io.TeeReader versus io.MultiWriter

multiwriter.go

package main

import (
	"bytes"
	"crypto/md5"
	"encoding/hex"
	"fmt"
	"io"
	"io/ioutil"
	"os"
)

func main() {
	sourceFile, _ := os.Open("source/ebook.pdf")

	process := func(sourceReader io.Reader) {
		targetFile, _ := os.Create("target/ebook.pdf")
		defer targetFile.Close()

		var buf1, buf2 bytes.Buffer
		w := io.MultiWriter(targetFile, &buf1, &buf2)

		if _, err := io.Copy(w, sourceReader); err != nil {
			fmt.Println(err)
		}

		fmt.Println(checksum(&buf1))
		fmt.Println(checksum(&buf2))
	}

	process(sourceFile)

}

func checksum(buf *bytes.Buffer) string {
	h := md5.New()
	b, _ := ioutil.ReadAll(buf)
	if _, err := h.Write(b); err != nil {
		fmt.Println(err)
	}
	return hex.EncodeToString(h.Sum(nil)[:16])
}

teereader.go

package main

import (
	"bytes"
	"crypto/md5"
	"encoding/hex"
	"fmt"
	"io"
	"io/ioutil"
	"os"
)

func main() {
	sourceFile, _ := os.Open("source/ebook.pdf")

	var buf bytes.Buffer
	tee := io.TeeReader(sourceFile, &buf)

	process := func(sourceReader io.Reader) {
		targetFile, _ := os.Create("target/ebook.pdf")
		defer targetFile.Close()
		if _, err := io.Copy(targetFile, sourceReader); err != nil {
			fmt.Println(err)
		}
	}

	process(tee)
	fmt.Println(checksum(&buf))
}

func checksum(buf *bytes.Buffer) string {
	h := md5.New()
	b, _ := ioutil.ReadAll(buf)
	if _, err := h.Write(b); err != nil {
		fmt.Println(err)
	}
	return hex.EncodeToString(h.Sum(nil)[:16])
}