[Backport 2.x] Fix bulk ingest NPE with empty pipeline

CHANGELOG.md

@@ -59,6 +59,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ### Fixed
 - Fix constraint bug which allows more primary shards than average primary shards per index ([#14908](https://github.com/opensearch-project/OpenSearch/pull/14908))
+- Fix NPE when bulk ingest with empty pipeline ([#15033](https://github.com/opensearch-project/OpenSearch/pull/15033))
 - Fix missing value of FieldSort for unsigned_long ([#14963](https://github.com/opensearch-project/OpenSearch/pull/14963))
 - Fix delete index template failed when the index template matches a data stream but is unused ([#15080](https://github.com/opensearch-project/OpenSearch/pull/15080))
 - Fix array_index_out_of_bounds_exception when indexing documents with field name containing only dot ([#15126](https://github.com/opensearch-project/OpenSearch/pull/15126))

server/src/main/java/org/opensearch/ingest/IngestService.java

@@ -997,7 +997,7 @@ private void innerBatchExecute(
         Consumer<List<IngestDocumentWrapper>> handler
     ) {
         if (pipeline.getProcessors().isEmpty()) {
-            handler.accept(null);
+            handler.accept(toIngestDocumentWrappers(slots, indexRequests));
             return;
         }
 
@@ -1271,6 +1271,14 @@ private static IngestDocumentWrapper toIngestDocumentWrapper(int slot, IndexRequ
         return new IngestDocumentWrapper(slot, toIngestDocument(indexRequest), null);
     }
 
+    private static List<IngestDocumentWrapper> toIngestDocumentWrappers(List<Integer> slots, List<IndexRequest> indexRequests) {
+        List<IngestDocumentWrapper> ingestDocumentWrappers = new ArrayList<>();
+        for (int i = 0; i < slots.size(); ++i) {
+            ingestDocumentWrappers.add(toIngestDocumentWrapper(slots.get(i), indexRequests.get(i)));
+        }
+        return ingestDocumentWrappers;
+    }
+
     private static Map<Integer, IndexRequest> createSlotIndexRequestMap(List<Integer> slots, List<IndexRequest> indexRequests) {
         Map<Integer, IndexRequest> slotIndexRequestMap = new HashMap<>();
         for (int i = 0; i < slots.size(); ++i) {

server/src/test/java/org/opensearch/ingest/IngestServiceTests.java

@@ -1995,6 +1995,42 @@ public void testExecuteBulkRequestInBatchWithDefaultBatchSize() {
         verify(mockCompoundProcessor, never()).execute(any(), any());
     }
 
+    public void testExecuteEmptyPipelineInBatch() throws Exception {
+        IngestService ingestService = createWithProcessors(emptyMap());
+        PutPipelineRequest putRequest = new PutPipelineRequest(
+            "_id",
+            new BytesArray("{\"processors\": [], \"description\": \"_description\"}"),
+            MediaTypeRegistry.JSON
+        );
+        ClusterState clusterState = ClusterState.builder(new ClusterName("_name")).build(); // Start empty
+        ClusterState previousClusterState = clusterState;
+        clusterState = IngestService.innerPut(putRequest, clusterState);
+        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
+        BulkRequest bulkRequest = new BulkRequest();
+        IndexRequest indexRequest1 = new IndexRequest("_index").id("_id1").source(emptyMap()).setPipeline("_id").setFinalPipeline("_none");
+        bulkRequest.add(indexRequest1);
+        IndexRequest indexRequest2 = new IndexRequest("_index").id("_id2").source(emptyMap()).setPipeline("_id").setFinalPipeline("_none");
+        bulkRequest.add(indexRequest2);
+        IndexRequest indexRequest3 = new IndexRequest("_index").id("_id3").source(emptyMap()).setPipeline("_id").setFinalPipeline("_none");
+        bulkRequest.add(indexRequest3);
+        IndexRequest indexRequest4 = new IndexRequest("_index").id("_id4").source(emptyMap()).setPipeline("_id").setFinalPipeline("_none");
+        bulkRequest.add(indexRequest4);
+        bulkRequest.batchSize(4);
+        final Map<Integer, Exception> failureHandler = new HashMap<>();
+        final Map<Thread, Exception> completionHandler = new HashMap<>();
+        ingestService.executeBulkRequest(
+            4,
+            bulkRequest.requests(),
+            failureHandler::put,
+            completionHandler::put,
+            indexReq -> {},
+            Names.WRITE,
+            bulkRequest
+        );
+        assertTrue(failureHandler.isEmpty());
+        assertEquals(Set.of(Thread.currentThread()), completionHandler.keySet());
+    }
+
     public void testPrepareBatches_same_index_pipeline() {
         IngestService.IndexRequestWrapper wrapper1 = createIndexRequestWrapper("index1", Collections.singletonList("p1"));
         IngestService.IndexRequestWrapper wrapper2 = createIndexRequestWrapper("index1", Collections.singletonList("p1"));