The actual issue was introduced in 61e7f3176b:
Session::non_realtime_stop() no longer unsets PostTransportStop
(other changes from that commit are not relevant).
The real issue however is a race-condition.
So far this only seems to happen on MacOS, Coreaudio.
It seems that non_realtime_stop() is called in the butler-thread
after exporting has started, even though the butler has been
paused in wait_until_finished().
Perhaps Coreaudio thread switches causes TransportFSM to
reinitialize and scheduling the butler?
The use of `usleep()` makes this rather a workaround.
However it's sufficient for the coreaudio rt thread to run
at least once.
cfd95340b1