Fix unsafe local mode to load rotated roots from disk

Previously, unsafe local mode would only use the initial trusted
root and wouldn't load any rotated roots from disk. This meant
that if roots were rotated while online, then switching to unsafe
local mode would fail to recognize the newer root versions.

This fix adds:
- loadRootFromDisk(): New method that loads rotated root metadata
  from local disk, similar to how online mode loads from remote
- Updated unsafeLocalRefresh(): Now calls loadRootFromDisk() to
  load any rotated roots before loading other metadata
- Updated loadRoot(): Persists versioned root files (N.root.json)
  to disk for offline use by unsafe local mode

This allows unsafe local mode to properly handle root rotations
when operating offline, as long as the rotated roots were
previously downloaded in online mode.

Author: kipz

metadata/updater/updater.go

@@ -148,7 +148,12 @@ func (update *Updater) onlineRefresh() error {
 // The metadata on disk are verified against the provided root though,
 // and expiration dates are verified.
 func (update *Updater) unsafeLocalRefresh() error {
-	// Root is already loaded
+	// Load any rotated roots from disk
+	err := update.loadRootFromDisk()
+	if err != nil {
+		return err
+	}
+
 	// load timestamp
 	var p = filepath.Join(update.cfg.LocalMetadataDir, metadata.TIMESTAMP)
 	data, err := update.loadLocalMetadata(p)
@@ -511,6 +516,45 @@ func (update *Updater) loadRoot() error {
 			if err != nil {
 				return err
 			}
+			// also persist versioned root for offline use (e.g., unsafe local mode)
+			versionedRootName := fmt.Sprintf("%d.%s", nextVersion, metadata.ROOT)
+			err = update.persistMetadata(versionedRootName, data)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// loadRootFromDisk loads root metadata from local disk. Sequentially loads and
+// verifies every newer root metadata version available on disk.
+// This is used by unsafe local mode to support root rotation when operating offline.
+func (update *Updater) loadRootFromDisk() error {
+	// calculate boundaries
+	lowerBound := update.trusted.Root.Signed.Version + 1
+	upperBound := lowerBound + update.cfg.MaxRootRotations
+
+	// loop until we find the latest available version of root on disk
+	for nextVersion := lowerBound; nextVersion < upperBound; nextVersion++ {
+		versionedRootName := fmt.Sprintf("%d.%s", nextVersion, metadata.ROOT)
+		rootPath := filepath.Join(update.cfg.LocalMetadataDir, fmt.Sprintf("%s.json", url.PathEscape(versionedRootName)))
+
+		// try to load versioned root from disk
+		data, err := os.ReadFile(rootPath)
+		if err != nil {
+			if os.IsNotExist(err) {
+				// no more root versions available on disk, stop the loop
+				break
+			}
+			// some other error occurred
+			return err
+		}
+
+		// verify and load the root metadata
+		_, err = update.trusted.UpdateRoot(data)
+		if err != nil {
+			return err
 		}
 	}
 	return nil