守护进程, 孤儿进程, 僵尸进程与waitpid

def fork_processes(num_processes, max_restarts=100):
    """Starts multiple worker processes.

    If ``num_processes`` is None or <= 0, we detect the number of cores
    available on this machine and fork that number of child
    processes. If ``num_processes`` is given and > 0, we fork that
    specific number of sub-processes.

    Since we use processes and not threads, there is no shared memory
    between any server code.

    Note that multiple processes are not compatible with the autoreload
    module (or the ``autoreload=True`` option to `tornado.web.Application`
    which defaults to True when ``debug=True``).
    When using multiple processes, no IOLoops can be created or
    referenced until after the call to ``fork_processes``.

    In each child process, ``fork_processes`` returns its *task id*, a
    number between 0 and ``num_processes``.  Processes that exit
    abnormally (due to a signal or non-zero exit status) are restarted
    with the same id (up to ``max_restarts`` times).  In the parent
    process, ``fork_processes`` returns None if all child processes
    have exited normally, but will otherwise only exit by throwing an
    exception.
    """
    global _task_id
    assert _task_id is None
    if num_processes is None or num_processes <= 0:
        num_processes = cpu_count()
    if ioloop.IOLoop.initialized():
        raise RuntimeError("Cannot run in multiple processes: IOLoop instance "
                           "has already been initialized. You cannot call "
                           "IOLoop.instance() before calling start_processes()")
    gen_log.info("Starting %d processes", num_processes)
    children = {}

    def start_child(i):
        pid = os.fork()
        if pid == 0:
            # child process
            _reseed_random()
            global _task_id
            _task_id = i
            return i
        else:
            children[pid] = i
            return None
    for i in range(num_processes):
        id = start_child(i)
        if id is not None:
            return id
    num_restarts = 0
    while children:
        try:
            pid, status = os.wait()
        except OSError as e:
            if errno_from_exception(e) == errno.EINTR:
                continue
            raise
        if pid not in children:
            continue
        id = children.pop(pid)
        if os.WIFSIGNALED(status):
            gen_log.warning("child %d (pid %d) killed by signal %d, restarting",
                            id, pid, os.WTERMSIG(status))
        elif os.WEXITSTATUS(status) != 0:
            gen_log.warning("child %d (pid %d) exited with status %d, restarting",
                            id, pid, os.WEXITSTATUS(status))
        else:
            gen_log.info("child %d (pid %d) exited normally", id, pid)
            continue
        num_restarts += 1
        if num_restarts > max_restarts:
            raise RuntimeError("Too many child restarts, giving up")
        new_id = start_child(id)
        if new_id is not None:
            return new_id
    # All child processes exited cleanly, so exit the master process
    # instead of just returning to right after the call to
    # fork_processes (which will probably just start up another IOLoop
    # unless the caller checks the return value).
    sys.exit(0)