Adding test for null checkpoint publisher and addreesing PR comments

Signed-off-by: Rishikesh1159 <rishireddy1159@gmail.com>

server/src/main/java/org/opensearch/index/IndexService.java

@@ -533,9 +533,7 @@ public synchronized IndexShard createShard(
                 () -> globalCheckpointSyncer.accept(shardId),
                 retentionLeaseSyncer,
                 circuitBreakerService,
-                this.indexSettings.isSegRepEnabled() && routing.primary()
-                    ? checkpointPublisher
-                    : SegmentReplicationCheckpointPublisher.EMPTY
+                this.indexSettings.isSegRepEnabled() && routing.primary() ? checkpointPublisher : null
             );
             eventListener.indexShardStateChanged(indexShard, null, indexShard.state(), "shard created");
             eventListener.afterIndexShardCreated(indexShard);

server/src/main/java/org/opensearch/index/shard/IndexShard.java

@@ -161,7 +161,7 @@
 import org.opensearch.indices.recovery.RecoveryTarget;
 import org.opensearch.indices.replication.checkpoint.PublishCheckpointRequest;
 import org.opensearch.indices.replication.checkpoint.SegmentReplicationCheckpointPublisher;
-import org.opensearch.indices.replication.copy.ReplicationCheckpoint;
+import org.opensearch.indices.replication.checkpoint.ReplicationCheckpoint;
 import org.opensearch.repositories.RepositoriesService;
 import org.opensearch.repositories.Repository;
 import org.opensearch.rest.RestStatus;
@@ -324,7 +324,7 @@ public IndexShard(
         final Runnable globalCheckpointSyncer,
         final RetentionLeaseSyncer retentionLeaseSyncer,
         final CircuitBreakerService circuitBreakerService,
-        final SegmentReplicationCheckpointPublisher checkpointPublisher
+        @Nullable final SegmentReplicationCheckpointPublisher checkpointPublisher
     ) throws IOException {
         super(shardRouting.shardId(), indexSettings);
         assert shardRouting.initializing();
@@ -407,7 +407,7 @@ public boolean shouldCache(Query query) {
         persistMetadata(path, indexSettings, shardRouting, null, logger);
         this.useRetentionLeasesInPeerRecovery = replicationTracker.hasAllPeerRecoveryRetentionLeases();
         this.refreshPendingLocationListener = new RefreshPendingLocationListener();
-        if (indexSettings.isSegRepEnabled() == true) {
+        if (checkpointPublisher != null) {
             this.checkpointRefreshListener = new CheckpointRefreshListener(this, checkpointPublisher);
         } else {
             this.checkpointRefreshListener = null;
@@ -1372,10 +1372,16 @@ public GatedCloseable<IndexCommit> acquireSafeIndexCommit() throws EngineExcepti
         }
     }
 
+    /**
+     * Returns the lastest Replication Checkpoint that shard received
+     */
     public ReplicationCheckpoint getLatestReplicationCheckpoint() {
         return new ReplicationCheckpoint(shardId, 0, 0, 0, 0);
     }
 
+    /**
+     * Invoked when a new checkpoint is received from a primary shard.  Starts the copy process.
+     */
     public synchronized void onNewCheckpoint(final PublishCheckpointRequest request) {
         assert shardRouting.primary() == false;
         // TODO

server/src/main/java/org/opensearch/indices/replication/checkpoint/PublishCheckpointRequest.java

@@ -11,7 +11,6 @@
 import org.opensearch.action.support.replication.ReplicationRequest;
 import org.opensearch.common.io.stream.StreamInput;
 import org.opensearch.common.io.stream.StreamOutput;
-import org.opensearch.indices.replication.copy.ReplicationCheckpoint;
 
 import java.io.IOException;
 

server/src/main/java/org/opensearch/indices/replication/copy/ReplicationCheckpoint.java → server/src/main/java/org/opensearch/indices/replication/checkpoint/ReplicationCheckpoint.java

@@ -6,7 +6,7 @@
  * compatible open source license.
  */
 
-package org.opensearch.indices.replication.copy;
+package org.opensearch.indices.replication.checkpoint;
 
 import org.opensearch.common.Nullable;
 import org.opensearch.common.io.stream.StreamInput;

server/src/test/java/org/opensearch/index/shard/IndexShardTests.java

@@ -3440,6 +3440,17 @@ public void testCheckpointRefreshListener() throws IOException {
         closeShards(shard);
     }
 
+    /**
+     * here we are mocking a SegmentReplicationcheckpointPublisher and testing that when a refresh happens on index shard if CheckpointRefreshListener is added to the InternalrefreshListerners List
+     */
+    public void testCheckpointRefreshListenerWithNull() throws IOException {
+        IndexShard shard = newStartedShard(p -> newShard(null), true);
+        shard.refresh("test");
+        List<ReferenceManager.RefreshListener> refreshListeners = shard.getEngine().config().getInternalRefreshListener();
+        assertFalse(refreshListeners.stream().anyMatch(e -> e instanceof CheckpointRefreshListener));
+        closeShards(shard);
+    }
+
     /**
      * creates a new initializing shard. The shard will will be put in its proper path under the
      * current node id the shard is assigned to.

server/src/test/java/org/opensearch/indices/replication/checkpoint/PublishCheckpointActionTests.java

@@ -23,7 +23,6 @@
 import org.opensearch.index.shard.ShardId;
 import org.opensearch.indices.IndicesService;
 import org.opensearch.indices.recovery.RecoverySettings;
-import org.opensearch.indices.replication.copy.ReplicationCheckpoint;
 import org.opensearch.test.OpenSearchTestCase;
 import org.opensearch.test.transport.CapturingTransport;
 import org.opensearch.threadpool.TestThreadPool;