Fix legacy ref files showing as orphans in cache list

Legacy-format ref files (written by v0.0.24, digest-only) were not
recognized by buildRefMap(), causing List() to report them as orphans
and gc --dry-run to incorrectly claim they would be deleted (while
actual GC via liveDigests() correctly preserved them).

Two fixes:
- LookupRef now upgrades legacy refs to extended format on cache hit
- buildRefMap now marks legacy refs as referenced with a placeholder

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JAORMX

image/cache.go

@@ -327,11 +327,14 @@ func (c *Cache) buildRefMap() map[string][]string {
 		if digest == "" {
 			continue
 		}
-		// Skip empty image refs from legacy-format files. The entry still
-		// counts as referenced for GC (via liveDigests), but we don't add
-		// an empty string to the Refs slice.
 		if imageRef != "" {
 			refMap[digest] = append(refMap[digest], imageRef)
+		} else if _, exists := refMap[digest]; !exists {
+			// Legacy-format ref (digest only, no image name). Mark the
+			// digest as referenced so it is not reported as orphaned.
+			// The placeholder is replaced once the ref is upgraded to
+			// extended format (e.g. on next LookupRef hit).
+			refMap[digest] = []string{"(unknown image)"}
 		}
 	}
 	return refMap
@@ -424,6 +427,10 @@ func (c *Cache) LookupRef(imageRef string) *RootFS {
 		return nil
 	}
 
+	// Upgrade legacy-format ref files (digest only) to extended format
+	// (imageRef\tdigest) so that List/GC can recover the image name.
+	c.putRef(imageRef, digest)
+
 	rootfsPath, ok := c.Get(digest)
 	if !ok {
 		return nil

image/cache_test.go

@@ -353,10 +353,10 @@ func TestCache_List_LegacyRefFormat(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, entries, 1)
 
-	// Legacy format: entry is referenced (not orphaned) but imageRef
-	// is not recoverable, so Refs is empty (no empty strings).
+	// Legacy format: entry is referenced (not orphaned) but the original
+	// image name is not recoverable, so a placeholder is used.
 	assert.Equal(t, "sha256:legacy", entries[0].Digest)
-	assert.Empty(t, entries[0].Refs)
+	assert.Equal(t, []string{"(unknown image)"}, entries[0].Refs)
 }
 
 // --- GC tests ---
@@ -584,6 +584,47 @@ func TestCache_GC_CorruptRefFiles(t *testing.T) {
 	assert.Equal(t, 1, removed)
 }
 
+func TestLookupRef_UpgradesLegacyRef(t *testing.T) {
+	t.Parallel()
+
+	cacheDir := t.TempDir()
+	c := NewCache(cacheDir)
+
+	digest := "sha256:legacyupgrade"
+	imageRef := "ghcr.io/org/image:latest"
+
+	// Create a rootfs entry with an OCI config.
+	rootfsDir := filepath.Join(cacheDir, "sha256-legacyupgrade")
+	require.NoError(t, os.MkdirAll(rootfsDir, 0o700))
+	require.NoError(t, os.WriteFile(
+		filepath.Join(rootfsDir, ".oci-config.json"),
+		[]byte(`{"Entrypoint":["/bin/sh"]}`), 0o600,
+	))
+
+	// Write a legacy-format ref file (digest only, no imageRef).
+	refsDir := filepath.Join(cacheDir, "refs")
+	require.NoError(t, os.MkdirAll(refsDir, 0o700))
+	p := c.refPath(imageRef)
+	require.NoError(t, os.WriteFile(p, []byte(digest+"\n"), 0o600))
+
+	// LookupRef should succeed (legacy format is readable).
+	result := c.LookupRef(imageRef)
+	require.NotNil(t, result)
+	assert.Equal(t, rootfsDir, result.Path)
+
+	// After LookupRef, the ref file should be upgraded to extended format.
+	data, err := os.ReadFile(p)
+	require.NoError(t, err)
+	assert.Equal(t, imageRef+"\t"+digest+"\n", string(data),
+		"LookupRef should upgrade legacy ref to extended format")
+
+	// List should now show the real image name instead of (unknown image).
+	entries, err := c.List()
+	require.NoError(t, err)
+	require.Len(t, entries, 1)
+	assert.Equal(t, []string{imageRef}, entries[0].Refs)
+}
+
 func TestGetRef_BackwardCompatible(t *testing.T) {
 	t.Parallel()