@taleinat Sadly, if the semaphore_tracker receives an invalid command, it raises and dies, making the test impossible:

                     cmd, name = line.strip().split(b':')
                     if cmd == b'REGISTER':
                         cache.add(name)
                     elif cmd == b'UNREGISTER':
                         cache.remove(name)
                     else:
                         raise RuntimeError('unrecognized command %r' % cmd)