Linux alarm signal (SIGALRM) to detach process isAlive

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题记

最近做项目遇到的问题，程序跑了多个process，每个process都是相互独立的，为了解耦，类似于微服务的架构，我们要求系统可以detach 到主线程跑飞，死循环等其他bug 问题，最初的设计方案是：每个process 都会给每一个monitor的process 去发送keep alive 消息，由monitor去收集每个module的keep alive消息，然后去判断是否process 跑飞等情况。但是这种方案，由于需要多一个monitor模块，在本来内存有限的嵌入式设备上，有点得不偿失，后来就想能否有linux 系统内部的实现可以达到我们的要求，也就是 SIGALRM

1. Signal & Semaphore 区别

Signal: 是通过软中断信号通知进程发生了异步事件。进程之间可以通过系统调用kill 发送软中断信号，内核也可以因为内部事件而给进程发送信号，通知进程发生了某个事件。

Semaphore: 信号量是用来操作系统进程间同步访问共享资源。信号量在创建时需要设置一个初始值，表示同时可以有几个任务可以访问该信号量保护的共享资源，初始值为1就变成互斥锁（Mutex），即同时只能有一个任务可以访问信号量保护的共享资源。

2. SIGALRM 以及python code 实现

SIGALRM是在定时器终止时发送给进程的信号，在进行阻塞式系统调用时，为避免进程陷入无限的等待，可以为阻塞式系统调用设置定时器。

#include <unistd.h>
unsigned int alarm(unsigned int seconds);

在alarm成功调用后，开始计时，超过该事件将触发SIGALARM信号，然后会调到handler 执行。如下是python的例子，

import signal,time,sys,thread,tracebackclass Example:def __init__(self):self.handler_counter = 0self.retry_counter = 3passdef timout_handler(self, signum, frame):'''timeout handler when failed to send signal alarmthere is a retry to make sure main thread hung'''self.handler_counter += 1print "call timeout_handler counter: " + str(self.handler_counter)if self.handler_counter == self.retry_counter:print("Have retry %s, exit process", self.retry_counter)traceback.print_stack(frame)  # print tracebacksys.exit()def monitor_alive(self, threadName, delay):'''monitor alive to send alarm message every (delay + 1) second, if after (delay + 1) doesn't receive response fromkernel, will interrupt timout_handler'''count = 0while True:time.sleep(delay)signal.alarm(delay + 1)print "sign_time count " + str(count)# below if logic to mock 3 time timeoutif count == 2:time.sleep(delay)if count == 4:time.sleep(delay)if count == 6:time.sleep(delay)count += 1print "%s: %s" % (threadName, time.ctime(time.time()))if __name__ == '__main__':example = Example()# register handler# only could set signal handler in main thread# https://stackoverflow.com/questions/44151888/why-only-main-thread-can-set-signal-handler-in-pythonsignal.signal(signal.SIGALRM, example.timout_handler)thread.start_new_thread(example.monitor_alive, ("Thread-1", 2,))while True:time.sleep(2)print('main thread ')

运行结果：

sign_time count 0
Thread-1: Sat Jun  9 10:52:43 2018
main thread 
sign_time count 1
Thread-1: Sat Jun  9 10:52:45 2018
main thread 
sign_time count 2
main thread 
Thread-1: Sat Jun  9 10:52:49 2018
main thread 
call timeout_handler counter: 1
main thread 
sign_time count 3
Thread-1: Sat Jun  9 10:52:51 2018
main thread 
sign_time count 4
main thread 
Thread-1: Sat Jun  9 10:52:55 2018
main thread 
call timeout_handler counter: 2
main thread 
sign_time count 5
Thread-1: Sat Jun  9 10:52:57 2018
main thread 
sign_time count 6
main thread 
Thread-1: Sat Jun  9 10:53:01 2018
main thread 
call timeout_handler counter: 3
('Have retry %s, exit process', 3)File "/home/odl/sereno/tests/singal.py", line 52, in <module>time.sleep(2)

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