Running python python_thread_bug.py -j4 often results in one of the threads failing to start until another thread finishes.

The bug appears to be that subprocess's pipe_cloexec function is racy: if another thread forks between os.pipe() and _set_cloexec_flag, then the resulting process could hang on to the write end of the pipe.  That will cause the Popen call that got rudely interrupted to wait until the whole resulting process tree dies.