Include resharding metadata in IndexMetadata.equals

Also add a basic BWC serialization test for resharding metadata.

Author: bcully

server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java

@@ -1483,6 +1483,12 @@ public boolean equals(Object o) {
         if (isSystem != that.isSystem) {
             return false;
         }
+        if (reshardingMetadata == null && that.reshardingMetadata != null) {
+            return false;
+        }
+        if (reshardingMetadata != null && reshardingMetadata.equals(that.reshardingMetadata) == false) {
+            return false;
+        }
         return true;
     }
 
@@ -1502,6 +1508,9 @@ public int hashCode() {
         result = 31 * result + rolloverInfos.hashCode();
         result = 31 * result + inferenceFields.hashCode();
         result = 31 * result + Boolean.hashCode(isSystem);
+        if (reshardingMetadata != null) {
+            result = 31 * result + reshardingMetadata.hashCode();
+        }
         return result;
     }
 

server/src/test/java/org/elasticsearch/cluster/metadata/IndexMetadataTests.java

@@ -9,6 +9,8 @@
 
 package org.elasticsearch.cluster.metadata;
 
+import org.elasticsearch.TransportVersion;
+import org.elasticsearch.TransportVersions;
 import org.elasticsearch.action.admin.indices.rollover.MaxAgeCondition;
 import org.elasticsearch.action.admin.indices.rollover.MaxDocsCondition;
 import org.elasticsearch.action.admin.indices.rollover.MaxPrimaryShardDocsCondition;
@@ -678,6 +680,34 @@ public void testInferenceFieldMetadata() {
         assertThat(idxMeta2.getInferenceFields(), equalTo(dynamicFields));
     }
 
+    public void testReshardingBWCSerialization() throws IOException {
+        final int numShards = randomIntBetween(1, 8);
+        final var settings = indexSettings(IndexVersion.current(), numShards, 0);
+        final var reshardingMetadata = IndexReshardingMetadata.newSplitByMultiple(numShards, randomIntBetween(2, 5));
+        IndexMetadata idx = IndexMetadata.builder("test").settings(settings).reshardingMetadata(reshardingMetadata).build();
+
+        // the version prior to TransportVersions.INDEX_RESHARDING_METADATA
+        final var version = TransportVersions.ML_INFERENCE_DEEPSEEK;
+        // should round trip
+        final var deserialized = roundTripWithVersion(idx, version);
+
+        // but be missing resharding metadata
+        assertNull(deserialized.getReshardingMetadata());
+        // but otherwise be equal
+        assertEquals(idx, IndexMetadata.builder(deserialized).reshardingMetadata(reshardingMetadata).build());
+    }
+
+    private IndexMetadata roundTripWithVersion(IndexMetadata indexMetadata, TransportVersion version) throws IOException {
+        try (BytesStreamOutput out = new BytesStreamOutput()) {
+            out.setTransportVersion(version);
+            indexMetadata.writeTo(out);
+            try (StreamInput in = new NamedWriteableAwareStreamInput(out.bytes().streamInput(), writableRegistry())) {
+                in.setTransportVersion(version);
+                return IndexMetadata.readFrom(in);
+            }
+        }
+    }
+
     private static Settings indexSettingsWithDataTier(String dataTier) {
         return indexSettings(IndexVersion.current(), 1, 0).put(DataTier.TIER_PREFERENCE, dataTier).build();
     }