How do I use proxy.ModifyResponse?

Question

I've implemented the most basic Reverse Proxy to pull a page and then add some content to the body. Unfortunately my attempt to add to the html isn't taking effect. The code below just shows the original page but without the "monkeys" I prepended to the response. What additional calls are needed to get this to work? I eventually want to use this to replace css for custom css.

package main

import (
    "log"
    "net/http"
    "net/http/httputil"
    "net/url"
    "fmt"
    "github.com/PuerkitoBio/goquery"
    "bytes"
)

type Director func(*http.Request)

func (f Director) Then(g Director) Director {
    return func(req *http.Request) {
        f(req)
        g(req)
    }
}

func hostDirector(host string) Director {
    return func(req *http.Request) {
        req.Host = host
    }
}

func UpdateResponse(r *http.Response) error {
    doc, err := goquery.NewDocumentFromReader(r.Body)
    if err != nil{
        //log.New("Research")
        log.Fatal("Bad doc %v", err)
        return err
    }
    html, err := goquery.OuterHtml(doc.First())
    if err != nil{
        log.Fatal("Bad html %v", err)
        return err
    }
    fmt.Printf("Body %v", html)
    r.Write(bytes.NewBufferString("monkeys"+html))

    return nil
}

func main() {
    url, _ := url.Parse("http://cnn.com/")
    proxy := httputil.NewSingleHostReverseProxy(url)

    d := proxy.Director
    // sequence the default director with our host director
    proxy.Director = Director(d).Then(hostDirector(url.Hostname()))
    proxy.ModifyResponse = UpdateResponse
    http.Handle("/", proxy)
    log.Fatal(http.ListenAndServe(":9090", nil))
}

Answer 1

Your ModifyResponse code is not modifying the response. In your code, you have:

r.Write(bytes.NewBufferString("monkeys"+html))

It seems that you are writing to the response, but nope. r is of *http.Response which has a Write method:

func (r *Response) Write(w io.Writer) error

The document says:

Write writes r to w in the HTTP/1.x server response format, including the status line, headers, body, and optional trailer.

Since bytes.Buffer also implements a io.Writer, the code compiles, but instead of write to the response from the buffer, it write the response to the buffer, which is absolutely undesired.

The fact is, http.Response simply don't provide a way to modify its Body. It is clear that Bodey is an io.ReadCloser. What you can do is create a new io.ReadCloser with the body. Remember to change Content-Length field in the Header or it may cause problem.

func UpdateResponse(r *http.Response) error {
    b, _ := ioutil.ReadAll(r.Body)
    buf := bytes.NewBufferString("Monkey")
    buf.Write(b)
    r.Body = ioutil.NopCloser(buf)
    r.Header["Content-Length"] = []string{fmt.Sprint(buf.Len())}
    return nil
}

Playground: https://play.golang.org/p/_Tyvo6GVN3x