// bucket.
func NewMonitoredReader(ctx context.Context, m *Monitor, r io.Reader, opts *MonitorReaderOptions) *MonitoredReader {
	reader := MonitoredReader{
		r:        r,
		throttle: m.throttle(opts.BucketOptions),
		m:        m,
		opts:     opts,
		ctx:      ctx,
	}
	reader.m.init(opts.BucketOptions)
	return &reader

// does not send more content than specified size.
func NewReader(ctx context.Context, src io.Reader, size int64, md5Hex, sha256Hex string, actualSize int64) (*Reader, error) {
	return newReader(ctx, src, size, md5Hex, sha256Hex, actualSize, false, nil)
}

// MD5 checksum of the content read from r as ETag.
//
// If the provided etag is not nil the returned
// Reader compares the etag with the computed
// MD5 sum once the r returns io.EOF.
func NewReader(ctx context.Context, r io.Reader, etag ETag, forceMD5 []byte) *Reader {
	if er, ok := r.(*Reader); ok {
		if er.readN == 0 && Equal(etag, er.checksum) {
			return er
		}
	}
	if len(forceMD5) != 0 {
		return &Reader{