Add a option to ReverseProxy, let the call to choose storing the body into buffer or not

As far as I can tell, you can already use the Director field to inject the io.TeeReader. The difficult part is how to pass the resulting buffer to the ErrorHandler function.

On the other hand, if you're assigning to outreq.Body anyway, then you could provide your own implementation than injects your own custom struct type, and use a type-switch to unpack that type within the ErrorHandler function. So it's not obvious to me that anything is needed here beyond the existing Director function.

CC @bradfitz (on leave) for httputil.

Add a option to ReverseProxy, let the call to choose storing the body into buffer or not

As far as I can tell, you can already use the Director field to inject the io.TeeReader. The difficult part is how to pass the resulting buffer to the ErrorHandler function.

Thank you!

Your way is much better, I should create a new "init" function like this:

func NewSingleHostBodyBufReverseProxy(target *url.URL, key string) *httputil.ReverseProxy {
	director := func(req *http.Request) {
		httputil.NewSingleHostReverseProxy(target).Director(req)
		if req.Body != nil && req.ContentLength != 0 {
			var buf bytes.Buffer
			tee := io.TeeReader(req.Body, &buf)
			req.Body = ioutil.NopCloser(tee)
			ctx := context.WithValue(req.Context(), key, &buf)
			r2 := req.WithContext(ctx)
			*req = *r2
		}
	}
	return &httputil.ReverseProxy{Director: director}
}

I inject the "buf" into context of request with the "key", so in my ErrorHandler, it could be

if buf, ok := r.Context().Value("bodybuf").(*bytes.Buffer); ok {
	r.Body = ioutil.NopCloser(buf)
}

the existing Director and ReverseProxy are good enough