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pythonmultithreadingtimeoutblocking
Anja V.
Anja V.

Reputation: 115

A thread is blocked by a blocking call - how do I make a timeout on the blocking call?

I have a python program which operates an external program and starts a timeout thread. Timeout thread should countdown for 10 minutes and if the script, which operates the external program isn't finished in that time, it should kill the external program.

My thread seems to work fine on the first glance, my main script and the thread run simultaneously with no issues. But if a pop up window appears in the external program, it stops my scripts, so that even the countdown thread stops counting, therefore totally failing it's job.

I assume the issue is that the script calls a blocking function in API for the external program, which is blocked by the pop up window. I understand why it blocks my main program, but don't understand why it blocks my countdown thread. So, one possible solution might be to run a separate script for the countdown, but I would like to keep it as clean as possible and it seems really messy to start a script for this.

I have searched everywhere for a clue, but I didn't find much. There was a reference to the gevent library here: background function in Python , but it seems like such a basic task, that I don't want to include external library for this.

I also found a solution which uses a windows multimedia timer here, but I've never worked with this before and am afraid the code won't be flexible with this. Script is Windows-only, but it should work on all Windows from XP on.

For Unix I found signal.alarm which seems to do exactly what I want, but it's not available for Windows. Any alternatives for this?

Any ideas on how to work with this in the most simplified manner?

This is the simplified thread I'm creating (run in IDLE to reproduce the issue):

import threading
import time

class timeToKill():
    def __init__(self, minutesBeforeTimeout):
        self.stop = threading.Event()
        self.countdownFrom = minutesBeforeTimeout * 60

    def startCountdown(self):
        self.countdownThread= threading.Thread(target=self.countdown, args=(self.countdownFrom,))
        self.countdownThread.start()

    def stopCountdown(self):
        self.stop.set()
        self.countdownThread.join()

    def countdown(self,seconds):
        for second in range(seconds):
            if(self.stop.is_set()):
                break
            else:
                print (second)
                time.sleep(1)

timeout = timeToKill(1)
timeout.startCountdown()
raw_input("Blocking call, waiting for input:\n")

Upvotes: 1

Views: 2717

Answers (2)

jfs
jfs

Reputation: 414119

One possible explanation for a function call to block another Python thread is that CPython uses global interpreter lock (GIL) and the blocking API call doesn't release it (NOTE: CPython releases GIL on blocking I/O calls therefore your raw_input() example should work as is).

If you can't make the buggy API call to release GIL then you could use a process instead of a thread e.g., multiprocessing.Process instead of threading.Thread (the API is the same). Different processes are not limited by GIL.

Upvotes: 1

Paul
Paul

Reputation: 7325

For quick and dirty threading, I usually resort to subprocess commands. it is quite robust and os independent. It does not give as fine grained control as the thread and queue modules but for external calls to programs generally does nicely. Note the shell=True must be used with caution. 

#this can be any command
p1 = subprocess.Popen(["python", "SUBSCRIPTS/TEST.py", "0"], shell=True)

#the thread p1 will run in the background - asynchronously.  If you want to kill it after some time, then you need 

#here do some other tasks/computations
time.sleep(10)

currentStatus = p1.poll()
if currentStatus is None: #then it is still running
  try:
    p1.kill() #maybe try os.kill(p1.pid,2) if p1.kill does not work
  except:
    #do something else if process is done running - maybe do nothing?
    pass

Upvotes: 1

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