The problem was two fault:
- flush_tables() wrongly gave errors when failing to open read only tables
- backup_block_ddl() didn't properly ignores errors from flush_tables()
The test case for this will be pushed in 10.5 as the test involves
S3 tables.
Part of MDEV-5336 Implement LOCK FOR BACKUP
- Changed check of Global_only_lock to also include BACKUP lock.
- We store latest MDL_BACKUP_DDL lock in thd->mdl_backup_ticket to be able
to downgrade lock during copy_data_between_tables()