All DDL operations must be atomic before we can really claim to support Atomic DDL (MDEV-17567).

DROP DATABASE was only made crash-safe in MDEV-24408; it is not atomic yet. With the following approach, it should become atomic on XA-capable storage engines:

An atomic implementation of DROP DATABASE would involve the following internal steps:

1. XA START
2. For each table in storage engines, invoke handler::delete_table().
3. If everything succeeded, invoke handlerton::drop_database in each storage engine that implements it.
4. XA PREPARE in each storage engine
5. XA COMMIT or XA ROLLBACK in each storage engine.