code: func actualContentLength(req *http.Request) int64 {
  	if req.Body == nil {
  		return 0
  	}
  	if req.ContentLength != 0 {
  		return req.ContentLength
  	}
  	return -1
  }
docs: // actualContentLength returns a sanitized version of
  // req.ContentLength, where 0 actually means zero (not unknown) and -1
  // means unknown.