Returns the maximum file descriptor limit.  This is guaranteed to
    return a useful int value.

def sys_maxfd():
    """
    Returns the maximum file descriptor limit.  This is guaranteed to
    return a useful int value.
"""
    maxfd = None
    try:
        maxfd = int(resource.getrlimit(resource.RLIMIT_NOFILE)[0])
        if maxfd == resource.RLIM_INFINITY:                                                     # pragma: no cover
            maxfd = None
    except: pass
    if maxfd is None:
        maxfd = sys_maxfd.fallback_maxfd
    return maxfd