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rootfs.go
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rootfs.go
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package demo
import (
"archive/tar"
"compress/gzip"
"encoding/json"
"fmt"
"io"
"net/http"
"os"
"os/exec"
"path/filepath"
"runtime"
"strings"
_ "crypto/sha256" // register SHA256 hash for digest
digest "github.com/opencontainers/go-digest"
specs "github.com/opencontainers/image-spec/specs-go"
v1 "github.com/opencontainers/image-spec/specs-go/v1"
)
const (
freeBSDDownload = "https://download.freebsd.org/ftp/releases/%s/%s/%s/base.txz"
)
// DownloadRootfs downloads a FreeBSD root filesystem
func DownloadRootfs(machine, arch, version string) (io.ReadCloser, int64, error) {
req, err := http.Get(fmt.Sprintf(freeBSDDownload, machine, arch, version))
if err != nil {
return nil, 0, err
}
if req.StatusCode < 200 || req.StatusCode > 299 {
return nil, 0, fmt.Errorf("download: unexpected status %s (%d)", req.Status, req.StatusCode)
}
return req.Body, req.ContentLength, nil
}
// MakeImage constructs a single-layer FreeBSD OCI image from a given input tar
func MakeImage(rootfsFilename string, outputFilename string, arch string) error {
tempDir, err := os.MkdirTemp("", "runj-demo-rootfs-")
if err != nil {
return err
}
defer os.RemoveAll(tempDir)
// setup directory structure
imageDir := filepath.Join(tempDir, "image")
blobDir := filepath.Join(imageDir, "blobs", "sha256")
err = os.MkdirAll(blobDir, 0755)
if err != nil {
return err
}
// extract
fmt.Println("extracting...")
err = unxz(rootfsFilename, filepath.Join(tempDir, "rootfs.tar"))
if err != nil {
return err
}
layerDigest, err := digestFile(filepath.Join(tempDir, "rootfs.tar"))
if err != nil {
return err
}
// re-compress
fmt.Println("compressing...")
layer, err := os.OpenFile(filepath.Join(tempDir, "rootfs.tar"), os.O_RDONLY, 0)
if err != nil {
return err
}
defer layer.Close()
compressedLayer, err := os.OpenFile(filepath.Join(tempDir, "rootfs.tar.gz"), os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0644)
if err != nil {
return err
}
defer compressedLayer.Close()
compressedWriter := gzip.NewWriter(compressedLayer)
_, err = io.Copy(compressedWriter, layer)
if err != nil {
return err
}
compressedWriter.Close()
compressedLayer.Sync()
compressedInfo, err := compressedLayer.Stat()
if err != nil {
return err
}
fmt.Println("computing layer digest...")
compressedDigest, err := digestFile(filepath.Join(tempDir, "rootfs.tar.gz"))
if err != nil {
return err
}
// blobs
err = os.Rename(filepath.Join(tempDir, "rootfs.tar.gz"), filepath.Join(blobDir, compressedDigest.Encoded()))
if err != nil {
return err
}
// oci-layout
err = os.WriteFile(filepath.Join(imageDir, "oci-layout"), []byte(`{"imageLayoutVersion": "1.0.0"}`), 0644)
if err != nil {
return err
}
// config
const freebsd = "freebsd"
if arch == "" {
arch = runtime.GOARCH
}
config := v1.Image{
Architecture: arch,
OS: freebsd,
RootFS: v1.RootFS{
Type: "layers",
DiffIDs: []digest.Digest{layerDigest},
},
Author: "runj <runj@sbk.wtf>",
}
configJSON, err := json.Marshal(config)
if err != nil {
return err
}
configDigest, err := writeBlob(blobDir, configJSON)
if err != nil {
return err
}
// manifest
manifest := v1.Manifest{
Versioned: specs.Versioned{SchemaVersion: 2},
Config: v1.Descriptor{
MediaType: v1.MediaTypeImageConfig,
Digest: configDigest,
Size: int64(len(configJSON)),
},
Layers: []v1.Descriptor{{
MediaType: v1.MediaTypeImageLayerGzip,
Digest: compressedDigest,
Size: compressedInfo.Size(),
}},
}
manifestJSON, err := json.Marshal(manifest)
if err != nil {
return err
}
manifestDigest, err := writeBlob(blobDir, manifestJSON)
if err != nil {
return err
}
// index
index := v1.Index{
Versioned: specs.Versioned{SchemaVersion: 2},
Manifests: []v1.Descriptor{{
MediaType: v1.MediaTypeImageManifest,
Digest: manifestDigest,
Size: int64(len(manifestJSON)),
Platform: &v1.Platform{
Architecture: arch,
OS: freebsd,
},
}},
}
indexJSON, err := json.Marshal(index)
if err != nil {
return err
}
// image index
err = os.WriteFile(filepath.Join(imageDir, "index.json"), indexJSON, 0644)
if err != nil {
return err
}
// tar
fmt.Println("tar...")
return makeTar(imageDir, outputFilename)
}
func writeBlob(blobDir string, blob []byte) (digest.Digest, error) {
d := digest.FromBytes(blob)
fmt.Println("writing blob", string(d))
err := os.WriteFile(filepath.Join(blobDir, d.Encoded()), blob, 0644)
if err != nil {
return "", err
}
return d, nil
}
func unxz(in, out string) error {
outFile, err := os.OpenFile(out, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0644)
if err != nil {
return err
}
defer func() {
outFile.Close()
if err != nil {
os.Remove(out)
}
}()
inFile, err := os.OpenFile(in, os.O_RDONLY, 0)
if err != nil {
return err
}
defer inFile.Close()
xz := exec.Command("xz", "--decompress", "-")
xz.Stdin = inFile
xz.Stdout = outFile
xz.Stderr = os.Stderr
return xz.Run()
}
func digestFile(in string) (digest.Digest, error) {
f, err := os.OpenFile(in, os.O_RDONLY, 0)
if err != nil {
return "", err
}
defer f.Close()
return digest.FromReader(f)
}
func makeTar(in string, out string) error {
outFile, err := os.OpenFile(out, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0644)
if err != nil {
return err
}
defer func() {
outFile.Close()
if err != nil {
os.Remove(out)
}
}()
tw := tar.NewWriter(outFile)
defer tw.Close()
prefix := in + string(os.PathSeparator)
return filepath.Walk(in, func(path string, info os.FileInfo, err error) error {
if err != nil {
return err
}
if info.IsDir() {
return nil
}
name := strings.TrimPrefix(path, prefix)
hdr := &tar.Header{
Name: name,
Size: info.Size(),
Mode: 0644,
}
err = tw.WriteHeader(hdr)
if err != nil {
return err
}
f, err := os.OpenFile(path, os.O_RDONLY, 0)
if err != nil {
return err
}
defer f.Close()
_, err = io.Copy(tw, f)
return err
})
}