Chaining io.Writers in Go: https://medium.com/@skdomino/writing-on-the-train-chaining-io-writers-in-go-1b39e07f71c9

publish.go

package main

import (
	"archive/tar"
	"compress/gzip"
	"crypto/md5"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"sync"

	api "github.com/pagodabox/nanobox-api-client"
)

//
var tw *tar.Writer

//
func main() {

	//
	h := md5.New()

	//
	pr, pw := io.Pipe()

	//
	mw := io.MultiWriter(h, pw)

	//
	gzw := gzip.NewWriter(mw)

	//
	tw = tar.NewWriter(gzw)

	//
	wg := &sync.WaitGroup{}

	wg.Add(1)

	//
	go func() {

		// defer is LIFO
		defer pw.Close()
		defer gzw.Close()
		defer tw.Close()

		for _, pf := range release.ProjectFiles {
			if stat, err := os.Stat(pf); err == nil {
				// if its a directory, walk the directory taring each file
				if stat.Mode().IsDir() {
					if err := filepath.Walk(pf, tarDir); err != nil {
						fmt.Println("ERROR", err)
					}

					// if its a file tar it
				} else {
					tarFile(pf)
				}
			}
		}

		wg.Done()
	}()

	obj := &Object{}

	// upload tarball
	if err := api.DoRawRequest(obj, "POST", "/server/uploads", pr, nil); err != nil {
		fmt.Println("ERROR", err)
	}

	wg.Wait()

	defer pr.Close()
}

// tarDir
func tarDir(path string, fi os.FileInfo, err error) error {
	if fi.Mode().IsDir() {
		return nil
	}

	return tarFile(path)
}

// tarFile
func tarFile(path string) error {

	// open the file/dir...
	f, err := os.Open(path)
	if err != nil {
		return err
	}
	defer f.Close()

	// stat the file
	if fi, err := f.Stat(); err == nil {

		// create header for this file
		header := &tar.Header{
			Name:    path,
			Size:    fi.Size(),
			Mode:    int64(fi.Mode()),
			ModTime: fi.ModTime(),
		}

		// write the header to the tarball archive
		if err := tw.WriteHeader(header); err != nil {
			return err
		}

		// copy the file data to the tarball
		if _, err := io.Copy(tw, f); err != nil {
			return err
		}
	}

	return nil
}