探寻python多线程ctrl+c退出问题解决方案

场景：

经常会遇到下述问题：很多io busy的应用采取多线程的方式来解决，但这时候会发现python命令行不响应ctrl-c 了，而对应的java代码则没有问题：

代码如下:

public class Test { 
    public static void main(String[] args) throws Exception { 
 
        new Thread(new Runnable() { 
 
            public void run() { 
                long start = System.currentTimeMillis(); 
                while (true) { 
                    try { 
                        Thread.sleep(1000); 
                    } catch (Exception e) { 
                    } 
                    System.out.println(System.currentTimeMillis()); 
                    if (System.currentTimeMillis() - start > 1000 * 100) break; 
                } 
            } 
        }).start(); 
 
    } 
} 
java Test

ctrl-c则会结束程序

而对应的python代码：

代码如下:

# -*- coding: utf-8 -*- 
import time 
import threading 
start=time.time() 
def foreverLoop(): 
    start=time.time() 
    while 1: 
        time.sleep(1) 
        print time.time() 
        if time.time()-start>100: 
            break 
              
thread_=threading.Thread(target=foreverLoop) 
#thread_.setDaemon(True) 
thread_.start()

python p.py

后ctrl-c则完全不起作用了。

不成熟的分析：

首先单单设置 daemon 为 true 肯定不行，就不解释了。当daemon为 false 时，导入python线程库后实际上，threading会在主线程执行完毕后，检查是否有不是 daemon 的线程，有的化就wait，等待线程结束了，在主线程等待期间，所有发送到主线程的信号也会被阻测，可以在上述代码加入signal模块验证一下：

代码如下:

def sigint_handler(signum,frame):   
    print "main-thread exit" 
    sys.exit()   
signal.signal(signal.SIGINT,sigint_handler)

在100秒内按下ctrl－c没有反应，只有当子线程结束后才会出现打印 "main-thread exit",可见 ctrl-c被阻测了

threading 中在主线程结束时进行的操作：

代码如下:

_shutdown = _MainThread()._exitfunc 
def _exitfunc(self): 
        self._Thread__stop() 
        t = _pickSomeNonDaemonThread() 
        if t: 
            if __debug__: 
                self._note("%s: waiting for other threads", self) 
        while t: 
            t.join() 
            t = _pickSomeNonDaemonThread() 
        if __debug__: 
            self._note("%s: exiting", self) 
        self._Thread__delete()

对所有的非daemon线程进行join等待，其中join中可自行察看源码，又调用了wait，同上文分析 ，主线程等待到了一把锁上。

不成熟的解决：

只能把线程设成daemon才能让主线程不等待，能够接受ctrl－c信号，但是又不能让子线程立即结束，那么只能采用传统的轮询方法了，采用sleep间歇省点cpu吧：

代码如下:

# -*- coding: utf-8 -*- 
import time,signal,traceback 
import sys 
import threading 
start=time.time() 
def foreverLoop(): 
    start=time.time() 
    while 1: 
        time.sleep(1) 
        print time.time() 
        if time.time()-start>5: 
            break 
             
thread_=threading.Thread(target=foreverLoop) 
thread_.setDaemon(True) 
thread_.start() 
 
#主线程wait住了,不能接受信号了 
#thread_.join() 
 
def _exitCheckfunc(): 
    print "ok" 
    try: 
        while 1: 
            alive=False 
            if thread_.isAlive(): 
                alive=True 
            if not alive: 
                break 
            time.sleep(1)   
    #为了使得统计时间能够运行，要捕捉  KeyboardInterrupt :ctrl-c       
    except KeyboardInterrupt, e: 
        traceback.print_exc() 
    print "consume time :",time.time()-start 
         
threading._shutdown=_exitCheckfunc

缺点：轮询总会浪费点cpu资源，以及battery.

有更好的解决方案敬请提出。

ps1: 进程监控解决方案 ：

用另外一个进程来接受信号后杀掉执行任务进程，牛

代码如下:

# -*- coding: utf-8 -*- 
import time,signal,traceback,os 
import sys 
import threading 
start=time.time() 
def foreverLoop(): 
    start=time.time() 
    while 1: 
        time.sleep(1) 
        print time.time() 
        if time.time()-start>5: 
            break 
 
class Watcher: 
    """this class solves two problems with multithreaded
    programs in Python, (1) a signal might be delivered
    to any thread (which is just a malfeature) and (2) if
    the thread that gets the signal is waiting, the signal
    is ignored (which is a bug).
 
    The watcher is a concurrent process (not thread) that
    waits for a signal and the process that contains the
    threads.  See Appendix A of The Little Book of Semaphores.
    http://greenteapress.com/semaphores/
 
    I have only tested this on Linux.  I would expect it to
    work on the Macintosh and not work on Windows.
    """ 
 
    def __init__(self): 
        """ Creates a child thread, which returns.  The parent
            thread waits for a KeyboardInterrupt and then kills
            the child thread.
        """ 
        self.child = os.fork() 
        if self.child == 0: 
            return 
        else: 
            self.watch() 
 
    def watch(self): 
        try: 
            os.wait() 
        except KeyboardInterrupt: 
            # I put the capital B in KeyBoardInterrupt so I can 
            # tell when the Watcher gets the SIGINT 
            print 'KeyBoardInterrupt' 
            self.kill() 
        sys.exit() 
 
    def kill(self): 
        try: 
            os.kill(self.child, signal.SIGKILL) 
        except OSError: pass 
 
Watcher()             
thread_=threading.Thread(target=foreverLoop) 
thread_.start()

注意 watch()一定要放在线程创建前，原因未知。。。。，否则立刻就结束