fastest way to extract tar files in side tar file using GO

I have a tar file that contains multiple tar files in it. I'm currently extracting these tars recursively using the tar Reader by moving manually over the files. This process is very heavy and slow, especially when dealing with large tar files that contain thousands of files and directories.

I didn't find any good package that is able to do this recursive extraction fast. plus I tried using the command tar -xf file.tar --same-owner" for the inner tars, but had a problem with permissions issue (which happens only on mac).

my question is:
Is there a way to parallelize the manual extraction process so that the inner tars will be extracted in parallel?

I have a method for the extraction task which I'm trying to make parallel:

	var wg sync.WaitGroup
	wg.Add(len(tarFiles))

	for {
		header, err := tarBallReader.Next()
		if err != nil {
			break
		}
		go extractFileAsync(parentFolder, header, tarBallReader, depth, &wg)
	}
	wg.Wait()

after adding the go routines, the files are getting corrupted and the process is stuck on an endless loop.

example of the main tar content:

1d2755f3375860aaaf2b5f0474692df2e0d4329569c1e8187595bf4b3bf3f3b9/
1d2755f3375860aaaf2b5f0474692df2e0d4329569c1e8187595bf4b3bf3f3b9/VERSION
1d2755f3375860aaaf2b5f0474692df2e0d4329569c1e8187595bf4b3bf3f3b9/json
1d2755f3375860aaaf2b5f0474692df2e0d4329569c1e8187595bf4b3bf3f3b9/layer.tar
348188998f2a69b4ac0ca96b42990292eef67c0abfa05412e2fb7857645f4280/
348188998f2a69b4ac0ca96b42990292eef67c0abfa05412e2fb7857645f4280/VERSION
348188998f2a69b4ac0ca96b42990292eef67c0abfa05412e2fb7857645f4280/json
348188998f2a69b4ac0ca96b42990292eef67c0abfa05412e2fb7857645f4280/layer.tar
54c027bf04447fdb035ddc13a6ae5493a3f997bdd3577607b0980954522efb9e.json
9dd3c29af50daaf86744a8ade86ecf12f6a5a6ffc27a5a7398628e4a21770ee3/
9dd3c29af50daaf86744a8ade86ecf12f6a5a6ffc27a5a7398628e4a21770ee3/VERSION
9dd3c29af50daaf86744a8ade86ecf12f6a5a6ffc27a5a7398628e4a21770ee3/json
9dd3c29af50daaf86744a8ade86ecf12f6a5a6ffc27a5a7398628e4a21770ee3/layer.tar
b6c49400b643245cdbe17b7a7eb14f0f7def5a93326b99560241715c1e95502e/
b6c49400b643245cdbe17b7a7eb14f0f7def5a93326b99560241715c1e95502e/VERSION
b6c49400b643245cdbe17b7a7eb14f0f7def5a93326b99560241715c1e95502e/json
b6c49400b643245cdbe17b7a7eb14f0f7def5a93326b99560241715c1e95502e/layer.tar
c662ec0dc487910e7b76b2a4d67ab1a9ca63ce1784f636c2637b41d6c7ac5a1e/
c662ec0dc487910e7b76b2a4d67ab1a9ca63ce1784f636c2637b41d6c7ac5a1e/VERSION
c662ec0dc487910e7b76b2a4d67ab1a9ca63ce1784f636c2637b41d6c7ac5a1e/json
c662ec0dc487910e7b76b2a4d67ab1a9ca63ce1784f636c2637b41d6c7ac5a1e/layer.tar
da87454b77f6ac7fab1f465c10a07a1eb4b46df8058d98892794618cac8eacdc/
da87454b77f6ac7fab1f465c10a07a1eb4b46df8058d98892794618cac8eacdc/VERSION
da87454b77f6ac7fab1f465c10a07a1eb4b46df8058d98892794618cac8eacdc/json
da87454b77f6ac7fab1f465c10a07a1eb4b46df8058d98892794618cac8eacdc/layer.tar
ea1c2adfdc777d8746e50ad3e679789893a991606739c9bc7e01f273fa0b6e12/
ea1c2adfdc777d8746e50ad3e679789893a991606739c9bc7e01f273fa0b6e12/VERSION
ea1c2adfdc777d8746e50ad3e679789893a991606739c9bc7e01f273fa0b6e12/json
ea1c2adfdc777d8746e50ad3e679789893a991606739c9bc7e01f273fa0b6e12/layer.tar
f3b6608e814053048d79e519be79f654a2e9364dfdc8fb87b71e2fc57bbff115/
f3b6608e814053048d79e519be79f654a2e9364dfdc8fb87b71e2fc57bbff115/VERSION
f3b6608e814053048d79e519be79f654a2e9364dfdc8fb87b71e2fc57bbff115/json
f3b6608e814053048d79e519be79f654a2e9364dfdc8fb87b71e2fc57bbff115/layer.tar
manifest.json
repositories

or simply you can run docker save <image>:<tag> -o image.tar and check the content of the tar.

Probably your code hangs on wg.Wait() due to the fact that the number of calls to wg.Done() during execution is not equal to len(tarFiles).

That should work:

    var wg sync.WaitGroup
    // wg.Add(len(tarFiles))

    for {
        header, err := tarBallReader.Next()
        if err != nil {
            break
        }
        wg.Add(1)
        go extractFileAsync(parentFolder, header, tarBallReader, depth, &wg)
    }
    wg.Wait()

func extractFileAsync(...) {
    defer wg.Done()

    // some code

}

UPD: correction of a possible race condition. Thanks @craigb

Here is my solution to a similar problem (simplified):

package main

import (
	"archive/tar"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"
	"sync"
)

type Semaphore struct {
	Wg sync.WaitGroup
	Ch chan int
}

// Limit on the number of simultaneously running goroutines.
// Depends on the number of processor cores, storage performance, amount of RAM, etc.
const grMax = 10

const tarFileName = "docker_image.tar"
const dstDir = "output/docker"

func extractTar(tarFileName string, dstDir string) error {
	f, err := os.Open(tarFileName)
	if err != nil {
		return err
	}

	sem := Semaphore{}
	sem.Ch = make(chan int, grMax)

	if err := Untar(dstDir, f, &sem, true); err != nil {
		return err
	}

	fmt.Println("extractTar: wait for complete")
	sem.Wg.Wait()
	return nil
}


func Untar(dst string, r io.Reader, sem *Semaphore, godeep bool) error {

	tr := tar.NewReader(r)

	for {
		header, err := tr.Next()

		switch {
		case err == io.EOF:
			return nil
		case err != nil:
			return err
		}

		// the target location where the dir/file should be created
		target := filepath.Join(dst, header.Name)

		switch header.Typeflag {

		// if its a dir and it doesn't exist create it
		case tar.TypeDir:
			if _, err := os.Stat(target); err != nil {
				if err := os.MkdirAll(target, 0755); err != nil {
					return err
				}
			}

		// if it's a file create it
		case tar.TypeReg:
			if err := saveFile(tr, target, os.FileMode(header.Mode)); err != nil {
				return err
			}
			ext := filepath.Ext(target)

			// if it's tar file and we are on top level, extract it
			if ext == ".tar" && godeep {
				sem.Wg.Add(1)
				// A buffered channel is used to limit the number of simultaneously running goroutines
				sem.Ch <- 1
				// the file is unpacked to a directory with the file name (without extension)
				newDir := filepath.Join(dst, strings.TrimSuffix(header.Name, ".tar"))
				if err := os.Mkdir(newDir, 0755); err != nil {
					return err
				}
				go func(target string, newDir string, sem *Semaphore) {
					fmt.Println("start goroutine, chan length:", len(sem.Ch))
					fmt.Println("START:", target)
					defer sem.Wg.Done()
					defer func() {<-sem.Ch}()
					// the internal tar file opens
					ft, err := os.Open(target)
					if err != nil {
						fmt.Println(err)
						return
					}
					defer ft.Close()
					// the godeep parameter is false here to avoid unpacking archives inside the current archive.
					if err := Untar(newDir, ft, sem, false); err != nil {
						fmt.Println(err)
						return
					}
					fmt.Println("DONE:", target)
				}(target, newDir, sem)
			}
		}
	}
	return nil
}

func saveFile(r io.Reader, target string, mode os.FileMode) error {
	f, err := os.OpenFile(target, os.O_CREATE|os.O_RDWR, mode)
	if err != nil {
		return err
	}
	defer f.Close()

	if _, err := io.Copy(f, r); err != nil {
		return err
	}
	
	return nil
}

func main() {
	err := extractTar(tarFileName, dstDir)
	if err != nil {
		fmt.Println(err)
	}
}