unpack/repack containerd snapshot manually

main.go

package main

import (
	"context"
	"fmt"
	"log"
	"os"
	"time"

	"github.com/containerd/containerd"
	"github.com/containerd/containerd/mount"
	"github.com/containerd/containerd/namespaces"
	"github.com/containerd/containerd/snapshots"
	"github.com/moby/moby/pkg/archive"
)

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

func run() error {
	ctx := namespaces.WithNamespace(context.Background(), "example")

	client, err := containerd.New("/run/containerd/containerd.sock")
	if err != nil {
		return err
	}
	defer client.Close()

	snapshotter := client.SnapshotService("overlayfs")
	tmpDir := "/tmp/unpack"

	err = os.MkdirAll(tmpDir, 0666)
	if err != nil {
		return err
	}

	defer os.RemoveAll(tmpDir)

	layerPath := "/tmp/layer/layer.tar" // just a path to layer tar file.
	noGcOpt := snapshots.WithLabels(map[string]string{
		"containerd.io/gc.root": time.Now().UTC().Format(time.RFC3339),
	})

	mounts, err := snapshotter.Prepare(ctx, "foo", "", noGcOpt)
	if err != nil {
		return fmt.Errorf("failed to prepare snapshot: %v", err)
	}

	fmt.Printf("%#+v\n", mounts)

	if err := mount.All(mounts, tmpDir); err != nil {
		return fmt.Errorf("failed to mount all: %v", err)
	}
	defer mount.UnmountAll(tmpDir, 0)

	layer, err := os.Open(layerPath)
	if err != nil {
		return fmt.Errorf("failed to open layer: %v", err)
	}

	_, err = archive.UnpackLayer(tmpDir, layer, nil) // unpack into layer location
	if err != nil {
		return fmt.Errorf("failed to unpack layer: %v", err)
	}

	// tmpDir would have the unpacked layer.
	// it would be a bindmount to an active snapshot dir

	if err := snapshotter.Commit(ctx, "bar", "foo", noGcOpt); err != nil {
		return fmt.Errorf("failed to commit snapshot: %v", err)
	}

	// at this point the original snapshot is gone, but the new one could be seen in storage
	// ctr -n example snapshot ls
	// to clean up:
	// ctr -n example snapshot rm foo

	return nil
}