rename worker probe, add comment

Author: mosche

server/src/main/java/org/elasticsearch/common/util/concurrent/EsExecutors.java

@@ -456,7 +456,7 @@ static class ExecutorScalingQueue<E> extends LinkedTransferQueue<E> {
 
         @Override
         public boolean offer(E e) {
-            if (e == EsThreadPoolExecutor.NOOP_PROBE) { // referential equality
+            if (e == EsThreadPoolExecutor.WORKER_PROBE) { // referential equality
                 // this probe ensures a worker is available after force queueing a task via ForceQueuePolicy
                 return super.offer(e);
             }
@@ -521,7 +521,7 @@ static class ForceQueuePolicy extends EsRejectedExecutionHandler {
 
         @Override
         public void rejectedExecution(Runnable task, ThreadPoolExecutor executor) {
-            if (task == EsThreadPoolExecutor.NOOP_PROBE) { // referential equality
+            if (task == EsThreadPoolExecutor.WORKER_PROBE) { // referential equality
                 return;
             }
             if (rejectAfterShutdown) {
@@ -549,7 +549,7 @@ private void put(ThreadPoolExecutor executor, Runnable task) {
                 if (probeWorkerPool && task == queue.peek()) { // referential equality
                     // If the task is at the head of the queue, we can assume the queue was previously empty. In this case available workers
                     // might have timed out in the meanwhile. To prevent the task from starving, we submit a noop probe to the executor.
-                    executor.execute(EsThreadPoolExecutor.NOOP_PROBE);
+                    executor.execute(EsThreadPoolExecutor.WORKER_PROBE);
                 }
             } catch (final InterruptedException e) {
                 assert false : "a scaling queue never blocks so a put to it can never be interrupted";

server/src/main/java/org/elasticsearch/common/util/concurrent/EsThreadPoolExecutor.java

@@ -29,8 +29,9 @@ public class EsThreadPoolExecutor extends ThreadPoolExecutor {
 
     private static final Logger logger = LogManager.getLogger(EsThreadPoolExecutor.class);
 
-    // probe to
-    static final Runnable NOOP_PROBE = () -> {};
+    // noop probe to prevent starvation of work in the work queue due to ForceQueuePolicy
+    // https://github.com/elastic/elasticsearch/issues/124667
+    static final Runnable WORKER_PROBE = () -> {};
 
     private final ThreadContext contextHolder;
 
@@ -81,7 +82,7 @@ public void setMaximumPoolSize(int maximumPoolSize) {
 
     @Override
     public void execute(Runnable command) {
-        final Runnable wrappedRunnable = command != NOOP_PROBE ? wrapRunnable(command) : NOOP_PROBE;
+        final Runnable wrappedRunnable = command != WORKER_PROBE ? wrapRunnable(command) : WORKER_PROBE;
         try {
             super.execute(wrappedRunnable);
         } catch (Exception e) {