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/solver/llbsolver/result.go

@@ -0,0 +1,74 @@
+package llbsolver
+
+import (
+	"bytes"
+	"context"
+	"path"
+
+	"github.com/moby/buildkit/cache/contenthash"
+	"github.com/moby/buildkit/solver"
+	"github.com/moby/buildkit/worker"
+	digest "github.com/opencontainers/go-digest"
+	"github.com/pkg/errors"
+	"golang.org/x/sync/errgroup"
+)
+
+type Selector struct {
+	Path        string
+	Wildcard    bool
+	FollowLinks bool
+}
+
+func NewContentHashFunc(selectors []Selector) solver.ResultBasedCacheFunc {
+	return func(ctx context.Context, res solver.Result) (digest.Digest, error) {
+		ref, ok := res.Sys().(*worker.WorkerRef)
+		if !ok {
+			return "", errors.Errorf("invalid reference: %T", res)
+		}
+
+		if len(selectors) == 0 {
+			selectors = []Selector{{}}
+		}
+
+		dgsts := make([][]byte, len(selectors))
+
+		eg, ctx := errgroup.WithContext(ctx)
+
+		for i, sel := range selectors {
+			// FIXME(tonistiigi): enabling this parallelization seems to create wrong results for some big inputs(like gobuild)
+			// func(i int) {
+			// 	eg.Go(func() error {
+			if !sel.Wildcard {
+				dgst, err := contenthash.Checksum(ctx, ref.ImmutableRef, path.Join("/", sel.Path), sel.FollowLinks)
+				if err != nil {
+					return "", err
+				}
+				dgsts[i] = []byte(dgst)
+			} else {
+				dgst, err := contenthash.ChecksumWildcard(ctx, ref.ImmutableRef, path.Join("/", sel.Path), sel.FollowLinks)
+				if err != nil {
+					return "", err
+				}
+				dgsts[i] = []byte(dgst)
+			}
+			// return nil
+			// })
+			// }(i)
+		}
+
+		if err := eg.Wait(); err != nil {
+			return "", err
+		}
+
+		return digest.FromBytes(bytes.Join(dgsts, []byte{0})), nil
+	}
+}
+
+func workerRefConverter(ctx context.Context, res solver.Result) (*solver.Remote, error) {
+	ref, ok := res.Sys().(*worker.WorkerRef)
+	if !ok {
+		return nil, errors.Errorf("invalid result: %T", res.Sys())
+	}
+
+	return ref.Worker.GetRemote(ctx, ref.ImmutableRef, true)
+}