Taking a readlock after a writelock in the same thread should result
in a deadlock, yet pthread on Linux returns EDEADLK and continues.
glib-2.42.0 ignores EDEADLK and assumes the lock was taken. Releasing
the lock later causes issues: "Calling g_rw_lock_writer_unlock() on a
lock that is not held by the current thread leads to undefined behaviour."
The issue at hand:
AudioStreamView::redisplay_track()
-> foreach_region() #<< WriteLock
-> add_region_view()
...
-> AudioRegionView::create_one_wave()
-> RegionView::update_coverage_frames
-> Playlist::top_unmuted_region_at() #<< ReadLock
All current users of Playlist::foreach_region() are in the GUI
and AFAICT read-only (display regions, update visuals)
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