Set additional settings at index level for knn enabled indexes

src/main/java/org/opensearch/knn/plugin/KNNPlugin.java

@@ -10,6 +10,7 @@
 import org.opensearch.common.ParseField;
 import org.opensearch.index.codec.CodecServiceFactory;
 import org.opensearch.index.engine.EngineFactory;
+import org.opensearch.index.shard.IndexSettingProvider;
 import org.opensearch.indices.SystemIndexDescriptor;
 import org.opensearch.knn.index.KNNCircuitBreaker;
 import org.opensearch.knn.index.KNNClusterUtil;
@@ -342,12 +343,35 @@ public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings sett
     }
 
     /**
-     * Plugin can provide additional node settings, that includes new settings or overrides for existing one from core.
+     * Plugin can provide additional index settings, that includes new settings or overrides for existing one from core.
      *
      * @return settings that are set by plugin
      */
     @Override
-    public Settings additionalSettings() {
+    public Collection<IndexSettingProvider> getAdditionalIndexSettingProviders() {
+        final IndexSettingProvider settingProvider = new IndexSettingProvider() {
+            @Override
+            public Settings getAdditionalIndexSettings(
+                final String indexName,
+                boolean isDataStreamIndex,
+                final Settings templateAndRequestSettings
+            ) {
+                return pluginAdditionalSettings(templateAndRequestSettings);
+            }
+        };
+        return List.of(settingProvider);
+    }
+
+    private Settings pluginAdditionalSettings(final Settings templateAndRequestSettings) {
+        final var settingsBuilder = Settings.builder();
+        boolean isKnnIndex = Boolean.parseBoolean(templateAndRequestSettings.get(KNNSettings.KNN_INDEX, Boolean.FALSE.toString()));
+        if (isKnnIndex) {
+            additionalMMapFileExtensionsForHybridFs(settingsBuilder);
+        }
+        return settingsBuilder.build();
+    }
+
+    private void additionalMMapFileExtensionsForHybridFs(final Settings.Builder settingsBuilder) {
         // We add engine specific extensions to the core list for HybridFS store type. We read existing values
         // and append ours because in core setting will be replaced by override.
         // Values are set as cluster defaults and are used at index creation time. Index specific overrides will take priority over values
@@ -359,6 +383,7 @@ public Settings additionalSettings() {
             IndexModule.INDEX_STORE_HYBRID_MMAP_EXTENSIONS.getDefault(Settings.EMPTY).stream(),
             engineSettings.stream()
         ).collect(Collectors.toList());
-        return Settings.builder().putList(IndexModule.INDEX_STORE_HYBRID_MMAP_EXTENSIONS.getKey(), combinedSettings).build();
+
+        settingsBuilder.putList(IndexModule.INDEX_STORE_HYBRID_MMAP_EXTENSIONS.getKey(), combinedSettings);
     }
 }

src/test/java/org/opensearch/knn/plugin/KNNPluginTests.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.knn.plugin;
+
+import org.opensearch.common.settings.Settings;
+import org.opensearch.index.IndexModule;
+import org.opensearch.index.shard.IndexSettingProvider;
+import org.opensearch.knn.KNNTestCase;
+import org.opensearch.knn.index.util.KNNEngine;
+
+import java.util.Collection;
+import java.util.List;
+
+public class KNNPluginTests extends KNNTestCase {
+
+    private static final String INDEX_NAME = "test_index";
+
+    public void testMMapFileExtensionsForHybridFs() {
+        final KNNPlugin knnPlugin = new KNNPlugin();
+
+        final Collection<IndexSettingProvider> additionalIndexSettingProviders = knnPlugin.getAdditionalIndexSettingProviders();
+
+        assertEquals(1, additionalIndexSettingProviders.size());
+
+        final IndexSettingProvider indexSettingProvider = additionalIndexSettingProviders.iterator().next();
+        // settings for knn enabled index
+        final Settings knnIndexSettings = indexSettingProvider.getAdditionalIndexSettings(INDEX_NAME, false, getKnnDefaultIndexSettings());
+        final List<String> mmapFileExtensionsForHybridFsKnnIndex = knnIndexSettings.getAsList(
+            IndexModule.INDEX_STORE_HYBRID_MMAP_EXTENSIONS.getKey()
+        );
+
+        assertNotNull(mmapFileExtensionsForHybridFsKnnIndex);
+        assertFalse(mmapFileExtensionsForHybridFsKnnIndex.isEmpty());
+
+        for (KNNEngine engine : KNNEngine.values()) {
+            assertTrue(mmapFileExtensionsForHybridFsKnnIndex.containsAll(engine.mmapFileExtensions()));
+        }
+
+        // settings for index without knn
+        final Settings nonKnnIndexSettings = indexSettingProvider.getAdditionalIndexSettings(INDEX_NAME, false, getNonKnnIndexSettings());
+        final List<String> mmapFileExtensionsForHybridFsNonKnnIndex = nonKnnIndexSettings.getAsList(
+            IndexModule.INDEX_STORE_HYBRID_MMAP_EXTENSIONS.getKey()
+        );
+
+        assertNotNull(mmapFileExtensionsForHybridFsNonKnnIndex);
+        assertTrue(mmapFileExtensionsForHybridFsNonKnnIndex.isEmpty());
+    }
+
+    private Settings getKnnDefaultIndexSettings() {
+        return Settings.builder().put("number_of_shards", 1).put("number_of_replicas", 0).put("index.knn", true).build();
+    }
+
+    private Settings getNonKnnIndexSettings() {
+        return Settings.builder().put("number_of_shards", 1).put("number_of_replicas", 0).build();
+    }
+}