 290E33 lock mode IX

RECORD LOCKS space id 82547 page no 3 n bits 72 index `PRIMARY` of table `drupal_test`.`x` trx id 290E33 lock_mode X locks rec but not gap

MariaDB [drupal_test]> insert into x values (DEFAULT);

Query OK, 1 row affected (0.00 sec)