net/http: http: ContentLength=222 with Body length 0

The problem is that the request body is read to the end on the first call to Do(). On subsequent calls to Do(), no data is read from the response body.

The fix is to move the creation of the body reader inside the for loop. This requires that the request also be created inside the for loop.

func Post(URL string, form url.Values, cl *http.Client) ([]byte, error) {
  body := form.Encode()
  for i := 0; i < 10; i++ {
    req, err := http.NewRequest("POST", URL, strings.NewReader(body))
    if err != nil {
        log.Error(err)
        return nil, err
    }
    req.Header.Set("User-Agent", ua)
    req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
    rsp, err := cl.Do(req)
    if err == nil {
        defer rsp.Body.Close()
        b, err := ioutil.ReadAll(rsp.Body)
        if err != nil {
            log.Error(err)
            return nil, err
        }
        return b, nil
    }

    if !IsErrorProxy(err) {
        return nil, err
    }

    log.Errorf("Proxy is slow or down ")
    time.Sleep(6 * time.Second)
  }
  return nil, fmt.Errorf("after 10 tries error: %v", err)
}

Let's see what does http.NewRequest do

rc, ok := body.(io.ReadCloser)
if !ok && body != nil {
    rc = ioutil.NopCloser(body)
}
// The host's colon:port should be normalized. See Issue 14836.
u.Host = removeEmptyPort(u.Host)
req := &Request{
    Method:     method,
    URL:        u,
    Proto:      "HTTP/1.1",
    ProtoMajor: 1,
    ProtoMinor: 1,
    Header:     make(Header),
    Body:       rc,
    Host:       u.Host,
}

body is type of io.Reader, and convert to io.ReaderCloser by ioutil.NopCloser. As @Cerise Limón said, the Request.Body had been read and the stream is closer, so when you Do() again, the Body Length is 0.

So, we could reset the Request.Body before invoke Do.

func Post(URL string, form url.Values, cl *http.Client) ([]byte, error) {
    requestBodyString := form.Encode()
    req, err := http.NewRequest("POST", URL, strings.NewReader(requestBodyString))
    // ...

    rsp, err := do(cl, req, requestBodyString)

    //...
    return b, nil
}

func do(cl *http.Client, req *http.Request, requestBodyString string)(*http.Response, error){
    rsp, err := cl.Do(req)
    for i := 0; IsErrProxy(err); i++ {
        log.Errorf("Proxy is slow or down ")
        time.Sleep(6 * time.Second)
        // reset Request.Body
        req.Body = ioutil.NopCloser(strings.NewReader(requestBodyString))
        rsp, err = cl.Do(&req)
        if err == nil{
            return rsp, nil
        }
        if i > 10 {

            return nil, fmt.Errorf("after %v tries error: %v", i, err)
        }
    }
    return rsp, err
}