[Backport main] Add fix to fromXContent and toXContent in ModelGraveyard

src/main/java/org/opensearch/knn/indices/ModelGraveyard.java

@@ -21,7 +21,8 @@
 import java.util.EnumSet;
 import java.util.HashSet;
 import java.util.Set;
-import java.util.stream.Collectors;
+import java.util.Iterator;
+
 import com.google.common.collect.Sets;
 
 /**
@@ -80,6 +81,13 @@ public void writeTo(StreamOutput out) throws IOException {
 
     @Override
     public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        Iterator model_ids = getModelIds().iterator();
+
+        builder.startArray("model_ids");
+        while (model_ids.hasNext()) {
+            builder.value(model_ids.next());
+        }
+        builder.endArray();
         return builder;
     }
 
@@ -143,7 +151,28 @@ public static NamedDiff readDiffFrom(StreamInput streamInput) throws IOException
      * @throws IOException
      */
     public static ModelGraveyard fromXContent(XContentParser xContentParser) throws IOException {
-        return new ModelGraveyard(xContentParser.list().stream().map(Object::toString).collect(Collectors.toSet()));
+        ModelGraveyard modelGraveyard = new ModelGraveyard();
+
+        // If it is a fresh parser, move to the first token
+        if (xContentParser.currentToken() == null) {
+            xContentParser.nextToken();
+        }
+
+        // on a start object move to next token
+        if (xContentParser.currentToken() == XContentParser.Token.START_OBJECT) {
+            xContentParser.nextToken();
+        }
+
+        if (xContentParser.currentToken() != XContentParser.Token.FIELD_NAME) {
+            throw new IllegalArgumentException("expected field name but got a " + xContentParser.currentToken());
+        }
+
+        while (xContentParser.nextToken() != XContentParser.Token.END_OBJECT) {
+            if (xContentParser.currentToken() == XContentParser.Token.VALUE_STRING) {
+                modelGraveyard.add(xContentParser.text());
+            }
+        }
+        return modelGraveyard;
     }
 
     /**

src/test/java/org/opensearch/knn/indices/ModelGraveyardTests.java

@@ -6,6 +6,9 @@
 package org.opensearch.knn.indices;
 
 import org.opensearch.common.io.stream.BytesStreamOutput;
+import org.opensearch.common.xcontent.ToXContent;
+import org.opensearch.common.xcontent.XContentBuilder;
+import org.opensearch.common.xcontent.XContentFactory;
 import org.opensearch.test.OpenSearchTestCase;
 
 import java.io.IOException;
@@ -57,6 +60,25 @@ public void testStreams() throws IOException {
         assertTrue(testModelGraveyardCopy.contains(testModelId));
     }
 
+    public void testXContentBuilder() throws IOException {
+        Set<String> modelIds = new HashSet<>();
+        String testModelId1 = "test-model-id1";
+        String testModelId2 = "test-model-id2";
+        modelIds.add(testModelId1);
+        modelIds.add(testModelId2);
+        ModelGraveyard testModelGraveyard = new ModelGraveyard(modelIds);
+
+        XContentBuilder xContentBuilder = XContentFactory.jsonBuilder();
+        xContentBuilder.startObject();
+        XContentBuilder builder = testModelGraveyard.toXContent(xContentBuilder, ToXContent.EMPTY_PARAMS);
+        builder.endObject();
+
+        ModelGraveyard testModelGraveyard2 = ModelGraveyard.fromXContent(createParser(builder));
+        assertEquals(2, testModelGraveyard2.size());
+        assertTrue(testModelGraveyard2.contains(testModelId1));
+        assertTrue(testModelGraveyard2.contains(testModelId2));
+    }
+
     public void testDiffStreams() throws IOException {
         Set<String> added = new HashSet<>();
         Set<String> removed = new HashSet<>();