Explicitly make a golden chunk for a given model `model_num`
    (not necessarily the most recent one).

      While we haven't yet got enough samples (EXAMPLES_PER_GENERATION)
      Add samples from the games of previous model.

def make_chunk_for(output_dir=LOCAL_DIR,
                   local_dir=LOCAL_DIR,
                   game_dir=None,
                   model_num=1,
                   positions=EXAMPLES_PER_GENERATION,
                   threads=8,
                   sampling_frac=0.02):
    """
    Explicitly make a golden chunk for a given model `model_num`
    (not necessarily the most recent one).

      While we haven't yet got enough samples (EXAMPLES_PER_GENERATION)
      Add samples from the games of previous model.
    """
    game_dir = game_dir or fsdb.selfplay_dir()
    ensure_dir_exists(output_dir)
    models = [model for model in fsdb.get_models() if model[0] < model_num]
    buf = ExampleBuffer(positions, sampling_frac=sampling_frac)
    files = []
    for _, model in sorted(models, reverse=True):
        local_model_dir = os.path.join(local_dir, model)
        if not tf.gfile.Exists(local_model_dir):
            print("Rsyncing", model)
            _rsync_dir(os.path.join(game_dir, model), local_model_dir)
        files.extend(tf.gfile.Glob(os.path.join(local_model_dir, '*.zz')))
        print("{}: {} games".format(model, len(files)))
        if len(files) * 200 * sampling_frac > positions:
            break

    print("Filling from {} files".format(len(files)))

    buf.parallel_fill(files, threads=threads)
    print(buf)
    output = os.path.join(output_dir, str(model_num) + '.tfrecord.zz')
    print("Writing to", output)
    buf.flush(output)