A code to upload multiple files with multipart with small ram usage

multifile-upload.go

package main

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"mime/multipart"
	"net/http"
	"os"
)

func main() {
	url := "https://postman-echo.com/post" // https://docs.postman-echo.com/?version=latest
	params := make(map[string]string)
	params["test1"] = "param1"
	params["test2"] = "param2"
	filename1_field := "file1"
	filename1 := "test.txt"
	filename2_field := "file2"
	filename2 := "file2.txt" // however this file will not be read from disk
	dataToSend := []byte("Hello!") // insted, send this from memory

	r, w := io.Pipe() // use pipe to transfer the data without high RAM usage
	m := multipart.NewWriter(w) // create a multipart structure that will use io.Pipe to write to http request
	// writing to pipe must be done inside a goroutine; otherwise the program will panic because of deadlock
	go func() {
		defer w.Close() // you have to at first close the multipart then the pipe
		defer m.Close() // note that this line runes first
		// write parameters
		for k, v := range params {
			_ = m.WriteField(k, v)
		}

		// send the first file that is present on the disk
		part, err := m.CreateFormFile(filename1_field, filename1)
		if err != nil{
			log.Fatal(err)
		}
		file1, _ := os.Open(filename1) // open the file to read from it
		io.Copy(part,file1) // copy the file to multipart; The multipart will use io.pipe to write them to the destination; No worries about the RAM, the buffer size is 32KB
		file1.Close()
		
		// now upload the second file from memory
		part2, err := m.CreateFormFile(filename2_field, filename2)
		if err != nil{
			log.Fatal(err)
		}
		io.Copy(part2,bytes.NewReader(dataToSend))
	}()

	req, _ := http.NewRequest("POST", url, r) // as you can see I have passed the pipe reader here
	req.Header.Set("Content-Type", m.FormDataContentType())
	resp, err := http.DefaultClient.Do(req) // do the request. The program will stop here until the upload is done
	if err != nil{
		log.Fatal(err)
	}
	data,_ := ioutil.ReadAll(resp.Body) // read the results
	fmt.Println(string(data)) // display the results
}

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