ioutil.ReadAll为tar读取器提供0个字节

Question

I create a tar file in memory:

var buf bytes.Buffer
tw := tar.NewWriter(&buf)
files := map[string][]byte{
    "1.txt": []byte("11"),
    "2.txt": []byte("2"),
}
for fileName, bStr := range files {
    b := []byte(bStr)
    hdr := &tar.Header{
        Name: fileName,
        Mode: 0600,
        Size: int64(len(b)),
    }
    log.Printf("include the file to the tar %+v
", hdr)
    if err := tw.WriteHeader(hdr); err != nil {
        err := fmt.Errorf("couldn't write a header: %+v", err)
        log.Println(err)
        return
    }
    if _, err := tw.Write(b); err != nil {
        err := fmt.Errorf("couldn't write file %+v to tar: %+v", fileName, err)
        log.Println(err)
        return
    }
}
if err := tw.Close(); err != nil {
    log.Println(err)
    return
}

But unable to convert this tar file to bytes. playground 1

    b, err := ioutil.ReadAll(tr)
    if err != nil {
        err = fmt.Errorf("could not read file data")
        log.Println(err)
        return
    }
    log.Printf("read %+v bytes", len(b))

It just prints 0 bytes:

2018/06/26 16:57:04 main.go:67: read 0 bytes

But I can read the content of the tar, file by file:

for {
    hdr, err := tr.Next()
    if err == io.EOF {
        break // End of archive
    }
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("Contents of %s:
", hdr.Name)
    if _, err := io.Copy(os.Stdout, tr); err != nil {
        log.Fatal(err)
    }
    fmt.Println()
}

playground 2

It prints

Contents of 2.txt:
2
Contents of 1.txt:
11

as expected.

So it means that my tar file is not empty. I just need to convert it to bytes for further pushing to glcoud storage. But looks like I mess around with readers and writers and can hardly figure out how to convert tar to bytes in the memory.

Answer 1

The program writes the archive to a bytes.Buffer. Call Buffer.Bytes() to get the archive bytes.