Avoid crashing on using the index.lifecycle.name in the API body (#1060)

* Avoid crashing on using the index.lifecycle.name in the API body

Signed-off-by: frotsch <frotsch@mailbox.org>

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

@@ -1044,7 +1044,10 @@ public class MetadataCreateIndexService {
         for (final String key : settings.keySet()) {
             final Setting<?> setting = indexScopedSettings.get(key);
             if (setting == null) {
-                assert indexScopedSettings.isPrivateSetting(key) : "expected [" + key + "] to be private but it was not";
+                // see: https://github.com/opensearch-project/OpenSearch/issues/1019
+                if(!indexScopedSettings.isPrivateSetting(key)) {
+                    validationErrors.add("expected [" + key + "] to be private but it was not");
+                }
             } else if (setting.isPrivateIndex()) {
                 validationErrors.add("private index setting [" + key + "] can not be set explicitly");
             }

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

@@ -66,6 +66,7 @@ import org.opensearch.common.unit.TimeValue;
 import org.opensearch.common.util.BigArrays;
 import org.opensearch.common.xcontent.NamedXContentRegistry;
 import org.opensearch.common.xcontent.XContentFactory;
+import org.opensearch.env.Environment;
 import org.opensearch.index.Index;
 import org.opensearch.index.IndexNotFoundException;
 import org.opensearch.index.IndexSettings;
@@ -963,6 +964,31 @@ public class MetadataCreateIndexServiceTests extends OpenSearchTestCase {
             + "and [index.translog.retention.size] are deprecated and effectively ignored. They will be removed in a future version.");
     }
 
+    public void testIndexLifecycleNameSetting() {
+        // see: https://github.com/opensearch-project/OpenSearch/issues/1019
+        final Settings ilnSetting = Settings.builder().put("index.lifecycle.name", "dummy").build();
+        withTemporaryClusterService(((clusterService, threadPool) -> {
+            MetadataCreateIndexService checkerService = new MetadataCreateIndexService(
+                Settings.EMPTY,
+                clusterService,
+                null,
+                null,
+                null,
+                createTestShardLimitService(randomIntBetween(1, 1000), clusterService),
+                new Environment(Settings.builder().put("path.home", "dummy").build(), null),
+                new IndexScopedSettings(ilnSetting, Collections.emptySet()),
+                threadPool,
+                null,
+                new SystemIndices(Collections.emptyMap()),
+                true
+            );
+
+            final List<String> validationErrors =  checkerService.getIndexSettingsValidationErrors(ilnSetting, true);
+            assertThat(validationErrors.size(), is(1));
+            assertThat(validationErrors.get(0), is("expected [index.lifecycle.name] to be private but it was not"));
+        }));
+    }
+
     private IndexTemplateMetadata addMatchingTemplate(Consumer<IndexTemplateMetadata.Builder> configurator) {
         IndexTemplateMetadata.Builder builder = templateMetadataBuilder("template1", "te*");
         configurator.accept(builder);