Attempt to acquire a POSIX file lock.

def flock(path):
    """Attempt to acquire a POSIX file lock.
    """
    with open(path, "w+") as lf:
        try:
            fcntl.flock(lf, fcntl.LOCK_EX | fcntl.LOCK_NB)
            acquired = True
            yield acquired

        except OSError:
            acquired = False
            yield acquired

        finally:
            if acquired:
                fcntl.flock(lf, fcntl.LOCK_UN)