Capture max processors in static init (#97119)

The number of processors available to the jvm can change over time.
However, most of Elasticsearch assumes this value is constant. Although
we could rework all code relying on the number of processors to
dynamically support updates and poll the jvm, doing so has little value
since the processors changing is an edge case. Instead, this commit
fixes validation of the node.processors setting (our internal number of
processors) to validate based on the max processors available at launch.

closes #97088

docs/changelog/97119.yaml

@@ -0,0 +1,6 @@
+pr: 97119
+summary: Capture max processors in static init
+area: Infra/Core
+type: bug
+issues:
+ - 97088

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

@@ -35,6 +35,9 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 public class EsExecutors {
 
+    // although the available processors may technically change, for node sizing we use the number available at launch
+    private static final int MAX_NUM_PROCESSORS = Runtime.getRuntime().availableProcessors();
+
     /**
      * Setting to manually control the number of allocated processors. This setting is used to adjust thread pool sizes per node. The
      * default value is {@link Runtime#availableProcessors()} but should be manually controlled if not all processors on the machine are
@@ -43,7 +46,7 @@ public class EsExecutors {
      */
     public static final Setting<Processors> NODE_PROCESSORS_SETTING = new Setting<>(
         "node.processors",
-        Double.toString(Runtime.getRuntime().availableProcessors()),
+        Double.toString(MAX_NUM_PROCESSORS),
         textValue -> {
             double numberOfProcessors = Double.parseDouble(textValue);
             if (Double.isNaN(numberOfProcessors) || Double.isInfinite(numberOfProcessors)) {
@@ -56,9 +59,8 @@ public class EsExecutors {
                 throw new IllegalArgumentException(err);
             }
 
-            final int maxNumberOfProcessors = Runtime.getRuntime().availableProcessors();
-            if (numberOfProcessors > maxNumberOfProcessors) {
-                String err = "Failed to parse value [" + textValue + "] for setting [node.processors] must be <= " + maxNumberOfProcessors;
+            if (numberOfProcessors > MAX_NUM_PROCESSORS) {
+                String err = "Failed to parse value [" + textValue + "] for setting [node.processors] must be <= " + MAX_NUM_PROCESSORS;
                 throw new IllegalArgumentException(err);
             }
             return Processors.of(numberOfProcessors);