在反向代理中多次重复http.Request

Question

I'm implementing a http.RoundTripper in Go, and as part of httputil.ReverseProxy implementation.

I need to buffer an incoming request, and repeat it several times, depending on the response I get from the backend. To do this, I use request.Write and http.ReadRequest. (I am actually not sure if this is a good idea, if there are any better ways, I'm interested.)

After deserializing request from []byte with http.ReadRequest and repeat it using the http.DefaultTransport’s roundtripper, I get this printed in my stderr:

2019/08/01 14:35:51 http: proxy error: unsupported protocol scheme ""

So it looks like for some reason I need to set req.URL again after deserializing to make it work.

Here's roughly how my code looks like:

func (s *myServer) RoundTrip(origReq *http.Request) (*http.Response, error) {
    var b bytes.Buffer
    if err := origReq.Write(&b); err != nil {
        return nil, errors.Wrap(err,"failed to buffer request")
    }
    for retries := 0; retries < s.maxRetries; retries++{
        req, err := http.ReadRequest(bufio.NewReader(bytes.NewReader(b.Bytes()))) // probably can be simplified
        if err != nil {
            return nil, errors.Wrap(err,"failed to un-buffer request")
        }
        req.URL = origReq.URL // <-- why is this necessary?

        resp, err := http.DefaultTransport.RoundTrip(req)
        if err != nil {
            return resp, err
        }
        if needRepeat(resp) {
            continue
        }
        return resp, nil
    }
}

Answer 1

ReadRequest reads a server request. Request.Write writes a client request. See the Request.URL documentation for how the Request.URL is handled differently in client and server requests.

Given that ReadRequest and Request.Write are not inverses of each other, a better approach is to copy the request body before the loop and create a new request on each iteration using data from the original request and the copied request body.