fix tests

Author: ywangd

server/src/main/java/org/elasticsearch/cluster/DiffableUtils.java

@@ -147,7 +147,10 @@ public static <K, T, M extends Map<K, T>> boolean hasKey(MapDiff<K, T, M> diff,
     }
 
     /**
-     * Create a new MapDiff by transforming the keys with the provided keyFunction
+     * Create a new JDK map backed MapDiff by transforming the keys with the provided keyFunction.
+     * @param diff Original MapDiff to transform
+     * @param keyFunction Function to transform the key
+     * @param keySerializer Serializer for the new key
      */
     public static <K1, K2, T extends Diffable<T>, M1 extends Map<K1, T>> MapDiff<K2, T, Map<K2, T>> jdkMapDiffWithUpdatedKeys(
         MapDiff<K1, T, M1> diff,

server/src/test/java/org/elasticsearch/snapshots/SnapshotsInProgressSerializationTests.java

@@ -105,7 +105,7 @@ public void testDiffSerializationBwc() throws IOException {
         out.setTransportVersion(oldVersion);
 
         final Custom before = createTestInstance(() -> randomSnapshot(ProjectId.DEFAULT));
-        final Custom after = makeTestChanges(before);
+        final Custom after = makeTestChanges(before, () -> randomSnapshot(ProjectId.DEFAULT));
         final Diff<Custom> diff = after.diff(before);
         diff.writeTo(out);
 
@@ -206,6 +206,10 @@ protected Writeable.Reader<Custom> instanceReader() {
 
     @Override
     protected Custom makeTestChanges(Custom testInstance) {
+        return makeTestChanges(testInstance, () -> randomSnapshot(randomProjectIdOrDefault()));
+    }
+
+    protected Custom makeTestChanges(Custom testInstance, Supplier<Entry> randomEntrySupplier) {
         final SnapshotsInProgress snapshots = (SnapshotsInProgress) testInstance;
         SnapshotsInProgress updatedInstance = SnapshotsInProgress.EMPTY;
         if (randomBoolean() && snapshots.count() > 1) {
@@ -226,7 +230,7 @@ protected Custom makeTestChanges(Custom testInstance) {
             // add some elements
             int addElements = randomInt(10);
             for (int i = 0; i < addElements; i++) {
-                updatedInstance = updatedInstance.withAddedEntry(randomSnapshot());
+                updatedInstance = updatedInstance.withAddedEntry(randomEntrySupplier.get());
             }
         }
         if (randomBoolean()) {