Why print operation within signal handler may change deadlock situation?
Question: I got simple program as below:

    
    
    import threading
    import time
    import signal
    
    WITH_DEADLOCK = 0
    
    lock = threading.Lock()
    
    def interruptHandler(signo, frame):
        print str(frame), 'received', signo
        lock.acquire()
        try:
            time.sleep(3)
        finally:
            if WITH_DEADLOCK:
                print str(frame), 'release'
            lock.release()
    
    signal.signal(signal.SIGINT, interruptHandler)
    for x in xrange(60):
        print time.strftime("%H:%M:%S"), 'main thread is working'
        time.sleep(1)
    

So, if you start that program and even Ctrl+C is pressed twice within 3
seconds, there is no deadlock. Each time you press Ctrl + C proper line is
displayed. If you change WITH_DEADLOCK=1 and you would press Ctrl+C twice
(withing 3 seconds) then program will be hung.

Does anybody may explain why print operation make such difference?

(My python version is 2.6.5)


Answer: To be honest I think J.F. Sebastian's comment is the most appropriate answer
here - you need to make your signal handler reentrant, which it currently
isn't, and it is mostly just surprising that it works anyway without the print
statement.

