code: func drainClientPreface(r io.Reader) error {
  	var buf bytes.Buffer
  	prefaceLen := int64(len([]byte(http2.ClientPreface)))
  	n, err := io.CopyN(&buf, r, prefaceLen)
  	if err != nil {
  		return err
  	}
  	if n != prefaceLen || buf.String() != http2.ClientPreface {
  		return fmt.Errorf("client never sent: %s", http2.ClientPreface)
  	}
  	return nil
  }
docs: // drainClientPreface reads a single instance of the HTTP/2 client preface from
  // the supplied reader.