Allow mapping service to be null for scenarios of shard recovery from translog

src/main/java/org/opensearch/knn/index/codec/KNNCodecVersion.java

@@ -56,7 +56,7 @@ public enum KNNCodecVersion {
         ),
         (userCodec, mapperService) -> KNN920Codec.builder()
             .delegate(userCodec)
-            .knnVectorsFormat(new KNN920PerFieldKnnVectorsFormat(Optional.of(mapperService)))
+            .knnVectorsFormat(new KNN920PerFieldKnnVectorsFormat(Optional.ofNullable(mapperService)))
             .build(),
         KNN920Codec::new
     ),
@@ -71,7 +71,7 @@ public enum KNNCodecVersion {
         ),
         (userCodec, mapperService) -> KNN940Codec.builder()
             .delegate(userCodec)
-            .knnVectorsFormat(new KNN940PerFieldKnnVectorsFormat(Optional.of(mapperService)))
+            .knnVectorsFormat(new KNN940PerFieldKnnVectorsFormat(Optional.ofNullable(mapperService)))
             .build(),
         KNN940Codec::new
     );

src/test/java/org/opensearch/knn/index/codec/KNNCodecServiceTests.java

@@ -0,0 +1,67 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.knn.index.codec;
+
+import org.apache.lucene.codecs.Codec;
+import org.opensearch.cluster.metadata.IndexMetadata;
+import org.opensearch.common.settings.Settings;
+import org.opensearch.index.Index;
+import org.opensearch.index.IndexSettings;
+import org.opensearch.index.codec.CodecServiceConfig;
+import org.opensearch.index.mapper.MapperService;
+import org.opensearch.knn.KNNTestCase;
+
+import org.apache.logging.log4j.Logger;
+
+import java.util.UUID;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * Test for KNNCodecService class with focus on codec by name lookup
+ */
+public class KNNCodecServiceTests extends KNNTestCase {
+    private static final String TEST_INDEX = "test-index";
+    private static final int NUM_OF_SHARDS = 1;
+    private static final UUID INDEX_UUID = UUID.randomUUID();
+
+    private IndexSettings indexSettings;
+
+    @Override
+    public void setUp() throws Exception {
+        super.setUp();
+        IndexMetadata indexMetadata = mock(IndexMetadata.class);
+        when(indexMetadata.getIndex()).thenReturn(new Index(TEST_INDEX, INDEX_UUID.toString()));
+        when(indexMetadata.getSettings()).thenReturn(Settings.EMPTY);
+        Settings settings = Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, Integer.toString(NUM_OF_SHARDS)).build();
+        indexSettings = new IndexSettings(indexMetadata, settings);
+    }
+
+    public void testGetCodecByName() {
+        MapperService mapperService = mock(MapperService.class);
+        Logger loggerMock = mock(Logger.class);
+        CodecServiceConfig codecServiceConfig = new CodecServiceConfig(indexSettings, mapperService, loggerMock);
+        KNNCodecService knnCodecService = new KNNCodecService(codecServiceConfig);
+        Codec codec = knnCodecService.codec(KNNCodecVersion.current().getCodecName());
+        assertNotNull(codec);
+    }
+
+    /**
+     * This test case covers scenarios when MapperService is null, for instance this may happen during index.close operation.
+     * In such scenario codec is not really required but is created as part of engine initialization, please check code references below:
+     * @see <a href="https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/index/engine/EngineConfig.java#L378">EngineConfig.java</a>
+     * @see <a href="https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/index/shard/IndexShard.java#L3315">IndexShard.java</a>
+     * @see <a href="https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/index/engine/Engine.java#L906">Engine.java</a>
+     */
+    public void testGetCodecByNameWithNoMapperService() {
+        Logger loggerMock = mock(Logger.class);
+        CodecServiceConfig codecServiceConfig = new CodecServiceConfig(indexSettings, null, loggerMock);
+        KNNCodecService knnCodecService = new KNNCodecService(codecServiceConfig);
+        Codec codec = knnCodecService.codec(KNNCodecVersion.current().getCodecName());
+        assertNotNull(codec);
+    }
+}