WSREP: BF lock wait long for trx

hi,

this might be duplicate if MDEV-15747, but is for Verision 10.2.12 so I ma not sure

We ran in this bug last month, but I had no time to look deeper into this until now.

It was on a Cluster with 2 nodes. The problems started on node "DB2", and shifted to node "DB1" when trying to restart "DB2". The restart failed, so I rebooted the node.

here is the timeline of what happend when on node DB2:
08:00:24 - problem on node DB2
10:27:58 - trying to restart mariadb (failed!)

from /var/log/messages:
Mar 13 10:29:28 DB2 systemd: mariadb.service stop-sigterm timed out. Skipping SIGKILL.
Mar 13 10:30:59 DB2 systemd: mariadb.service stop-final-sigterm timed out. Skipping SIGKILL. Entering failed mode.
Mar 13 10:30:59 DB2 systemd: Stopped MariaDB 10.2.12 database server.
Mar 13 10:30:59 DB2 systemd: Unit mariadb.service entered failed state.
Mar 13 10:30:59 DB2 systemd: mariadb.service failed.
Mar 13 10:31:13 DB2 systemd: Starting MariaDB 10.2.12 database server...
Mar 13 10:32:43 DB2 systemd: mariadb.service start-pre operation timed out. Terminating.
Mar 13 10:32:43 DB2 systemd: Failed to start MariaDB 10.2.12 database server.
Mar 13 10:32:43 DB2 systemd: Unit mariadb.service entered failed state.
Mar 13 10:32:43 DB2 systemd: mariadb.service failed.

10:41 - rebooted node DB2