'Return whether this nonce is valid.
str -> bool'

def useNonce(self, server_url, timestamp, salt):

    if (abs((timestamp - time.time())) > nonce.SKEW):
        return False
    if server_url:
        (proto, rest) = server_url.split('://', 1)
    else:
        (proto, rest) = ('', '')
    domain = _filenameEscape(rest.split('/', 1)[0])
    url_hash = _safe64(server_url)
    salt_hash = _safe64(salt)
    filename = ('%08x-%s-%s-%s-%s' % (timestamp, proto, domain, url_hash, salt_hash))
    filename = os.path.join(self.nonce_dir, filename)
    try:
        fd = os.open(filename, ((os.O_CREAT | os.O_EXCL) | os.O_WRONLY), 128)
    except OSError as why:
        if (why.errno == EEXIST):
            return False
        else:
            raise
    else:
        os.close(fd)
        return True
