When using the export-tool, the very first callback may already be
freewheeling. In this case the first call to the butler also happens
directly from the freewheel process-callback and initial session events
are handled there. Setting PostTransportAdjustPlaybackBuffering
took the process-lock, which caused a deadlock:
Glib::Threads::Mutex::Lock::Lock(Glib::Threads::Mutex&) at /usr/include/glibmm-2.4/glibmm/threads.h:687
ARDOUR::Session::butler_transport_work() at ../libs/ardour/session_transport.cc:1157
ARDOUR::Session::process_export_fw(unsigned int) at ../libs/ardour/session_export.cc:303
ARDOUR::AudioEngine::process_callback(unsigned int) at ../libs/ardour/audioengine.cc:486
ARDOUR::DummyAudioBackend::main_process_thread() at ../libs/backends/dummy/dummy_audiobackend.cc:951
f8b5424d9f