Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/util/contentutil/copy.go

@@ -0,0 +1,81 @@
+package contentutil
+
+import (
+	"context"
+	"io"
+	"sync"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/remotes"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+)
+
+func Copy(ctx context.Context, ingester content.Ingester, provider content.Provider, desc ocispec.Descriptor) error {
+	if _, err := remotes.FetchHandler(ingester, &localFetcher{provider})(ctx, desc); err != nil {
+		return err
+	}
+	return nil
+}
+
+type localFetcher struct {
+	content.Provider
+}
+
+func (f *localFetcher) Fetch(ctx context.Context, desc ocispec.Descriptor) (io.ReadCloser, error) {
+	r, err := f.Provider.ReaderAt(ctx, desc)
+	if err != nil {
+		return nil, err
+	}
+	return &rc{ReaderAt: r}, nil
+}
+
+type rc struct {
+	content.ReaderAt
+	offset int
+}
+
+func (r *rc) Read(b []byte) (int, error) {
+	n, err := r.ReadAt(b, int64(r.offset))
+	r.offset += n
+	if n > 0 && err == io.EOF {
+		err = nil
+	}
+	return n, err
+}
+
+func CopyChain(ctx context.Context, ingester content.Ingester, provider content.Provider, desc ocispec.Descriptor) error {
+	var m sync.Mutex
+	manifestStack := []ocispec.Descriptor{}
+
+	filterHandler := images.HandlerFunc(func(ctx context.Context, desc ocispec.Descriptor) ([]ocispec.Descriptor, error) {
+		switch desc.MediaType {
+		case images.MediaTypeDockerSchema2Manifest, ocispec.MediaTypeImageManifest,
+			images.MediaTypeDockerSchema2ManifestList, ocispec.MediaTypeImageIndex:
+			m.Lock()
+			manifestStack = append(manifestStack, desc)
+			m.Unlock()
+			return nil, images.ErrStopHandler
+		default:
+			return nil, nil
+		}
+	})
+	handlers := []images.Handler{
+		images.ChildrenHandler(provider),
+		filterHandler,
+		remotes.FetchHandler(ingester, &localFetcher{provider}),
+	}
+
+	if err := images.Dispatch(ctx, images.Handlers(handlers...), nil, desc); err != nil {
+		return errors.WithStack(err)
+	}
+
+	for i := len(manifestStack) - 1; i >= 0; i-- {
+		if err := Copy(ctx, ingester, provider, manifestStack[i]); err != nil {
+			return errors.WithStack(err)
+		}
+	}
+
+	return nil
+}