concurrently.

There is a deadlock between

C1 mariabackup's connection that holds MDL locks
C2 Online ALTER TABLE that wants to have MDL exclusively
   and tries to upgrade its mdl lock.
C3 another mariabackup's connection that does FLUSH TABLES (or FTWRL)

C3 waits waits for C2,  which waits for C1, which waits for C3,
thus the deadlock.


MDL locks cannot be released until FLUSH  succeeds, because
otherwise it would allow ALTER to sneak in, causing backup to abort and
breaking lock-ddl-per-table's promise.

The fix here workarounds the deadlock, by killing connections in
"Waiting for metadata lock" status (i.e ALTER). This killing continues
until FTWRL succeeds.

Killing connections is skipped in case --no-locks parameter
was  passed to backup, because there won't be a FLUSH.

For the reference,in Percona's xtrabackup --lock-ddl-per-connection
silently implies --no-lock ie FLUSH is always skipped there.

A rather large part of fix is introducing DBUG capability to start
a query  the new connection at the right moment of backup
compensating somewhat for mariabackup' lack of send_query or DBUG_SYNC.