如何在Golang中倒带io.ReadCloser？

Question

I'm perpetually caught out by reading an io.ReadCloser and then forgetting that I've read it before, and when I read it again, I get an empty payload. I wish there was some lint check for my stupidity. Nonetheless, I think I can use TeeReader, but it fails to meet my expectations here:

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        buf := &bytes.Buffer{}
        tee := io.TeeReader(r.Body, buf)
        body, err := ioutil.ReadAll(tee)
        if err != nil {
            http.Error(w, err.Error(), http.StatusBadRequest)
            return
        }
        log.Println("body", string(body))
        payload, err := httputil.DumpRequest(r, true)
        if err != nil {
            http.Error(w, err.Error(), http.StatusInternalServerError)
            return
        }
        log.Println("dump request", string(payload))
        w.WriteHeader(http.StatusOK)
    })
    log.Fatal(http.ListenAndServe(":8080", nil))
}

The body is missing from my "dump request" log line.

I.e. when I run curl -i -X POST --data '{"username":"xyz","password":"xyz"}' http://localhost:8080

I want the original request in full:

2019/01/14 11:09:50 dump request POST / HTTP/1.1
Host: localhost:8080
Accept: */*
Content-Length: 35
Content-Type: application/x-www-form-urlencoded
User-Agent: curl/7.63.0

{"username":"xyz","password":"xyz"}

What am I missing?

Answer 1

How do I rewind a io.ReadCloser in Go[...]?

You cannot. A ReadCloser can be read and closed. Unless the actual underlying type has some method to rewind you simply cannot. (For your case you may just use the bytes.Buffer, possibly after adding a Close method via io/ioutil.ReadCloser as the Request.Body; but this is not "rewinding" but "replacing".)