MariaDB [test]> CREATE TABLE t1 (f1 INT NOT NULL, f2 INT NOT NULL) ENGINE=innodb;

Query OK, 0 rows affected (0.09 sec)

MariaDB [test]> SET debug_dbug='d,innodb_alter_commit_crash_before_commit';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> INSERT INTO t1 VALUES(1, 1);

Query OK, 1 row affected (0.16 sec)

MariaDB [test]> INSERT INTO t1 VALUES(2, 2);

Query OK, 1 row affected (0.08 sec)

MariaDB [test]> ALTER TABLE t1 ADD PRIMARY KEY (f2, f1);

ERROR 2013 (HY000): Lost connection to MySQL server during query

.. restart server & reconnect.. 

MariaDB [test]> ALTER TABLE t1 ADD PRIMARY KEY (f2, f1);

Query OK, 0 rows affected (0.20 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [test]> SHOW CREATE TABLE t1;

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

| Table | Create Table                                                                                                                           |

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

| t1    | CREATE TABLE `t1` (

  `f1` int(11) NOT NULL,

  `f2` int(11) NOT NULL,

  PRIMARY KEY (`f2`,`f1`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

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

1 row in set (0.03 sec)

MariaDB [test]> SELECT * FROM INFORMATION_SCHEMA.INNODB_SYS_TABLES;

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

| TABLE_ID | NAME                      | FLAG | N_COLS | SPACE | FILE_FORMAT | ROW_FORMAT | ZIP_PAGE_SIZE |

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

...

|       19 | test/#sql-ib18-1827491244 |    1 |      5 |     5 | Antelope    | Compact    |             0 |

|       20 | test/t1                   |    1 |      5 |     6 | Antelope    | Compact    |             0 |

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

9 rows in set (0.00 sec)

MariaDB [test]> \! ls data1/test

#sql-426c_3.frm  #sql-ib18-1827491244.ibd  t1.frm  t1.ibd