893a0ed dodged reentrant allocation due to pthread_mutex_lock() on macOS by using OSSpinLocks, and since then we've also added support for os_unfair_lock as a result of the former becoming deprecated. It appears that our reentrant support (see #501) may be sufficient to allow pthread_mutex_lock() calls. If so, we can expand background thread support (#149) to macOS by using pthreads mutexes and removing the non-standard API support.