[ML] Followup to annotations index creation (#36824)

Fixes two minor problems reported after merge of #36731:

1. Name the creation method to make clear it only creates
   if necessary
2. Avoid multiple simultaneous in-flight creation requests

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/annotations/AnnotationIndex.java

@@ -38,12 +38,12 @@ public class AnnotationIndex {
     public static final String INDEX_NAME = ".ml-annotations-6";
 
     /**
-     * Create the .ml-annotations index with correct mappings.
-     * This index is read and written by the UI results views,
-     * so needs to exist when there might be ML results to view.
+     * Create the .ml-annotations index with correct mappings if it does not already
+     * exist. This index is read and written by the UI results views, so needs to
+     * exist when there might be ML results to view.
      */
-    public static void createAnnotationsIndex(Settings settings, Client client, ClusterState state,
-                                              final ActionListener<Boolean> finalListener) {
+    public static void createAnnotationsIndexIfNecessary(Settings settings, Client client, ClusterState state,
+                                                         final ActionListener<Boolean> finalListener) {
 
         final ActionListener<Boolean> createAliasListener = ActionListener.wrap(success -> {
             final IndicesAliasesRequest request = client.admin().indices().prepareAliases()

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/MlInitializationService.java

@@ -19,6 +19,8 @@
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.xpack.core.ml.annotations.AnnotationIndex;
 
+import java.util.concurrent.atomic.AtomicBoolean;
+
 class MlInitializationService implements LocalNodeMasterListener, ClusterStateListener {
 
     private static final Logger logger = LogManager.getLogger(MlInitializationService.class);
@@ -27,6 +29,7 @@ class MlInitializationService implements LocalNodeMasterListener, ClusterStateLi
     private final ThreadPool threadPool;
     private final ClusterService clusterService;
     private final Client client;
+    private final AtomicBoolean isIndexCreationInProgress = new AtomicBoolean(false);
 
     private volatile MlDailyMaintenanceService mlDailyMaintenanceService;
 
@@ -55,14 +58,20 @@ public void clusterChanged(ClusterChangedEvent event) {
             return;
         }
 
-        if (event.localNodeMaster()) {
-            AnnotationIndex.createAnnotationsIndex(settings, client, event.state(), ActionListener.wrap(
+        // The atomic flag prevents multiple simultaneous attempts to create the
+        // index if there is a flurry of cluster state updates in quick succession
+        if (event.localNodeMaster() && isIndexCreationInProgress.compareAndSet(false, true)) {
+            AnnotationIndex.createAnnotationsIndexIfNecessary(settings, client, event.state(), ActionListener.wrap(
                 r -> {
+                    isIndexCreationInProgress.set(false);
                     if (r) {
                         logger.info("Created ML annotations index and aliases");
                     }
                 },
-                e -> logger.error("Error creating ML annotations index or aliases", e)));
+                e -> {
+                    isIndexCreationInProgress.set(false);
+                    logger.error("Error creating ML annotations index or aliases", e);
+                }));
         }
     }