python forking after threading?

现在需要对 mirrord 进行单元测试，但 mirrord 进行了 daemon switch，因此会 fork 子进程并退出父进程。而在单元测试中，我的测试程序只能通过创建子进程或子线程来运行工作代码(否则运行到 daemon 状态就阻塞进入无限循环，测试代码就没办法往下走了)，为了得到工作代码的状态来进行检测，需要能够访问到工作代码的一些变量，所以简单的就是使用线程。

但问题是，如果在 threading 之后又进行了 forking，而 fork 后的父进程退出，那么线程是否退出呢？写一个原型代码看看：

#!/usr/bin/env python
# -*- encoding: utf-8 -*-

import os,sys
import time
import threading

def run_thread():
    pid = os.fork()
    if pid == 0:
        print "child process ..."
        while True:
            time.sleep(5)
            print "child process is running ..."
        print "main process exiting ..."
        sys.exit(0)

that = threading.Thread(target=run_thread, name="child")
that.setDaemon(1)
that.start()
# #1:
that.join()
print "main thread end"

运行结果如下：

sh$ python mirrord_5_ut.py
child process ...
main process exiting ...
main thread end
[Ctrl^C]
sh$ child process is running ...
child process is running ...
child process is running ...
[Ctrl^C]
sh$ child process is running ...

由此可见，fork 后的子进程还在运行，但是主进程退出了，也导致调度它的线程退出了！

但有一个问题，如果在 thread code 中 fork 的子进程也退出，那么在上面 #1 后面的代码会不会执行？因为进程是复制地址空间的，例如：

#!/usr/bin/env python
# -*- encoding: utf-8 -*-

"""
If the main process exits after forking,
    while the forking is execute by a child thread,
    will the main/child threads end? -- Yes
Author: Roc Zhou
Date: 2007-07-31
Email: chowroc.z@gmail.com
"""

import os,sys
import time
import threading

var = 0

def run_thread():
    global var
    var = 1
    print "var is '%d' in thread" % var
    pid = os.fork()
    if pid == 0:
        print "child process ..."
        i = 0
        while i < 10:
            var = i * 2
            time.sleep(1)
            print "child process is running %d and var is '%d' ..." % (i, var)
            i += 1
    else:
        print "main process ..."
        j = 0
        while j < 10:
            var = j * 2 + 1
            time.sleep(1)
            print "main process is running %d and var is '%d' ..." % (j, var)
            j += 1
        # sys.exit(0)
    print "var is '%d' at the end of thread code" % var

print "var is '%d' at the beginning" % var
# #a:
# run_thread()
# ------------
# #b:
that = threading.Thread(target=run_thread, name="child")
that.setDaemon(1)
that.start()
time.sleep(5)
# #1:
that.join()
# ------------
print "var is '%d' at the main thread end" % var

当按照 #a 来运行的时候，没有启动线程，那么输出结果是：

sh$ python mirrord_5_ut.py
var is '0' at the beginning
var is '1' in thread
child process ...
main process ...
main process is running 0 and var is '1' ...
child process is running 0 and var is '0' ...
main process is running 1 and var is '3' ...
child process is running 1 and var is '2' ...
main process is running 2 and var is '5' ...
child process is running 2 and var is '4' ...
main process is running 3 and var is '7' ...
child process is running 3 and var is '6' ...
main process is running 4 and var is '9' ...
child process is running 4 and var is '8' ...
main process is running 5 and var is '11' ...
child process is running 5 and var is '10' ...
main process is running 6 and var is '13' ...
child process is running 6 and var is '12' ...
main process is running 7 and var is '15' ...
child process is running 7 and var is '14' ...
main process is running 8 and var is '17' ...
child process is running 8 and var is '16' ...
main process is running 9 and var is '19' ...
var is '19' at the end of thread code
var is '19' at the main thread end
sh$ child process is running 9 and var is '18' ...
var is '18' at the end of thread code
var is '18' at the main thread end

可以看到，最后的代码运行了两次！但是在调用线程之后就不是，按 #b 运行，输出结果如下：

sh$ python mirrord_5_ut.py
var is '0' at the beginning
var is '1' in thread
child process ...
main process ...
child process is running 0 and var is '0' ...
main process is running 0 and var is '1' ...
child process is running 1 and var is '2' ...
main process is running 1 and var is '3' ...
child process is running 2 and var is '4' ...
main process is running 2 and var is '5' ...
child process is running 3 and var is '6' ...
main process is running 3 and var is '7' ...
child process is running 4 and var is '8' ...
main process is running 4 and var is '9' ...
child process is running 5 and var is '10' ...
main process is running 5 and var is '11' ...
child process is running 6 and var is '12' ...
main process is running 6 and var is '13' ...
child process is running 7 and var is '14' ...
main process is running 7 and var is '15' ...
child process is running 8 and var is '16' ...
main process is running 8 and var is '17' ...
child process is running 9 and var is '18' ...
var is '18' at the end of thread code
main process is running 9 and var is '19' ...
var is '19' at the end of thread code
var is '19' at the main thread end

这里最后的代码只运行了一次！

另外，是否使用 #1 处的 that.join() 也是有差别的！

sh$ python mirrord_5_ut.py
var is '0' at the beginning
var is '1' in thread
child process ...
main process ...
child process is running 0 and var is '0' ...
main process is running 0 and var is '1' ...
child process is running 1 and var is '2' ...
main process is running 1 and var is '3' ...
child process is running 2 and var is '4' ...
main process is running 2 and var is '5' ...
child process is running 3 and var is '6' ...
main process is running 3 and var is '7' ...
var is '9' at the main thread end
sh$ child process is running 4 and var is '8' ...
child process is running 5 and var is '10' ...
child process is running 6 and var is '12' ...
child process is running 7 and var is '14' ...
child process is running 8 and var is '16' ...
child process is running 9 and var is '18' ...
var is '18' at the end of thread code

所以，最终，将 mirrord.py 这个 module 中的 daemon 的代码移到 mirrord 这个 script 中！