Semi-sync replication hangs when master opens new binlog file

Hi,

I am having an issue with MariaDB semi-sync replication.
Setting up replication between master and slaves works as expected but when the master begins writing to a new binlog file replication stalls on waiting for an ack from the semi-sync slave.
I have the fallback to async set very high because I basically don't want the server to ever commit writes to the master that aren't committed to the slave.

I have discussed this somewhat on the mailing list and have created this gist which reliably reproduces the issue: https://gist.github.com/josephglanville/c9a69b02de319ee06aef1a8d787bcce3

It's possible this is a configuration issue rather than a bug, config for master and slave is generated in the scripts in the gist so if that is the case hopefully it's an easy fix.

Joseph.