3 way lock : ALTER, MDL, BACKUP STAGE BLOCK_DDL

Here is an unexpected wait with "Backup locks" (similar to the one described in MDEV-15636, FTWRL-related, but Backup locks were supposed to be better than FTWRL, and be instant in most cases) Backup locks was supposed not to wait for SELECTs, or ALTER in progress, but in this case, it does. Moreover, in the case below, it waits until the end of a transaction, while no DDL, DML or SELECT is currently running (even if example has a DDL command, it is waiting and it did not start to run yet).

create table t1(i int) engine innodb;

• Connection 1 ( Acquire MDL lock)

MariaDB [test]> start transaction;
Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> select 1 from t1; # <-- acquires MDL lock
Empty set (0.001 sec)

• Connection 2 (ALTER TABLE)

MariaDB [test]> alter table t1 add column (j int); # <-- waits on MDL

• Connection 3 (BACKUP STAGE ...)

MariaDB [(none)]> backup stage start;
Query OK, 0 rows affected (0.000 sec)

MariaDB [(none)]> backup stage flush;
Query OK, 0 rows affected (0.002 sec)

MariaDB [(none)]> backup stage block_ddl; # <-- waits on something