urlfetch/urlfetch.go
func (t *Transport) RoundTrip(req *http.Request) (res *http.Response, err error) {
	methNum, ok := pb.URLFetchRequest_RequestMethod_value[req.Method]
	if !ok {
		return nil, fmt.Errorf("urlfetch: unsupported HTTP method %q", req.Method)
	}

	method := pb.URLFetchRequest_RequestMethod(methNum)

	freq := &pb.URLFetchRequest{
		Method:                        &method,
		Url:                           proto.String(urlString(req.URL)),
		FollowRedirects:               proto.Bool(false), // http.Client's responsibility
		MustValidateServerCertificate: proto.Bool(!t.AllowInvalidServerCertificate),
	}
	if deadline, ok := t.Context.Deadline(); ok {
		freq.Deadline = proto.Float64(deadline.Sub(time.Now()).Seconds())
	}

	for k, vals := range req.Header {
		for _, val := range vals {
			freq.Header = append(freq.Header, &pb.URLFetchRequest_Header{
				Key:   proto.String(k),
				Value: proto.String(val),
			})
		}
	}
	if methodAcceptsRequestBody[req.Method] && req.Body != nil {
		// Avoid a []byte copy if req.Body has a Bytes method.
		switch b := req.Body.(type) {
		case interface {
			Bytes() []byte
		}:
			freq.Payload = b.Bytes()
		default:
			freq.Payload, err = ioutil.ReadAll(req.Body)
			if err != nil {
				return nil, err
			}
		}
	}

	fres := &pb.URLFetchResponse{}
	if err := internal.Call(t.Context, "urlfetch", "Fetch", freq, fres); err != nil {
		return nil, err
	}

	res = &http.Response{}
	res.StatusCode = int(*fres.StatusCode)
	res.Status = fmt.Sprintf("%d %s", res.StatusCode, statusCodeToText(res.StatusCode))
	res.Header = make(http.Header)
	res.Request = req

	// Faked:
	res.ProtoMajor = 1
	res.ProtoMinor = 1
	res.Proto = "HTTP/1.1"
	res.Close = true

	for _, h := range fres.Header {
		hkey := http.CanonicalHeaderKey(*h.Key)
		hval := *h.Value
		if hkey == "Content-Length" {
			// Will get filled in below for all but HEAD requests.
			if req.Method == "HEAD" {
				res.ContentLength, _ = strconv.ParseInt(hval, 10, 64)
			}
			continue
		}
		res.Header.Add(hkey, hval)
	}

	if req.Method != "HEAD" {
		res.ContentLength = int64(len(fres.Content))
	}

	truncated := fres.GetContentWasTruncated()
	res.Body = &bodyReader{content: fres.Content, truncated: truncated}
	return
}
func (t *Transport) RoundTrip(req *http.Request) (res *http.Response, err error) {
	methNum, ok := pb.URLFetchRequest_RequestMethod_value[req.Method]
	if !ok {
		return nil, fmt.Errorf("urlfetch: unsupported HTTP method %q", req.Method)
	}

	method := pb.URLFetchRequest_RequestMethod(methNum)

	freq := &pb.URLFetchRequest{
		Method:                        &method,
		Url:                           proto.String(urlString(req.URL)),
		FollowRedirects:               proto.Bool(false), // http.Client's responsibility
		MustValidateServerCertificate: proto.Bool(!t.AllowInvalidServerCertificate),
	}
	if deadline, ok := t.Context.Deadline(); ok {
		freq.Deadline = proto.Float64(deadline.Sub(time.Now()).Seconds())
	}

	for k, vals := range req.Header {
		for _, val := range vals {
			freq.Header = append(freq.Header, &pb.URLFetchRequest_Header{
				Key:   proto.String(k),
				Value: proto.String(val),
			})
		}
	}
	if methodAcceptsRequestBody[req.Method] && req.Body != nil {
		// Avoid a []byte copy if req.Body has a Bytes method.
		switch b := req.Body.(type) {
		case interface {
			Bytes() []byte
		}:
			freq.Payload = b.Bytes()
		default:
			freq.Payload, err = ioutil.ReadAll(req.Body)
			if err != nil {
				return nil, err
			}
		}
	}

	fres := &pb.URLFetchResponse{}
	if err := internal.Call(t.Context, "urlfetch", "Fetch", freq, fres); err != nil {
		return nil, err
	}

	res = &http.Response{}
	res.StatusCode = int(*fres.StatusCode)
	res.Status = fmt.Sprintf("%d %s", res.StatusCode, statusCodeToText(res.StatusCode))
	res.Header = make(http.Header)
	res.Request = req

	// Faked:
	res.ProtoMajor = 1
	res.ProtoMinor = 1
	res.Proto = "HTTP/1.1"
	res.Close = true

	for _, h := range fres.Header {
		hkey := http.CanonicalHeaderKey(*h.Key)
		hval := *h.Value
		if hkey == "Content-Length" {
			// Will get filled in below for all but HEAD requests.
			if req.Method == "HEAD" {
				res.ContentLength, _ = strconv.ParseInt(hval, 10, 64)
			}
			continue
		}
		res.Header.Add(hkey, hval)
	}

	if req.Method != "HEAD" {
		res.ContentLength = int64(len(fres.Content))
	}

	truncated := fres.GetContentWasTruncated()
	res.Body = &bodyReader{content: fres.Content, truncated: truncated}
	return
}
// RoundTrip issues a single HTTP request and returns its response. Per the
// http.RoundTripper interface, RoundTrip only returns an error if there
// was an unsupported request or the URL Fetch proxy fails.
// Note that HTTP response codes such as 5xx, 403, 404, etc are not
// errors as far as the transport is concerned and will be returned
// with err set to nil.