The reason of the double lock was an extraneous ha_flush_logs().
Unlike the upstream it is unnecessary in Mariadb that exploits a binlog
checkpoint mechanism for not letting PURGE or RESET-MASTER to trouble
transaction recovery. That is in case should a trx
be prepared but its binlog file gone, the trx then is committed on disk too.
Those facts have been always verified by existing tests of

  binlog.binlog_{checkpoint,xa_recover}.test.

A regression test for the bug is included though.