Golang multipart/form-data File Upload

file-upload-multipart.go

package main

import (
  "net/http"
  "os"
  "bytes"
  "path"
  "path/filepath"
  "mime/multipart"
  "io"
)

func main() {
  fileDir, _ := os.Getwd()
  fileName := "upload-file.txt"
  filePath := path.Join(fileDir, fileName)

  file, _ := os.Open(filePath)
  defer file.Close()

  body := &bytes.Buffer{}
  writer := multipart.NewWriter(body)
  part, _ := writer.CreateFormFile("file", filepath.Base(file.Name()))
  io.Copy(part, file)
  writer.Close()

  r, _ := http.NewRequest("POST", "http://example.com", body)
  r.Header.Add("Content-Type", writer.FormDataContentType())
  client := &http.Client{}
  client.Do(r)
}

Will this code use sendfile to achieve zero copy?

No It doesn't.
If you want to achieve zero copy, you can use os.pipe instead of sendfile.
sendfile is fit to tcp package so, it is hard to use with http package.
See this code snippet and benchmarks of it.