title: How to transfer large file to http client in golang
body: <p>I have a handler function which looks like below:</p>
  
  <pre><code>func DownloadFileHandler(w http.ResponseWriter, r *http.Request) {
      SetResponseHeaders(w, r)
      if r.Method == "OPTIONS" {
          return
      }
      var err error
      var responseCode int = http.StatusBadRequest
      vars := mux.Vars(r)
      file := vars["file"]
      var n int64 = 0
      var reader io.Reader
      Filename := "images/" + file
      f, err := os.Open(Filename)
      if err != nil {
          err = errors.New("File not found")
          goto Error
      }
      reader = bufio.NewReader(f)
      settings.WriteDebugLog(time.Now())
      n, err = io.Copy(w, reader) // this takes a long time if the file is large
      settings.WriteDebugLog(time.Now())
      settings.WriteDebugLog(n / 1024 / 1024)
      responseCode = http.StatusOK
      settings.WriteLog(r, responseCode, nil, GetCurrentFuncName())
      return
  Error:
      WriteErrorResponse(w, responseCode, err)
      settings.WriteLog(r, responseCode, err, GetCurrentFuncName())
  }
  </code></pre>
  
  <p>But if it is a large file (more than 1GB), the io.Copy takes quite a long time (best case: 1G 13 seconds shown below) and the download process in browser doesn't show up until the process of io.Copy completes. Is there a way to achieve my goal more efficiently? I want the download process to show up in my browser right after I send the download request. </p>
  
  <pre><code>2017/05/16 22:32:29 Debug: 2017-05-16 22:32:29.967499719 +0900 JST
  2017/05/16 22:32:36 Debug: 2017-05-16 22:32:36.980914163 +0900 JST
  2017/05/16 22:32:36 Debug: 1097
  </code></pre>
  
  <p>I tried calling io.Copy with "go io.Copy" then the download process shows up soon but the file size is 0.</p>