Make ActionListener.notifyOnce a little cheaper

server/src/main/java/org/elasticsearch/action/ActionListener.java

@@ -275,29 +275,7 @@ static <Response> ActionListener<Response> runBefore(ActionListener<Response> de
      * and {@link #onFailure(Exception)} of the provided listener will be called at most once.
      */
     static <Response> ActionListener<Response> notifyOnce(ActionListener<Response> delegate) {
-        final var delegateRef = new AtomicReference<>(delegate);
-        return new ActionListener<>() {
-            @Override
-            public void onResponse(Response response) {
-                final var acquired = delegateRef.getAndSet(null);
-                if (acquired != null) {
-                    acquired.onResponse(response);
-                }
-            }
-
-            @Override
-            public void onFailure(Exception e) {
-                final var acquired = delegateRef.getAndSet(null);
-                if (acquired != null) {
-                    safeOnFailure(acquired, e);
-                }
-            }
-
-            @Override
-            public String toString() {
-                return "notifyOnce[" + delegateRef.get() + "]";
-            }
-        };
+        return new ActionListenerImplementations.NotifyOnceActionListener<>(delegate);
     }
 
     /**

server/src/main/java/org/elasticsearch/action/ActionListenerImplementations.java

@@ -14,6 +14,7 @@
 import org.elasticsearch.core.Releasable;
 import org.elasticsearch.core.Releasables;
 
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 
@@ -250,4 +251,35 @@ public String toString() {
             return super.toString() + "/" + runBefore;
         }
     }
+
+    // Extend AtomicReference directly for minimum memory overhead and indirection.
+    static final class NotifyOnceActionListener<Response> extends AtomicReference<ActionListener<Response>>
+        implements
+            ActionListener<Response> {
+
+        NotifyOnceActionListener(ActionListener<Response> delegate) {
+            super(delegate);
+        }
+
+        @Override
+        public void onResponse(Response response) {
+            final var acquired = getAndSet(null);
+            if (acquired != null) {
+                acquired.onResponse(response);
+            }
+        }
+
+        @Override
+        public void onFailure(Exception e) {
+            final var acquired = getAndSet(null);
+            if (acquired != null) {
+                safeOnFailure(acquired, e);
+            }
+        }
+
+        @Override
+        public String toString() {
+            return "notifyOnce[" + get() + "]";
+        }
+    }
 }