I don't think hang.py is correct.  If you launch a process using subprocess, the subprocess owns the child process.  You should not call waitpid() or os.kill() separately.  Also, since you don't keep a reference to the subprocess.Popen object, its destructor will run and may call waitpid() on the child process.  Depending on when exactly the destructor runs, it can interfere with your own waitpid() call.