python僵尸进程产生的原因
在 unix 或 unix-like 的系统中，当一个子进程退出后，它就会变成一个僵尸进程，如果父进程没有通过 wait 系统调用来读取这个子进程的退出状态的话，这个子进程就会一直维持僵尸进程状态。
Zombie process - Wikipedia 中是这样描述的：
On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the "Terminated state". This occurs for child processes, where the entry is still needed to allow the parent process to read its child's exit status: once the exit status is read via the wait system call, the zombie's entry is removed from the process table and it is said to be "reaped". A child process always first becomes a zombie before being removed from the resource table. In most cases, under normal system operation zombies are immediately waited on by their parent and then reaped by the system ?C processes that stay zombies for a long time are generally an error and cause a resource leak.
并且僵尸进程无法通过 kill 命令来清除。
本文将探讨如何手动制造一个僵尸进程以及清除僵尸进程的办法。
手动制造一个僵尸进程
为了便于后面讲解清除僵尸进程的方法，我们使用日常开发中经常使用的 multiprocessing 模块来制造僵尸进程（准确的来说是制造一个长时间维持僵尸进程状态的子进程）：
$ cat test_a.py
from multiprocessing import Process, current_process
import logging
import os
import time
logging.basicConfig(
level=logging.DEBUG,
format='%(asctime)-15s - %(levelname)s - %(message)s'
)
def run():
logging.info('exit child process %s', current_process().pid)
os._exit(3)
p = Process(target=run)
p.start()
time.sleep(100)
测试：
$ python test_a.py & [1] 10091 $ 2017-07-20 21:28:14,792 - INFO - exit child process 10106 $ ps aux |grep 10106 mozillazg 10126 0.0 0.0 2434836 740 s006 R+ 0:00.00 grep 10106 mozillazg 10106 0.0 0.0 0 0 s006 Z 0:00.00 (Python)
可以看到，子进程 10091 变成了僵尸进程。
既然已经可以控制僵尸进程的产生了，那我们就可以进入下一步如何清除僵尸进程了。
清除僵尸进程有两种方法：
•第一种方法就是结束父进程。当父进程退出的时候僵尸进程随后也会被清除。
• 第二种方法就是通过 wait 调用来读取子进程退出状态。我们可以通过处理 SIGCHLD 信号，在处理程序中调用 wait 系统调用来清除僵尸进程。
处理 SIGCHLD 信号
子进程退出时系统会向父进程发送 SIGCHLD 信号，父进程可以通过注册 SIGCHLD 信号处理程序，在信号处理程序中调用 wait
系统调用来清理僵尸进程。 $ cat test_b.py
import errno
from multiprocessing import Process, current_process
import logging
import os
import signal
import time
logging.basicConfig(
level=logging.DEBUG,
format='%(asctime)-15s - %(levelname)s - %(message)s'
)
def run():
exitcode = 3
logging.info('exit child process %s with exitcode %s',
current_process().pid, exitcode)
os._exit(exitcode)
def wait_child(signum, frame):
logging.info('receive SIGCHLD')
try:
while True:
# -1 表示任意子进程
# os.WNOHANG 表示如果没有可用的需要 wait 退出状态的子进程，立即返回不阻塞
cpid, status = os.waitpid(-1, os.WNOHANG)
if cpid == 0:
logging.info('no child process was immediately available')
break
exitcode = status >> 8
logging.info('child process %s exit with exitcode %s', cpid, exitcode)
except OSError as e:
if e.errno == errno.ECHILD:
logging.error('current process has no existing unwaited-for child processes.')
else:
raise
logging.info('handle SIGCHLD end')
signal.signal(signal.SIGCHLD, wait_child)
p = Process(target=run)
p.start()
while True:
time.sleep(100)
效果：
$ python test_b.py & [1] 10159 $ 2017-07-20 21:28:56,085 - INFO - exit child process 10174 with exitcode 3 2017-07-20 21:28:56,088 - INFO - receive SIGCHLD 2017-07-20 21:28:56,089 - INFO - child process 10174 exit with exitcode 3 2017-07-20 21:28:56,090 - ERROR - current process has no existing unwaited-for child processes. 2017-07-20 21:28:56,090 - INFO - handle SIGCHLD end $ ps aux |grep 10174 mozillazg 10194 0.0 0.0 2432788 556 s006 R+ 0:00.00 grep 10174
可以看到，子进程退出变成僵尸进程后，系统给父进程发送了 SIGCHLD 信号，我们在 SIGCHLD 信号的处理程序中通过 os.waitpid 调用 wait 系统调用后阻止了子进程一直处于僵尸进程状态，从而实现了清除僵尸进程的效果。
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