Alternative for timeout signals in python threads

Question

I have an application that relies on signals for timeouts for a number of blocking actions.

For example:

def wait_timeout(signum, frame):
    raise Exception("timeout")

signal.signal(signal.SIGALRM, wait_timeout)
signal.setitimer(signal.ITIMER_REAL, 5)

try:
    while true:
        print("zzz")
        sleep(1)
except Exception as e:
    # timeout
    print("Time's up")

Now I've implemented multithreading using the same approach, but with all threads I get ValueError: signal only works in main thread .

I assume the approach for timeouts with signals is not applicable in threads.

Unfortunately I can't use something like this:

timeout = 5
start = time.time()

while true:
    print("zzz")
    sleep(1)
    if time.time() <= start+timeout:
        print("Time's up)
        break

As the operations in the while loop may be blocking and possibly last forever, therefore the loop may never reach the if clause.

Q: How may I implement a timeout, like I used to do it with signals, in threads?

edit: I've come across this blog post , showing a similar solution to setTimeout() in JavaScript in python. I assume this may be a possible solution, but I'm really not sure how this may be used.

edit2: I start the threads in the main as follows:

p = Popen(["tool", "--param", arg], stdin=PIPE, stdout=PIPE, stderr=STDOUT)
t = Thread(target=process_thread, daemon=True, args=(p,arg1,arg2))
t.start()

The process_thread function processes the stdout of the tool , by doing the following:

for line in p.stdout:
    # process line of the processes stdout

This process may take forever, eg once the tool doesn't produce any output. I only want the output of the tool for, let's say 5 seconds, so the for loop needs to be broken, after a specific timeout.

That's what I used signals for, but apparently they don't work in threads.

edit3: I've created a more elaborate and accurate example on how I intend to use the singals in threads. See the gist here

Answer 1

What you are looking for is a Watchdog .

def watchdog(queue):
    while True:
        watch = queue.get()
        time.sleep(watch.seconds)

        try:
            watch = queue.get_nowait()
            # No except, got queue message, 
            #   do noting wait for next watch

        except queue.Empty:
            os.kill(watch.pid, signal.SIGKILL)

def workload_thread(queue):
    pid = os.getpid()
    queue.put({'pid':pid, 'seconds':5})

    # do your work
    # Test Watchdog
    # time.sleep(6)

    queue.put({'pid':pid, 'done':True})

Note: Code not tested, may have Syntax errors!

Answer 2

This implements a class Terminator , which sends a signal.SIG... after a given timeout=5 to Threads Popen process . Multiple different pid are possible.

class Terminator(object):
    class WObj():
        def __init__(self, process, timeout=0, sig=signal.SIGABRT):
            self.process = process
            self.timeout = timeout
            self.sig = sig

    def __init__(self):
        self.__queue = queue.Queue()
        self.__t = Thread(target=self.__sigterm_thread, args=(self.__queue,))
        self.__t.start()
        time.sleep(0.1)

    def __sigterm_thread(self, q):
        w = {}
        t = 0
        while True:
            time.sleep(0.1);
            t += 1
            try:
                p = q.get_nowait()
                if p.process == 0 and p.sig == signal.SIGTERM:
                    # Terminate sigterm_thread
                    return 1

                if p.process.pid not in w:
                    if p.timeout > 0 and p.sig != signal.SIGABRT:
                        w[p.process.pid] = p
                else:
                    if p.sig == signal.SIGABRT:
                        del (w[p.process.pid])
                    else:
                        w[p.process.pid].timeout = p.timeout

            except queue.Empty:
                pass

            if t == 10:
                for key in list(w.keys()):
                    p = w[key]
                    p.timeout -= 1
                    if p.timeout == 0:
                        """ A None value indicates that the process hasn't terminated yet. """
                        if p.process.poll() == None:
                            p.process.send_signal(p.sig)
                        del (w[p.process.pid])
                t = 0
            # end if t == 10
        # end while True

    def signal(self, process, timeout=0, sig=signal.SIGABRT):
        self.__queue.put(self.WObj(process, timeout, sig))
        time.sleep(0.1)

    def close(self, process):
        self.__queue.put(self.WObj(process, 0, signal.SIGABRT))
        time.sleep(0.1)

    def terminate(self):
        while not self.__queue.empty():
            trash = self.__queue.get()

        if self.__t.is_alive():
            self.__queue.put(self.WObj(0, 0, signal.SIGTERM))

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.__del__()

    def __del__(self):
        self.terminate()  

This is the workload for instance:

def workload(n, sigterm):
    print('Start workload(%s)' % n)
    arg = str(n)
    p = Popen(["tool", "--param", arg], stdin=PIPE, stdout=PIPE, stderr=STDOUT)

    sigterm.signal(p, timeout=4, sig=signal.SIGTERM)
    while True:
        for line in p.stdout:
            # process line of the processes stdout
            print(line.strip())
            time.sleep(1)

        if p.poll() != None:
            break

    sigterm.close(p)
    time.sleep(0.1)
    print('Exit workload(%s)' % n)

if __name__ == '__main__':
    with Terminator() as sigterm:
        p1 = Thread(target=workload, args=(1, sigterm)); p1.start(); time.sleep(0.1)
        p2 = Thread(target=workload, args=(2, sigterm)); p2.start(); time.sleep(0.1)
        p3 = Thread(target=workload, args=(3, sigterm)); p3.start(); time.sleep(0.1)
        p1.join(); p2.join(); p3.join()

        time.sleep(0.5)
    print('EXIT __main__')

Tested with Python:3.4.2 and Python:2.7.9