python僵尸進程產(chǎn)生的原因

更新時間：2017年07月21日 08:54:27 作者：Huang Huang

這篇文章主要給大家講解的是在Python中是如何產(chǎn)生僵尸進程的，以及如何清除僵尸進程的方法，有需要的小伙伴可以參考下

在 unix 或 unix-like 的系統(tǒng)中，當(dāng)一個子進程退出后，它就會變成一個僵尸進程，如果父進程沒有通過 wait 系統(tǒng)調(diào)用來讀取這個子進程的退出狀態(tài)的話，這個子進程就會一直維持僵尸進程狀態(tài)。

Zombie process - Wikipedia 中是這樣描述的：

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.

并且僵尸進程無法通過 kill 命令來清除。

本文將探討如何手動制造一個僵尸進程以及清除僵尸進程的辦法。

手動制造一個僵尸進程

為了便于后面講解清除僵尸進程的方法，我們使用日常開發(fā)中經(jīng)常使用的 multiprocessing 模塊來制造僵尸進程（準(zhǔn)確的來說是制造一個長時間維持僵尸進程狀態(tài)的子進程）：

$ 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)

測試：

$ 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，子進程 10091 變成了僵尸進程。

既然已經(jīng)可以控制僵尸進程的產(chǎn)生了，那我們就可以進入下一步如何清除僵尸進程了。

清除僵尸進程有兩種方法：

•第一種方法就是結(jié)束父進程。當(dāng)父進程退出的時候僵尸進程隨后也會被清除。
• 第二種方法就是通過 wait 調(diào)用來讀取子進程退出狀態(tài)。我們可以通過處理 SIGCHLD 信號，在處理程序中調(diào)用 wait 系統(tǒng)調(diào)用來清除僵尸進程。

處理 SIGCHLD 信號

子進程退出時系統(tǒng)會向父進程發(fā)送 SIGCHLD 信號，父進程可以通過注冊 SIGCHLD 信號處理程序，在信號處理程序中調(diào)用 wait
系統(tǒng)調(diào)用來清理僵尸進程。 $ 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 表示任意子進程
      # os.WNOHANG 表示如果沒有可用的需要 wait 退出狀態(tài)的子進程，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，子進程退出變成僵尸進程后，系統(tǒng)給父進程發(fā)送了 SIGCHLD 信號，我們在 SIGCHLD 信號的處理程序中通過 os.waitpid 調(diào)用 wait 系統(tǒng)調(diào)用后阻止了子進程一直處于僵尸進程狀態(tài)，從而實現(xiàn)了清除僵尸進程的效果。