Add UnmodifiableOnRestore property to index.knn setting

src/main/java/org/opensearch/knn/index/KNNSettings.java

@@ -44,6 +44,7 @@
 import static org.opensearch.common.settings.Setting.Property.IndexScope;
 import static org.opensearch.common.settings.Setting.Property.NodeScope;
 import static org.opensearch.common.settings.Setting.Property.Final;
+import static org.opensearch.common.settings.Setting.Property.UnmodifiableOnRestore;
 import static org.opensearch.common.unit.MemorySizeValue.parseBytesSizeValueOrHeapRatio;
 import static org.opensearch.core.common.unit.ByteSizeValue.parseBytesSizeValue;
 import static org.opensearch.knn.common.featureflags.KNNFeatureFlags.getFeatureFlags;
@@ -269,7 +270,13 @@ public class KNNSettings {
     /**
      * This setting identifies KNN index.
      */
-    public static final Setting<Boolean> IS_KNN_INDEX_SETTING = Setting.boolSetting(KNN_INDEX, false, IndexScope, Final);
+    public static final Setting<Boolean> IS_KNN_INDEX_SETTING = Setting.boolSetting(
+        KNN_INDEX,
+        false,
+        IndexScope,
+        Final,
+        UnmodifiableOnRestore
+    );
 
     /**
      * index_thread_quantity - the parameter specifies how many threads the nms library should use to create the graph.

src/test/java/org/opensearch/knn/index/RestoreSnapshotIT.java

@@ -0,0 +1,96 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.knn.index;
+
+import org.opensearch.client.Request;
+import org.opensearch.client.Response;
+import org.opensearch.client.ResponseException;
+import org.opensearch.common.settings.Settings;
+import org.opensearch.common.xcontent.json.JsonXContent;
+import org.opensearch.common.xcontent.support.XContentMapValues;
+import org.opensearch.core.xcontent.XContentBuilder;
+import org.opensearch.test.rest.OpenSearchRestTestCase;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.notNullValue;
+import static org.hamcrest.Matchers.hasSize;
+
+public class RestoreSnapshotIT extends OpenSearchRestTestCase {
+
+    private String index;
+    private String snapshot;
+    private String repository;
+
+    private void setupSnapshotRestore() throws Exception {
+        Request clusterSettingsRequest = new Request("GET", "/_cluster/settings");
+        clusterSettingsRequest.addParameter("include_defaults", "true");
+        Response clusterSettingsResponse = client().performRequest(clusterSettingsRequest);
+        Map<String, Object> clusterSettings = entityAsMap(clusterSettingsResponse);
+
+        @SuppressWarnings("unchecked")
+        List<String> pathRepos = (List<String>) XContentMapValues.extractValue("defaults.path.repo", clusterSettings);
+        assertThat(pathRepos, notNullValue());
+        assertThat(pathRepos, hasSize(1));
+
+        final String pathRepo = pathRepos.get(0);
+
+        index = "test-index";
+        snapshot = "snapshot-" + index;
+        repository = "repo";
+
+        // create index
+        Settings indexSettings = Settings.builder().put("number_of_shards", 1).put("number_of_replicas", 1).put("index.knn", true).build();
+        createIndex(index, indexSettings);
+
+        // create repo
+        Settings repoSettings = Settings.builder().put("compress", randomBoolean()).put("location", pathRepo).build();
+        registerRepository(repository, "fs", true, repoSettings);
+
+        // create snapshot
+        createSnapshot(repository, snapshot, true);
+    }
+
+    public void testKnnSettingIsUnmodifiableOnRestore() throws Exception {
+        setupSnapshotRestore();
+
+        // invalid restore
+        XContentBuilder restoreCommand = JsonXContent.contentBuilder().startObject();
+        restoreCommand.field("indices", index);
+        restoreCommand.field("rename_pattern", index);
+        restoreCommand.field("rename_replacement", "restored-" + index);
+        restoreCommand.startObject("index_settings");
+        {
+            restoreCommand.field("index.knn", false);
+        }
+        restoreCommand.endObject();
+        restoreCommand.endObject();
+        Request restoreRequest = new Request("POST", "/_snapshot/" + repository + "/" + snapshot + "/_restore");
+        restoreRequest.addParameter("wait_for_completion", "true");
+        restoreRequest.setJsonEntity(restoreCommand.toString());
+        final ResponseException error = expectThrows(ResponseException.class, () -> client().performRequest(restoreRequest));
+        assertThat(error.getMessage(), containsString("cannot modify UnmodifiableOnRestore setting [index.knn]" + " on restore"));
+    }
+
+    public void testKnnSettingCannotBeIgnoredDuringRestore() throws Exception {
+        setupSnapshotRestore();
+
+        // invalid restore
+        XContentBuilder restoreCommand = JsonXContent.contentBuilder().startObject();
+        restoreCommand.field("indices", index);
+        restoreCommand.field("rename_pattern", index);
+        restoreCommand.field("rename_replacement", "restored-" + index);
+        restoreCommand.field("ignore_index_settings", "index.knn");
+        restoreCommand.endObject();
+        Request restoreRequest = new Request("POST", "/_snapshot/" + repository + "/" + snapshot + "/_restore");
+        restoreRequest.addParameter("wait_for_completion", "true");
+        restoreRequest.setJsonEntity(restoreCommand.toString());
+        final ResponseException error = expectThrows(ResponseException.class, () -> client().performRequest(restoreRequest));
+        assertThat(error.getMessage(), containsString("cannot remove UnmodifiableOnRestore setting [index.knn] on restore"));
+    }
+}