Threaded Python application does not react on Ctrl+C

Question

I have a simple Python app that runs in two threads. One is SMTP server, the other is HTTP server. When I start it in terminal it does not react on Ctrl+C. Here is the code:

import asyncore
import threading
import SimpleHTTPServer
import SocketServer

from smtpd import SMTPServer


class MailHoleSMTP(SMTPServer):
    def process_message(self, peer, mailfrom, rcpttos, data):
        pass


def run_smtp():
    MailHoleSMTP(('localhost', 1025), None)
    asyncore.loop()


def run_http():
    handler = SimpleHTTPServer.SimpleHTTPRequestHandler
    httpd = SocketServer.TCPServer(('localhost', 1080), handler)
    httpd.serve_forever()


if __name__ == '__main__':
    http_thread = threading.Thread(target=run_http)
    smtp_thread = threading.Thread(target=run_smtp)
    http_thread.start()
    smtp_thread.start()

    http_thread.join()
    smtp_thread.join()

I suspect that something might be wrong with that serve_forever() call, maybe it does not play well with threads or something. What can I do to make it react on Ctrl+C?

UPD: It does not work (for both threads) even if I run only one of the two threads.

3 threads, not 2. You're not counting the main thread, which is the one that catches the keyboard interrupt. — Joel Cornett
– Joel Cornett, Commented Sep 1, 2014 at 19:05
Related question: python - threading ignores KeyboardInterrupt exception - Stack Overflow — user202729
– user202729, Commented Dec 13, 2023 at 7:31

roippi · Accepted Answer · 2014-09-01 19:04:18Z

3

So there are two problems to address here. First is that a joined thread will not respond to any signals. Fix that by looping to check for a signal once every second:

while http_thread.is_alive():
    http_thread.join(1)
#similar for smtp_thread

This will make your program respond to KeyboardInterrupt signals, but now you will notice that the whole thing doesn't shut down cleanly. The reason: signals are not propagated to other threads, and the whole process is not allowed to shut down until your worker threads finish (because they are non-daemonic). To fix, the simplest way is to just make them daemon threads:

http_thread = threading.Thread(target=run_http)
http_thread.daemon = True
smtp_thread = threading.Thread(target=run_smtp)
smtp_thread.daemon = True
...

Signaling is a pretty common topic in multithreaded applications, so you may want to do more reading.

edited Sep 1, 2014 at 19:04

answered Sep 1, 2014 at 18:42

roippi

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1 Comment

Messa Over a year ago

I've tried this but it looks that thread.join() blocks processing signals. But try to replace it with while thread.is_alive(): time.sleep(1) - works for me

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