python - 如何等待父进程退出子进程？

Question

我想 fork 一个进程，等待父进程退出，然后再对子进程做一些事情。

Answer 1

天真的方法是这样一个繁忙的循环:

# BAD WAY!
pid = os.fork()
if pid == 0:
    while True:
        if os.path.isdir("/proc/%s" % pid):
            break
        time.sleep(0.1)
    # Parent is dead there.

但这很容易受到 PID 重用问题的影响。如果在父进程退出并获取其 PID 后立即创建另一个进程，则子进程将永远不会退出。

另一种方法是对特定文件使用 flock()。但这是行不通的，因为 child 与 parent 共享相同的锁。

一个万无一失的方法是使用一个特殊的技巧:在父级中创建一个管道，在子级中，您只需等待直到获得 EOF。

# Good way
read_fd, write_fd = os.pipe()

pid = os.fork()
if pid > 0:
    # Close the read pipe, so that only the child is the reader.
    os.close(read_fd)

    # It is important to voluntarily leak write_fd there,
    # so that the kernel will close it for the parent process
    # when it will exit, triggering our trick.

elif pid == 0:
    # Daemon ourselves first.
    os.setsid()
    for fd in {0, 1, 2}:
        os.close(fd)

    # Close the write pipe so that the parent is the only writer.
    # This will make sure we then get an EOF when the only writer exits.
    os.close(write_fd)

    # Now, we're waiting on the read pipe for an EOF.
    # PS: the assert is not necessary in production...
    assert os.read(read_fd, 1) == ""
    os.close(read_fd)

    # At this point, the parent is dead.