python僵尸进程产生的原因_Python

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 – 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