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/cache/contenthash/tarsum.go

@@ -0,0 +1,63 @@
+package contenthash
+
+import (
+	"archive/tar"
+	"io"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// WriteV1TarsumHeaders writes a tar header to a writer in V1 tarsum format.
+func WriteV1TarsumHeaders(h *tar.Header, w io.Writer) {
+	for _, elem := range v1TarHeaderSelect(h) {
+		w.Write([]byte(elem[0] + elem[1]))
+	}
+}
+
+// Functions below are from docker legacy tarsum implementation.
+// There is no valid technical reason to continue using them.
+
+func v0TarHeaderSelect(h *tar.Header) (orderedHeaders [][2]string) {
+	return [][2]string{
+		{"name", h.Name},
+		{"mode", strconv.FormatInt(h.Mode, 10)},
+		{"uid", strconv.Itoa(h.Uid)},
+		{"gid", strconv.Itoa(h.Gid)},
+		{"size", strconv.FormatInt(h.Size, 10)},
+		{"mtime", strconv.FormatInt(h.ModTime.UTC().Unix(), 10)},
+		{"typeflag", string([]byte{h.Typeflag})},
+		{"linkname", h.Linkname},
+		{"uname", h.Uname},
+		{"gname", h.Gname},
+		{"devmajor", strconv.FormatInt(h.Devmajor, 10)},
+		{"devminor", strconv.FormatInt(h.Devminor, 10)},
+	}
+}
+
+func v1TarHeaderSelect(h *tar.Header) (orderedHeaders [][2]string) {
+	// Get extended attributes.
+	xAttrKeys := make([]string, len(h.Xattrs))
+	for k := range h.Xattrs {
+		if k == "security.capability" || !strings.HasPrefix(k, "security.") && !strings.HasPrefix(k, "system.") {
+			xAttrKeys = append(xAttrKeys, k)
+		}
+	}
+	sort.Strings(xAttrKeys)
+
+	// Make the slice with enough capacity to hold the 11 basic headers
+	// we want from the v0 selector plus however many xattrs we have.
+	orderedHeaders = make([][2]string, 0, 11+len(xAttrKeys))
+
+	// Copy all headers from v0 excluding the 'mtime' header (the 5th element).
+	v0headers := v0TarHeaderSelect(h)
+	orderedHeaders = append(orderedHeaders, v0headers[0:5]...)
+	orderedHeaders = append(orderedHeaders, v0headers[6:]...)
+
+	// Finally, append the sorted xattrs.
+	for _, k := range xAttrKeys {
+		orderedHeaders = append(orderedHeaders, [2]string{k, h.Xattrs[k]})
+	}
+
+	return
+}