[Backport 2.x] Add additional handling in SearchTemplateRequest when simulate is set to true

CHANGELOG.md

@@ -34,6 +34,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - [BWC and API enforcement] Introduce checks for enforcing the API restrictions ([#11175](https://github.com/opensearch-project/OpenSearch/pull/11175))
 - Maintainer approval check ([#11378](https://github.com/opensearch-project/OpenSearch/pull/11378))
 - Create separate transport action for render search template action ([#11170](https://github.com/opensearch-project/OpenSearch/pull/11170))
+- Add additional handling in SearchTemplateRequest when simulate is set to true ([#11591](https://github.com/opensearch-project/OpenSearch/pull/11591))
 
 ### Dependencies
 - Bumps jetty version to 9.4.52.v20230823 to fix GMS-2023-1857 ([#9822](https://github.com/opensearch-project/OpenSearch/pull/9822))

modules/lang-mustache/src/main/java/org/opensearch/script/mustache/SearchTemplateRequest.java

@@ -259,16 +259,25 @@ public void writeTo(StreamOutput out) throws IOException {
 
     @Override
     public String[] indices() {
+        if (request == null) {
+            return new String[0];
+        }
         return request.indices();
     }
 
     @Override
     public IndicesOptions indicesOptions() {
+        if (request == null) {
+            return SearchRequest.DEFAULT_INDICES_OPTIONS;
+        }
         return request.indicesOptions();
     }
 
     @Override
     public IndicesRequest indices(String... indices) {
+        if (request == null) {
+            return new SearchRequest(new String[0]).indices(indices);
+        }
         return request.indices(indices);
     }
 }

modules/lang-mustache/src/test/java/org/opensearch/script/mustache/SearchTemplateRequestTests.java

@@ -32,6 +32,7 @@
 
 package org.opensearch.script.mustache;
 
+import org.opensearch.action.search.SearchRequest;
 import org.opensearch.core.common.io.stream.Writeable;
 import org.opensearch.script.ScriptType;
 import org.opensearch.search.RandomSearchRequestGenerator;
@@ -110,4 +111,19 @@ public static SearchTemplateRequest createRandomRequest() {
         request.setRequest(RandomSearchRequestGenerator.randomSearchRequest(SearchSourceBuilder::searchSource));
         return request;
     }
+
+    public void testSimulatedSearchTemplateRequest() {
+        SearchTemplateRequest request = new SearchTemplateRequest();
+        request.setSimulate(true);
+
+        assertEquals(0, request.indices().length);
+        assertEquals(SearchRequest.DEFAULT_INDICES_OPTIONS, request.indicesOptions());
+        assertEquals(2, request.indices("index1", "index2").indices().length);
+
+        SearchTemplateRequest randomRequest = createRandomRequest();
+        int expectedIndicesLength = randomRequest.indices().length;
+        request.setSimulate(true);
+
+        assertEquals(expectedIndicesLength, randomRequest.indices().length);
+    }
 }