Deprecate the listener thread pool (#53266)

The listener thread pool is being removed from use in the server
codebase. This commit deprecates configuring the listener thread pool.

docs/reference/migration/migrate_8_0/settings.asciidoc

@@ -37,3 +37,10 @@ processors. As this leads to more context switches and more threads but without
 an increase in the number of physical CPUs on which to schedule these additional
 threads, the `node.processors` setting is now bounded by the number of available
 processors.
+
+[float]
+==== `thread_pool.listener.size` and `thread_pool.listener.queue_size` have been deprecated
+
+The listener thread pool is no longer used internally by Elasticsearch.
+Therefore, these settings have been deprecated. You can safely remove these
+settings from the configuration of your nodes.

server/src/main/java/org/elasticsearch/threadpool/FixedExecutorBuilder.java

@@ -51,7 +51,28 @@ public final class FixedExecutorBuilder extends ExecutorBuilder<FixedExecutorBui
      * @param trackEWMA whether to track the exponentially weighted moving average of the task execution time
      */
     FixedExecutorBuilder(final Settings settings, final String name, final int size, final int queueSize, final boolean trackEWMA) {
-        this(settings, name, size, queueSize, "thread_pool." + name, trackEWMA);
+        this(settings, name, size, queueSize, trackEWMA, false);
+    }
+
+    /**
+     * Construct a fixed executor builder; the settings will have the key prefix "thread_pool." followed by the executor name.
+     *
+     * @param settings   the node-level settings
+     * @param name       the name of the executor
+     * @param size       the fixed number of threads
+     * @param queueSize  the size of the backing queue, -1 for unbounded
+     * @param trackEWMA  whether to track the exponentially weighted moving average of the task execution time
+     * @param deprecated whether or not the thread pool is deprecated
+     */
+    FixedExecutorBuilder(
+        final Settings settings,
+        final String name,
+        final int size,
+        final int queueSize,
+        final boolean trackEWMA,
+        final boolean deprecated
+    ) {
+        this(settings, name, size, queueSize, "thread_pool." + name, trackEWMA, deprecated);
     }
 
     /**
@@ -66,16 +87,45 @@ public final class FixedExecutorBuilder extends ExecutorBuilder<FixedExecutorBui
      */
     public FixedExecutorBuilder(final Settings settings, final String name, final int size, final int queueSize, final String prefix,
                                 final boolean trackEWMA) {
+        this(settings, name, size, queueSize, prefix, trackEWMA, false);
+    }
+
+    /**
+     * Construct a fixed executor builder.
+     *
+     * @param settings   the node-level settings
+     * @param name       the name of the executor
+     * @param size       the fixed number of threads
+     * @param queueSize  the size of the backing queue, -1 for unbounded
+     * @param prefix     the prefix for the settings keys
+     * @param trackEWMA  whether to track the exponentially weighted moving average of the task execution time
+     * @param deprecated whether or not the thread pool is deprecated
+     */
+    public FixedExecutorBuilder(
+        final Settings settings,
+        final String name,
+        final int size,
+        final int queueSize,
+        final String prefix,
+        final boolean trackEWMA,
+        final boolean deprecated
+    ) {
         super(name);
         final String sizeKey = settingsKey(prefix, "size");
+        final Setting.Property[] properties;
+        if (deprecated) {
+            properties = new Setting.Property[]{Setting.Property.NodeScope, Setting.Property.Deprecated};
+        } else {
+            properties = new Setting.Property[]{Setting.Property.NodeScope};
+        }
         this.sizeSetting =
-                new Setting<>(
-                        sizeKey,
-                        s -> Integer.toString(size),
-                        s -> Setting.parseInt(s, 1, applyHardSizeLimit(settings, name), sizeKey),
-                        Setting.Property.NodeScope);
+            new Setting<>(
+                sizeKey,
+                s -> Integer.toString(size),
+                s -> Setting.parseInt(s, 1, applyHardSizeLimit(settings, name), sizeKey),
+                properties);
         final String queueSizeKey = settingsKey(prefix, "queue_size");
-        this.queueSizeSetting = Setting.intSetting(queueSizeKey, queueSize, Setting.Property.NodeScope);
+        this.queueSizeSetting = Setting.intSetting(queueSizeKey, queueSize, properties);
         this.trackEWMA = trackEWMA;
     }
 

server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -68,7 +68,7 @@ public class ThreadPool implements Scheduler {
     public static class Names {
         public static final String SAME = "same";
         public static final String GENERIC = "generic";
-        public static final String LISTENER = "listener";
+        @Deprecated public static final String LISTENER = "listener";
         public static final String GET = "get";
         public static final String ANALYZE = "analyze";
         public static final String WRITE = "write";
@@ -172,7 +172,7 @@ public class ThreadPool implements Scheduler {
         builders.put(Names.MANAGEMENT, new ScalingExecutorBuilder(Names.MANAGEMENT, 1, 5, TimeValue.timeValueMinutes(5)));
         // no queue as this means clients will need to handle rejections on listener queue even if the operation succeeded
         // the assumption here is that the listeners should be very lightweight on the listeners side
-        builders.put(Names.LISTENER, new FixedExecutorBuilder(settings, Names.LISTENER, halfProcMaxAt10, -1, false));
+        builders.put(Names.LISTENER, new FixedExecutorBuilder(settings, Names.LISTENER, halfProcMaxAt10, -1, false, true));
         builders.put(Names.FLUSH, new ScalingExecutorBuilder(Names.FLUSH, 1, halfProcMaxAt5, TimeValue.timeValueMinutes(5)));
         builders.put(Names.REFRESH, new ScalingExecutorBuilder(Names.REFRESH, 1, halfProcMaxAt10, TimeValue.timeValueMinutes(5)));
         builders.put(Names.WARMER, new ScalingExecutorBuilder(Names.WARMER, 1, halfProcMaxAt5, TimeValue.timeValueMinutes(5)));

server/src/test/java/org/elasticsearch/threadpool/FixedThreadPoolTests.java

@@ -22,6 +22,7 @@ package org.elasticsearch.threadpool;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.common.util.concurrent.EsRejectedExecutionException;
+import org.elasticsearch.threadpool.ThreadPool.Names;
 
 import java.util.concurrent.CountDownLatch;
 
@@ -88,6 +89,10 @@ public class FixedThreadPoolTests extends ESThreadPoolTestCase {
         } finally {
             terminateThreadPoolIfNeeded(threadPool);
         }
+
+        if (Names.LISTENER.equals(threadPoolName)) {
+            assertSettingDeprecationsAndWarnings(new String[]{"thread_pool.listener.queue_size", "thread_pool.listener.size"});
+        }
     }
 
 }

server/src/test/java/org/elasticsearch/threadpool/UpdateThreadPoolSettingsTests.java

@@ -118,6 +118,10 @@ public class UpdateThreadPoolSettingsTests extends ESThreadPoolTestCase {
         } finally {
             terminateThreadPoolIfNeeded(threadPool);
         }
+
+        if (Names.LISTENER.equals(threadPoolName)) {
+            assertSettingDeprecationsAndWarnings(new String[]{"thread_pool.listener.size"});
+        }
     }
 
     public void testScalingExecutorType() throws InterruptedException {
@@ -173,6 +177,10 @@ public class UpdateThreadPoolSettingsTests extends ESThreadPoolTestCase {
         } finally {
             terminateThreadPoolIfNeeded(threadPool);
         }
+
+        if (Names.LISTENER.equals(threadPoolName)) {
+            assertSettingDeprecationsAndWarnings(new String[]{"thread_pool.listener.queue_size"});
+        }
     }
 
     public void testCustomThreadPool() throws Exception {