Updates the flagfile at `flags_path`, changing the value for
    `resign_threshold` to `new_threshold`

def update_flagfile(flags_path, new_threshold):
    """Updates the flagfile at `flags_path`, changing the value for
    `resign_threshold` to `new_threshold`
    """
    if abs(new_threshold) > 1:
        raise ValueError("Invalid new percentile for resign threshold")
    with tf.gfile.GFile(flags_path) as f:
        lines = f.read()
    if new_threshold > 0:
        new_threshold *= -1
    if not RESIGN_FLAG_REGEX.search(lines):
        print("Resign threshold flag not found in flagfile {}!  Aborting.".format(flags_path))
        sys.exit(1)
    old_threshold = RESIGN_FLAG_REGEX.search(lines).groups(1)
    lines = re.sub(RESIGN_FLAG_REGEX, "--resign_threshold={:.3f}".format(new_threshold), lines)

    if abs(float(old_threshold[0]) - new_threshold) < 0.001:
        print("Not updating percentiles; {} ~= {:.3f}".format(
                old_threshold[0], new_threshold), flush=True)
    else:
        print("Updated percentile from {} to {:.3f}".format(
                old_threshold[0], new_threshold), flush=True)
        with tf.gfile.GFile(flags_path, 'w') as f:
            f.write(lines)