if sc.sawClientPreface {
		return nil
	}
	errc := make(chan error, 1)
	go func() {
		// Read the client preface
		buf := make([]byte, len(http2ClientPreface))
		if _, err := io.ReadFull(sc.conn, buf); err != nil {
			errc <- err
		} else if !bytes.Equal(buf, http2clientPreface) {
			errc <- fmt.Errorf("bogus greeting %q", buf)
		} else {
			errc <- nil
		}
	}()