Ensure index templates are not applied to system indices

CHANGELOG.md

@@ -97,6 +97,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix typo super->sb in method toString() of RemoteStoreNodeAttribute ([#15362](https://github.com/opensearch-project/OpenSearch/pull/15362))
 - [Workload Management] Fixing Create/Update QueryGroup TransportActions to execute from non-cluster manager nodes ([16422](https://github.com/opensearch-project/OpenSearch/pull/16422))
 - Fix flaky test in `testApproximateRangeWithSizeOverDefault` by adjusting totalHits assertion logic ([#16434](https://github.com/opensearch-project/OpenSearch/pull/16434#pullrequestreview-2386999409))
+- Ensure index templates are not applied to system indices ([#16418](https://github.com/opensearch-project/OpenSearch/pull/16418))
 
 ### Security
 

server/src/main/java/org/opensearch/cluster/metadata/MetadataCreateIndexService.java

@@ -434,15 +434,21 @@ public ClusterState applyCreateIndexRequest(
             // in which case templates don't apply, so create the index from the source metadata
             return applyCreateIndexRequestWithExistingMetadata(currentState, request, silent, sourceMetadata, metadataTransformer);
         } else {
+            // The backing index may have a different name or prefix than the data stream name.
+            final String name = request.dataStreamName() != null ? request.dataStreamName() : request.index();
+
+            // Do not apply any templates to system indices
+            if (systemIndices.isSystemIndex(name)) {
+                return applyCreateIndexRequestWithNoTemplates(currentState, request, silent, metadataTransformer);
+            }
+
             // Hidden indices apply templates slightly differently (ignoring wildcard '*'
             // templates), so we need to check to see if the request is creating a hidden index
             // prior to resolving which templates it matches
             final Boolean isHiddenFromRequest = IndexMetadata.INDEX_HIDDEN_SETTING.exists(request.settings())
                 ? IndexMetadata.INDEX_HIDDEN_SETTING.get(request.settings())
                 : null;
 
-            // The backing index may have a different name or prefix than the data stream name.
-            final String name = request.dataStreamName() != null ? request.dataStreamName() : request.index();
             // Check to see if a v2 template matched
             final String v2Template = MetadataIndexTemplateService.findV2Template(
                 currentState.metadata(),
@@ -676,7 +682,18 @@ public void addRemoteStoreCustomMetadata(IndexMetadata.Builder tmpImdBuilder, bo
         tmpImdBuilder.putCustom(IndexMetadata.REMOTE_STORE_CUSTOM_KEY, remoteCustomData);
     }
 
-    private ClusterState applyCreateIndexRequestWithV1Templates(
+    ClusterState applyCreateIndexRequestWithNoTemplates(
+        final ClusterState currentState,
+        final CreateIndexClusterStateUpdateRequest request,
+        final boolean silent,
+        final BiConsumer<Metadata.Builder, IndexMetadata> metadataTransformer
+    ) throws Exception {
+        // Using applyCreateIndexRequestWithV1Templates with empty list instead of applyCreateIndexRequestWithV2Template
+        // with null template as applyCreateIndexRequestWithV2Template has assertions when template is null
+        return applyCreateIndexRequestWithV1Templates(currentState, request, silent, Collections.emptyList(), metadataTransformer);
+    }
+
+    ClusterState applyCreateIndexRequestWithV1Templates(
         final ClusterState currentState,
         final CreateIndexClusterStateUpdateRequest request,
         final boolean silent,

server/src/test/java/org/opensearch/cluster/metadata/MetadataCreateIndexServiceTests.java

@@ -184,7 +184,11 @@
 import static org.hamcrest.Matchers.notNullValue;
 import static org.hamcrest.Matchers.nullValue;
 import static org.hamcrest.Matchers.startsWith;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 public class MetadataCreateIndexServiceTests extends OpenSearchTestCase {
@@ -2559,6 +2563,62 @@ public void testApplyContextWithSettingsOverlap() throws IOException {
         }
     }
 
+    public void testApplyCreateIndexRequestWithNoTemplates() throws Exception {
+        BiConsumer<Metadata.Builder, IndexMetadata> mockMetadataTransformer = mock(BiConsumer.class);
+        SystemIndices mockSystemIndices = mock(SystemIndices.class);
+        when(mockSystemIndices.isSystemIndex(anyString())).thenReturn(true);
+
+        ClusterService clusterService = mock(ClusterService.class);
+        ClusterState clusterState = ClusterState.builder(org.opensearch.cluster.ClusterName.CLUSTER_NAME_SETTING.getDefault(Settings.EMPTY))
+            .metadata(Metadata.EMPTY_METADATA)
+            .build();
+
+        ThreadPool threadPool = new TestThreadPool(getTestName());
+
+        ClusterSettings clusterSettings = new ClusterSettings(Settings.EMPTY, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
+        when(clusterService.getSettings()).thenReturn(Settings.EMPTY);
+        when(clusterService.getClusterSettings()).thenReturn(clusterSettings);
+        when(clusterService.state()).thenReturn(clusterState);
+
+        MetadataCreateIndexService metadataCreateIndexService = spy(
+            new MetadataCreateIndexService(
+                Settings.EMPTY,
+                clusterService,
+                indicesServices,
+                null,
+                null,
+                createTestShardLimitService(randomIntBetween(1, 1000), false, clusterService),
+                new Environment(Settings.builder().put("path.home", "dummy").build(), null),
+                IndexScopedSettings.DEFAULT_SCOPED_SETTINGS,
+                threadPool,
+                null,
+                mockSystemIndices,
+                true,
+                new AwarenessReplicaBalance(Settings.EMPTY, clusterService.getClusterSettings()),
+                DefaultRemoteStoreSettings.INSTANCE,
+                repositoriesServiceSupplier
+            )
+        );
+
+        metadataCreateIndexService.applyCreateIndexRequest(clusterState, request, false, mockMetadataTransformer);
+        verify(metadataCreateIndexService).applyCreateIndexRequestWithNoTemplates(
+            eq(clusterState),
+            eq(request),
+            eq(false),
+            eq(mockMetadataTransformer)
+        );
+
+        verify(metadataCreateIndexService).applyCreateIndexRequestWithV1Templates(
+            eq(clusterState),
+            eq(request),
+            eq(false),
+            eq(Collections.emptyList()),
+            eq(mockMetadataTransformer)
+        );
+
+        threadPool.shutdown();
+    }
+
     private IndexTemplateMetadata addMatchingTemplate(Consumer<IndexTemplateMetadata.Builder> configurator) {
         IndexTemplateMetadata.Builder builder = templateMetadataBuilder("template1", "te*");
         configurator.accept(builder);