net/http: Wrong ContentLength when use http.Request.Body as the third parameter of http.NewRequest()

[rtxu]

func (h *RedirectHttpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    body, _ := r.Body.(io.Reader)
    rLogger.Infof("%T", body)   // print: *http.body

    req, _ := http.NewRequest(r.Method, h.Dest+r.URL.String(), r.Body)
    rLogger.Infof("%d", req.ContentLength)  // print: 0

    ......
}

Here's the net/http/request.go implementation:

// NewRequest returns a new Request given a method, URL, and optional body.
//
// If the provided body is also an io.Closer, the returned
// Request.Body is set to body and will be closed by the Client
// methods Do, Post, and PostForm, and Transport.RoundTrip.
func NewRequest(method, urlStr string, body io.Reader) (*Request, error) {
        u, err := url.Parse(urlStr)
        if err != nil {
                return nil, err
        }
        rc, ok := body.(io.ReadCloser)
        if !ok && body != nil {
                rc = ioutil.NopCloser(body)
        }
        req := &Request{
                Method:     method,
                URL:        u,
                Proto:      "HTTP/1.1",
                ProtoMajor: 1,
                ProtoMinor: 1,
                Header:     make(Header),
                Body:       rc,
                Host:       u.Host,
        }
        if body != nil {
                switch v := body.(type) {
                case *bytes.Buffer:
                        req.ContentLength = int64(v.Len())
                case *bytes.Reader:
                        req.ContentLength = int64(v.Len())
                case *strings.Reader:
                        req.ContentLength = int64(v.Len())
                }
        }

        return req, nil
}

[bradfitz]

NewRequest has a few special cases for common types in the standard library with known lengths, but it doesn't do everything. You'll have to set your own lengths when it matters. Usually chunked encoding should be good enough.