tr.mu.Lock()
	if tr.err == nil {
		tr.err = err
	}
	tr.mu.Unlock()
}

// useRegisteredProtocol reports whether an alternate protocol (as registered
// with Transport.RegisterProtocol) should be respected for this request.
func (t *Transport) useRegisteredProtocol(req *Request) bool {
	if req.URL.Scheme == "https" && req.requiresHTTP1() {
		// If this request requires HTTP/1, don't use the