perl ./mtr mariabackup.lock_ddl_per_table --mem --mysqld=--alter-algorithm=copy

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

| Id | User        | Host            | db   | Command | Time | State                           | Info                                              | Progress |

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

|  1 | system user |                 | NULL | Daemon  | NULL | InnoDB purge coordinator        | NULL                                              |    0.000 |

|  4 | system user |                 | NULL | Daemon  | NULL | InnoDB purge worker             | NULL                                              |    0.000 |

|  2 | system user |                 | NULL | Daemon  | NULL | InnoDB purge worker             | NULL                                              |    0.000 |

|  3 | system user |                 | NULL | Daemon  | NULL | InnoDB purge worker             | NULL                                              |    0.000 |

|  5 | system user |                 | NULL | Daemon  | NULL | InnoDB shutdown handler         | NULL                                              |    0.000 |

|  9 | root        | localhost       | test | Sleep   |   52 |                                 | NULL                                              |    0.000 |

| 10 | root        | localhost       | NULL | Query   |   51 | Waiting for backup lock         | BACKUP STAGE BLOCK_COMMIT                         |    0.000 |

| 11 | root        | localhost       | NULL | Sleep   |   52 |                                 | NULL                                              |    0.000 |

| 12 | root        | localhost       | NULL | Query   |   52 | Waiting for table metadata lock | ALTER TABLE test.t ADD COLUMN mdl_lock_column int |    0.000 |

| 13 | root        | localhost:33124 | NULL | Query   |    0 | Init                            | show processlist                                  |    0.000 |

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

MariaDB [(none)]> select * from information_schema.metadata_lock_info;

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

| THREAD_ID | LOCK_MODE               | LOCK_DURATION | LOCK_TYPE            | TABLE_SCHEMA | TABLE_NAME           |

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

|        12 | MDL_BACKUP_DDL          | NULL          | Backup lock          |              |                      |

|        10 | MDL_BACKUP_WAIT_FLUSH   | NULL          | Backup lock          |              |                      |

|        11 | MDL_SHARED_READ         | NULL          | Table metadata lock  | test         | bobby`tables         |

|        11 | MDL_SHARED_READ         | NULL          | Table metadata lock  | test         | t                    |

|        12 | MDL_SHARED_NO_WRITE     | NULL          | Table metadata lock  | test         | t                    |

|        11 | MDL_SHARED_READ         | NULL          | Table metadata lock  | mysql        | transaction_registry |

|        11 | MDL_SHARED_READ         | NULL          | Table metadata lock  | mysql        | innodb_table_stats   |

|        11 | MDL_SHARED_READ         | NULL          | Table metadata lock  | mysql        | innodb_index_stats   |

|        12 | MDL_INTENTION_EXCLUSIVE | NULL          | Schema metadata lock | test         |                      |

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