Update tuf updater api surface

sigstore-java/src/main/java/dev/sigstore/tuf/Updater.java

@@ -22,6 +22,7 @@
 import dev.sigstore.encryption.Keys;
 import dev.sigstore.encryption.signers.Verifiers;
 import dev.sigstore.tuf.model.*;
+import dev.sigstore.tuf.model.TargetMeta.TargetData;
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.security.InvalidKeyException;
@@ -91,13 +92,15 @@ public static Builder builder() {
     return new Builder();
   }
 
+  /** Update metadata and download all targets. */
   public void update()
       throws IOException, NoSuchAlgorithmException, InvalidKeySpecException, InvalidKeyException {
     updateMeta();
     downloadTargets(trustedMetaStore.getTargets());
   }
 
-  void updateMeta() throws IOException, NoSuchAlgorithmException, InvalidKeySpecException {
+  /** Update just metadata but do not download targets. */
+  public void updateMeta() throws IOException, NoSuchAlgorithmException, InvalidKeySpecException {
     updateRoot();
     var oldTimestamp = trustedMetaStore.findTimestamp();
     updateTimestamp();
@@ -112,6 +115,16 @@ void updateMeta() throws IOException, NoSuchAlgorithmException, InvalidKeySpecEx
     updateTargets();
   }
 
+  /** Download a single target defined in targets. Does not handle delegated targets. */
+  public void downloadTarget(String targetName)
+      throws IOException, NoSuchAlgorithmException, InvalidKeySpecException {
+    var targetData = trustedMetaStore.getTargets().getSignedMeta().getTargets().get(targetName);
+    if (targetData == null) {
+      throw new TargetMetadataMissingException(targetName);
+    }
+    downloadTarget(targetName, targetData);
+  }
+
   // https://theupdateframework.github.io/specification/latest/#detailed-client-workflow
   void updateRoot()
       throws IOException, RoleExpiredException, NoSuchAlgorithmException, InvalidKeySpecException,
@@ -304,7 +317,6 @@ void updateTimestamp()
             localTimestamp.getSignedMeta().getVersion(), timestamp.getSignedMeta().getVersion());
       }
       if (localTimestamp.getSignedMeta().getVersion() == timestamp.getSignedMeta().getVersion()) {
-        trustedMetaStore.setTimestamp(localTimestamp);
         return;
       }
     }
@@ -459,24 +471,28 @@ void downloadTargets(Targets targets)
         throw new TargetMetadataMissingException(targetName);
       }
       TargetMeta.TargetData targetData = entry.getValue();
-      // 9) Download target up to length specified in bytes. verify against hash.
-      String versionedTargetName;
-      if (targetData.getHashes().getSha512() != null) {
-        versionedTargetName = targetData.getHashes().getSha512() + "." + targetName;
-      } else {
-        versionedTargetName = targetData.getHashes().getSha256() + "." + targetName;
-      }
+      downloadTarget(targetName, targetData);
+    }
+  }
 
-      var targetBytes = targetFetcher.fetchResource(versionedTargetName, targetData.getLength());
-      if (targetBytes == null) {
-        throw new FileNotFoundException(targetName, targetFetcher.getSource());
-      }
-      verifyHashes(entry.getKey(), targetBytes, targetData.getHashes());
+  void downloadTarget(String targetName, TargetData targetData) throws IOException {
+    // 9) Download target up to length specified in bytes. verify against hash.
+    String versionedTargetName;
+    if (targetData.getHashes().getSha512() != null) {
+      versionedTargetName = targetData.getHashes().getSha512() + "." + targetName;
+    } else {
+      versionedTargetName = targetData.getHashes().getSha256() + "." + targetName;
+    }
 
-      // when persisting targets use the targetname without sha512 prefix
-      // https://theupdateframework.github.io/specification/latest/index.html#fetch-target
-      targetStore.writeTarget(targetName, targetBytes);
+    var targetBytes = targetFetcher.fetchResource(versionedTargetName, targetData.getLength());
+    if (targetBytes == null) {
+      throw new FileNotFoundException(targetName, targetFetcher.getSource());
     }
+    verifyHashes(targetName, targetBytes, targetData.getHashes());
+
+    // when persisting targets use the targetname without sha512 prefix
+    // https://theupdateframework.github.io/specification/latest/index.html#fetch-target
+    targetStore.writeTarget(targetName, targetBytes);
   }
 
   @VisibleForTesting

sigstore-java/src/main/java/dev/sigstore/tuf/model/Root.java

@@ -24,6 +24,7 @@
 @Value.Immutable
 public interface Root extends SignedTufMeta<RootMeta> {
   @Override
+  @Gson.Ignore
   @Derived
   default RootMeta getSignedMeta() {
     return getSignedMeta(RootMeta.class);

sigstore-java/src/main/java/dev/sigstore/tuf/model/SignedTufMeta.java

@@ -38,6 +38,7 @@ public interface SignedTufMeta<T extends TufMeta> {
 
   /** An internal helper to translate raw signed json to a useable type. */
   @Derived
+  @Gson.Ignore
   default T getSignedMeta(Class<T> type) {
     return GsonSupplier.GSON.get().fromJson(getRawSignedMeta(), type);
   }

sigstore-java/src/main/java/dev/sigstore/tuf/model/Snapshot.java

@@ -25,6 +25,7 @@
 public interface Snapshot extends SignedTufMeta<SnapshotMeta> {
   @Override
   @Derived
+  @Gson.Ignore
   default SnapshotMeta getSignedMeta() {
     return getSignedMeta(SnapshotMeta.class);
   }

sigstore-java/src/main/java/dev/sigstore/tuf/model/SnapshotMeta.java

@@ -59,6 +59,7 @@ interface SnapshotTarget {
 
     /** The length in bytes of the given target's metadata, or a default if not present */
     @Derived
+    @Gson.Ignore
     default Integer getLengthOrDefault() {
       return getLength().orElse(DEFAULT_MAX_LENGTH);
     }

sigstore-java/src/main/java/dev/sigstore/tuf/model/Targets.java

@@ -25,6 +25,7 @@
 public interface Targets extends SignedTufMeta<TargetMeta> {
   @Override
   @Derived
+  @Gson.Ignore
   default TargetMeta getSignedMeta() {
     return getSignedMeta(TargetMeta.class);
   }

sigstore-java/src/main/java/dev/sigstore/tuf/model/Timestamp.java

@@ -26,6 +26,7 @@ public interface Timestamp extends SignedTufMeta<TimestampMeta> {
 
   @Override
   @Derived
+  @Gson.Ignore
   default TimestampMeta getSignedMeta() {
     return getSignedMeta(TimestampMeta.class);
   }