pkg/tarsum: migrate to (tar.Header).PAXRecords

Fix a silly bug in the implementation which had the effect of
len(h.Xattrs) blank entries being inserted in the middle of
orderedHeaders. Luckily this is not a load-bearing bug: empty headers
are ignored as the tarsum digest is computed by concatenating header
keys and values without any intervening delimiter.

Signed-off-by: Cory Snider <csnider@mirantis.com>

pkg/tarsum/versioning.go

@@ -115,15 +115,18 @@ func v0TarHeaderSelect(h *tar.Header) (orderedHeaders [][2]string) {
 
 func v1TarHeaderSelect(h *tar.Header) (orderedHeaders [][2]string) {
 	// Get extended attributes.
-	xAttrKeys := make([]string, len(h.Xattrs))
-	for k := range h.Xattrs {
-		xAttrKeys = append(xAttrKeys, k)
+	const paxSchilyXattr = "SCHILY.xattr."
+	var xattrs [][2]string
+	for k, v := range h.PAXRecords {
+		if xattr, ok := strings.CutPrefix(k, paxSchilyXattr); ok {
+			xattrs = append(xattrs, [2]string{xattr, v})
+		}
 	}
-	sort.Strings(xAttrKeys)
+	sort.Slice(xattrs, func(i, j int) bool { return xattrs[i][0] < xattrs[j][0] })
 
 	// 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))
+	orderedHeaders = make([][2]string, 0, 11+len(xattrs))
 
 	// Copy all headers from v0 excluding the 'mtime' header (the 5th element).
 	v0headers := v0TarHeaderSelect(h)
@@ -131,9 +134,7 @@ func v1TarHeaderSelect(h *tar.Header) (orderedHeaders [][2]string) {
 	orderedHeaders = append(orderedHeaders, v0headers[6:]...)
 
 	// Finally, append the sorted xattrs.
-	for _, k := range xAttrKeys {
-		orderedHeaders = append(orderedHeaders, [2]string{k, h.Xattrs[k]})
-	}
+	orderedHeaders = append(orderedHeaders, xattrs...)
 
 	return
 }