Make some constant SubscribableListener instances cheaper (elastic#124452)

We can just use a real constant for the `null` case, avoiding any
non-plain stores in all cases. This should be somewhat helpful for the
security interceptors.

Author: original-brownbear

server/src/main/java/org/elasticsearch/action/admin/cluster/stats/TransportClusterStatsAction.java

@@ -157,15 +157,15 @@ public TransportClusterStatsAction(
     protected SubscribableListener<AdditionalStats> createActionContext(Task task, ClusterStatsRequest request) {
         assert task instanceof CancellableTask;
         final var cancellableTask = (CancellableTask) task;
-        final var additionalStatsListener = new SubscribableListener<AdditionalStats>();
         if (request.isRemoteStats() == false) {
+            final var additionalStatsListener = new SubscribableListener<AdditionalStats>();
             final AdditionalStats additionalStats = new AdditionalStats();
             additionalStats.compute(cancellableTask, request, additionalStatsListener);
+            return additionalStatsListener;
         } else {
             // For remote stats request, we don't need to compute anything
-            additionalStatsListener.onResponse(null);
+            return SubscribableListener.nullSuccess();
         }
-        return additionalStatsListener;
     }
 
     @Override

server/src/main/java/org/elasticsearch/action/support/SubscribableListener.java

@@ -586,4 +586,15 @@ private Runnable scheduleTimeout(TimeValue timeout, ThreadPool threadPool, Execu
     private Object compareAndExchangeState(Object expectedValue, Object newValue) {
         return VH_STATE_FIELD.compareAndExchange(this, expectedValue, newValue);
     }
+
+    @SuppressWarnings("rawtypes")
+    private static final SubscribableListener NULL_SUCCESS = newSucceeded(null);
+
+    /**
+     * Same as {@link #newSucceeded(Object)} but always returns the same instance with result value {@code null}.
+     */
+    @SuppressWarnings("unchecked")
+    public static <T> SubscribableListener<T> nullSuccess() {
+        return NULL_SUCCESS;
+    }
 }

server/src/main/java/org/elasticsearch/indices/cluster/IndicesClusterStateService.java

@@ -243,7 +243,7 @@ protected void doClose() {}
      * Kind of a hack tbh, we can't be sure the shard locks are fully released when this is completed so there's all sorts of retries and
      * other lenience to handle that. It'd be better to wait for the shard locks to be released and then delete the data. See #74149.
      */
-    private volatile SubscribableListener<Void> lastClusterStateShardsClosedListener = SubscribableListener.newSucceeded(null);
+    private volatile SubscribableListener<Void> lastClusterStateShardsClosedListener = SubscribableListener.nullSuccess();
 
     @Nullable // if not currently applying a cluster state
     private RefCountingListener currentClusterStateShardsClosedListeners;
@@ -397,7 +397,7 @@ private void deleteIndices(final ClusterChangedEvent event) {
                 );
             } else if (project.isPresent() && project.get().hasIndex(index)) {
                 // The deleted index was part of the previous cluster state, but not loaded on the local node
-                indexServiceClosedListener = SubscribableListener.newSucceeded(null);
+                indexServiceClosedListener = SubscribableListener.nullSuccess();
                 final IndexMetadata metadata = project.get().index(index);
                 indexSettings = new IndexSettings(metadata, settings);
                 indicesService.deleteUnassignedIndex("deleted index was not assigned to local node", metadata, state);
@@ -411,7 +411,7 @@ private void deleteIndices(final ClusterChangedEvent event) {
                 // previous cluster state is not initialized/recovered.
                 assert state.metadata().projects().values().stream().anyMatch(p -> p.indexGraveyard().containsIndex(index))
                     || previousState.blocks().hasGlobalBlock(GatewayService.STATE_NOT_RECOVERED_BLOCK);
-                indexServiceClosedListener = SubscribableListener.newSucceeded(null);
+                indexServiceClosedListener = SubscribableListener.nullSuccess();
                 final IndexMetadata metadata = indicesService.verifyIndexIsDeleted(index, event.state());
                 if (metadata != null) {
                     indexSettings = new IndexSettings(metadata, settings);

test/framework/src/main/java/org/elasticsearch/indices/recovery/RecoveryClusterStateDelayListeners.java

@@ -64,7 +64,7 @@ public SubscribableListener<Void> getClusterStateDelayListener(long clusterState
                 refCounted.decRef();
             }
         } else {
-            return SubscribableListener.newSucceeded(null);
+            return SubscribableListener.nullSuccess();
         }
     }
 

test/framework/src/main/java/org/elasticsearch/test/TestCluster.java

@@ -130,8 +130,7 @@ private void deleteTemplates(Set<String> excludeTemplates, ActionListener<Void>
         SubscribableListener
 
             // dummy start step for symmetry
-            .newSucceeded(null)
-
+            .nullSuccess()
             // delete composable templates
             .<GetComposableIndexTemplateAction.Response>andThen(getComposableTemplates::addListener)
             .<AcknowledgedResponse>andThen((l, r) -> {

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/Operator.java

@@ -92,7 +92,7 @@ default IsBlockedResult isBlocked() {
         return NOT_BLOCKED;
     }
 
-    IsBlockedResult NOT_BLOCKED = new IsBlockedResult(SubscribableListener.newSucceeded(null), "not blocked");
+    IsBlockedResult NOT_BLOCKED = new IsBlockedResult(SubscribableListener.nullSuccess(), "not blocked");
 
     /**
      * A factory for creating intermediate operators.

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/exchange/ExchangeService.java

@@ -357,7 +357,7 @@ public void fetchPageAsync(boolean allSourcesFinished, ActionListener<ExchangeRe
             }
             doFetchPageAsync(false, ActionListener.wrap(r -> {
                 if (r.finished()) {
-                    completionListenerRef.compareAndSet(null, SubscribableListener.newSucceeded(null));
+                    completionListenerRef.compareAndSet(null, SubscribableListener.nullSuccess());
                 }
                 listener.onResponse(r);
             }, e -> close(ActionListener.running(() -> listener.onFailure(e)))));

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authz/interceptor/BulkShardRequestInterceptor.java

@@ -81,6 +81,6 @@ public SubscribableListener<Void> intercept(
                 }
             }
         }
-        return SubscribableListener.newSucceeded(null);
+        return SubscribableListener.nullSuccess();
     }
 }

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authz/interceptor/DlsFlsLicenseRequestInterceptor.java

@@ -101,6 +101,6 @@ public SubscribableListener<Void> intercept(
                 }
             }
         }
-        return SubscribableListener.newSucceeded(null);
+        return SubscribableListener.nullSuccess();
     }
 }

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authz/interceptor/FieldAndDocumentLevelSecurityRequestInterceptor.java

@@ -77,7 +77,7 @@ && supports(indicesRequest)
                 return listener;
             }
         }
-        return SubscribableListener.newSucceeded(null);
+        return SubscribableListener.nullSuccess();
     }
 
     abstract void disableFeatures(