如何使用SingleFlight共享下载的大文件？

I'm proxying a bunch of http GET calls through singleflight. But returned response is only seen by the first request.

I also noticed a problem in my test. If the first request times out, the response will be lost.

Let's say r1,r2,r3 are requests that come in order. They are all grouped in one groupKey. If r1 time out , r2 and r3 will wait until the shared HTTP call returns or until their own timeout.

proxy code (credits to here)

// add auth to the requst and proxy to target host
var serveReverseProxy = func(target string, res http.ResponseWriter, req *http.Request) {
    log.Println("new request!")
    requestURL, _ := url.Parse(target)
    proxy := httputil.NewSingleHostReverseProxy(requestURL)
    req1, _ := http.NewRequest(req.Method, req.RequestURI, req.Body)
    for k, v := range req.Header {
        for _, vv := range v {
            req1.Header.Add(k, vv)
        }
    }
    req1.Header.Set("Authorization", "Bearer "+"some token")
    req1.Host = requestURL.Host

    proxy.ServeHTTP(res, req1)
}

var requestGroup singleflight.Group
mockBackend := httptest.NewServer(http.HandlerFunc(func(res http.ResponseWriter, req *http.Request) {
    groupKey := req.Host + req.RequestURI
    name := req.Header.Get("From")
    ch := requestGroup.DoChan(groupKey, func() (interface{}, error) {
        //increase key retention to 20s to make sure r1,r2,r3 are all in one group
        go func() {
            time.Sleep(20 * time.Second)
            requestGroup.Forget(groupKey)
            log.Println("Key deleted :", groupKey)
        }()

        // proxy to some host and expect the result to be written in res
        serveReverseProxy("https://somehost.com", res, req)
        return nil, nil
    })

    timeout := time.After(15 * time.Second)

    var result singleflight.Result
    select {
    case <-timeout: // Timeout elapsed, send a timeout message (504)
        log.Println(name, " timed out")
        http.Error(res, "request timed out", http.StatusGatewayTimeout)
        return
    case result = <-ch: // Received result from channel
    }

    if result.Err != nil {
        http.Error(res, result.Err.Error(), http.StatusInternalServerError)
        return
    }

    if result.Shared {
        log.Println(name, " is shared")
    } else {
        log.Println(name, " not shared")
    }
}))

I'd expect r2,r3 to either

1. at least see the result from their own reponseWriter
2. timeout along with r1

https://github.com/golang/net/blob/master/http2/h2demo/h2demo.go#L181-L219 this works. Turns out I need to return handler in singleFlight.Group.Do instead of the response. I don't know why