Python's GIL should be released when calling ground

[rkaminsk]

No description provided.

[rkaminsk]

@javier-romero

Signals in Python are a weird thing. So the code you posted in #83 cannot be made to work. With the above commit the following program can be interrupted:

import clingo
import signal
import sys
import os

from threading import Thread
from time import sleep

def signal_handler(signum, frame):
    print("signal caught")
    os._exit(1)

def ground():
    ctl = clingo.Control()
    ctl.add("base",[],"""
    #const n=500.
    b(1..n,1..n,1..n).
    """)
    ctl.ground([("base",[])])

signal.signal(signal.SIGINT, signal_handler)

thread = Thread(target=ground)
thread.start()
thread.join(float("inf"))

Grounding has to be started in a thread and you have to wait for completion with a timeout. Whether this works also depends on which python/platform you are using. Also note that os_.exit is used instead of sys.exit, which is important because, otherwise, python will wait until grounding completed.