fcntl.flock - how to implement a timeout?

Question

I am using python 2.7

I want to create a wrapper function around fcntl.flock() that will timeout after a set interval:

wrapper_function(timeout):
         


        

Answer 1

Timeouts for system calls are done with signals. Most blocking system calls return with EINTR when a signal happens, so you can use alarm to implement timeouts.

Here's a context manager that works with most system calls, causing IOError to be raised from a blocking system call if it takes too long.

import signal, errno
from contextlib import contextmanager
import fcntl

@contextmanager
def timeout(seconds):
    def timeout_handler(signum, frame):
        pass

    original_handler = signal.signal(signal.SIGALRM, timeout_handler)

    try:
        signal.alarm(seconds)
        yield
    finally:
        signal.alarm(0)
        signal.signal(signal.SIGALRM, original_handler)

with timeout(1):
    f = open("test.lck", "w")
    try:
        fcntl.flock(f.fileno(), fcntl.LOCK_EX)
    except IOError, e:
        if e.errno != errno.EINTR:
            raise e
        print "Lock timed out"