Question

Is there some way to close a specific hanging recv zmq socket from another thread in Python without clobbering other sockets that may be in use? The following code doesn’t seem to exit; replacing the sock.close() with ctx.destroy() causes it to exit, but that would obviously destroy all sockets in the context:

import zmq
import time
import threading as th
ctx = zmq.Context()
sock = ctx.socket(zmq.ROUTER)
sock.bind('tcp://*:6000')
def shutdown():
    time.sleep(5)
    print 'closing'
    sock.close()
t = th.Thread(target=shutdown)
t.start()
try:
    sock.recv()
except zmq.ZMQError:
print 'closed'

Answer 1

One possibility is to perform the socket operations within a context manager that shuts down sockets using a signal handler that raises an exception when a signal is detected:

import zmq
import signal, time
import threading as th
from contextlib import contextmanager
@contextmanager
def ZMQErrorOnAlarm():
    def handler(signum, frame):
        raise Exception('ALARM detected')
    signal.signal(signal.SIGALRM, handler)
    yield
ctx = zmq.Context()
sock = ctx.socket(zmq.ROUTER)
sock.bind('tcp://*:6000')
def shutdown():
    signal.alarm(5)
    print 'closing'
t = th.Thread(target=shutdown)
t.start()
with ZMQErrorOnAlarm():
    try:
        sock.send('x')
        sock.recv()
    except:
        print 'closed'