Remove unused ThreadPool.Names#SAME (#107249)

`SAME` is a distinguished threadpool name that callers could use to
obtain a special `ExecutorService` that runs tasks immediately, directly
on the calling thread. In fact there are no callers that use this name
any more, so we can remove it and all the associated special handling.

Relates #106279

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

@@ -27,6 +27,7 @@
 import org.elasticsearch.common.util.concurrent.ThreadContext;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.TimeValue;
+import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.node.Node;
 import org.elasticsearch.node.ReportingService;
 import org.elasticsearch.telemetry.metric.Instrument;
@@ -64,7 +65,6 @@ public class ThreadPool implements ReportingService<ThreadPoolInfo>, Scheduler {
     private static final Logger logger = LogManager.getLogger(ThreadPool.class);
 
     public static class Names {
-        public static final String SAME = "same";
         public static final String GENERIC = "generic";
         public static final String CLUSTER_COORDINATION = "cluster_coordination";
         public static final String GET = "get";
@@ -99,9 +99,13 @@ public static class Names {
     public static final String THREAD_POOL_METRIC_NAME_REJECTED = ".threads.rejected.total";
 
     public enum ThreadPoolType {
+        @Deprecated(forRemoval = true)
+        @UpdateForV9 // no longer used, remove in v9
         DIRECT("direct"),
         FIXED("fixed"),
-        FIXED_AUTO_QUEUE_SIZE("fixed_auto_queue_size"), // TODO: remove in 9.0
+        @Deprecated(forRemoval = true)
+        @UpdateForV9 // no longer used, remove in v9
+        FIXED_AUTO_QUEUE_SIZE("fixed_auto_queue_size"),
         SCALING("scaling");
 
         private final String type;
@@ -127,7 +131,6 @@ public static ThreadPoolType fromType(String type) {
     }
 
     public static final Map<String, ThreadPoolType> THREAD_POOL_TYPES = Map.ofEntries(
-        entry(Names.SAME, ThreadPoolType.DIRECT),
         entry(Names.GENERIC, ThreadPoolType.SCALING),
         entry(Names.GET, ThreadPoolType.FIXED),
         entry(Names.ANALYZE, ThreadPoolType.FIXED),
@@ -335,16 +338,10 @@ public ThreadPool(final Settings settings, MeterRegistry meterRegistry, final Ex
             executors.put(entry.getKey(), executorHolder);
         }
 
-        executors.put(Names.SAME, new ExecutorHolder(EsExecutors.DIRECT_EXECUTOR_SERVICE, new Info(Names.SAME, ThreadPoolType.DIRECT)));
         this.executors = Map.copyOf(executors);
         this.executors.forEach((k, v) -> instruments.put(k, setupMetrics(meterRegistry, k, v)));
         this.instruments = instruments;
-        final List<Info> infos = executors.values()
-            .stream()
-            .filter(holder -> holder.info.getName().equals("same") == false)
-            .map(holder -> holder.info)
-            .toList();
-        this.threadPoolInfo = new ThreadPoolInfo(infos);
+        this.threadPoolInfo = new ThreadPoolInfo(executors.values().stream().map(holder -> holder.info).toList());
         this.scheduler = Scheduler.initScheduler(settings, "scheduler");
         this.slowSchedulerWarnThresholdNanos = SLOW_SCHEDULER_TASK_WARN_THRESHOLD_SETTING.get(settings).nanos();
         this.cachedTimeThread = new CachedTimeThread(
@@ -481,10 +478,6 @@ public ThreadPoolStats stats() {
         List<ThreadPoolStats.Stats> stats = new ArrayList<>();
         for (ExecutorHolder holder : executors.values()) {
             final String name = holder.info.getName();
-            // no need to have info on "same" thread pool
-            if ("same".equals(name)) {
-                continue;
-            }
             int threads = -1;
             int queue = -1;
             int active = -1;

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

@@ -20,8 +20,7 @@ protected final ThreadPool.Info info(final ThreadPool threadPool, final String n
                 return info;
             }
         }
-        assert "same".equals(name);
-        return null;
+        return fail(null, "unknown threadpool name: " + name);
     }
 
     protected final ThreadPoolStats.Stats stats(final ThreadPool threadPool, final String name) {
@@ -30,10 +29,10 @@ protected final ThreadPoolStats.Stats stats(final ThreadPool threadPool, final S
                 return stats;
             }
         }
-        throw new IllegalArgumentException(name);
+        return fail(null, "unknown threadpool name: " + name);
     }
 
-    protected final void terminateThreadPoolIfNeeded(final ThreadPool threadPool) throws InterruptedException {
+    protected final void terminateThreadPoolIfNeeded(final ThreadPool threadPool) {
         if (threadPool != null) {
             terminate(threadPool);
         }

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

@@ -29,7 +29,7 @@
 
 public class UpdateThreadPoolSettingsTests extends ESThreadPoolTestCase {
 
-    public void testCorrectThreadPoolTypePermittedInSettings() throws InterruptedException {
+    public void testCorrectThreadPoolTypePermittedInSettings() {
         String threadPoolName = randomThreadPoolName();
         ThreadPool.ThreadPoolType correctThreadPoolType = ThreadPool.THREAD_POOL_TYPES.get(threadPoolName);
         ThreadPool threadPool = null;
@@ -41,13 +41,7 @@ public void testCorrectThreadPoolTypePermittedInSettings() throws InterruptedExc
                     .build(),
                 MeterRegistry.NOOP
             );
-            ThreadPool.Info info = info(threadPool, threadPoolName);
-            if (ThreadPool.Names.SAME.equals(threadPoolName)) {
-                assertNull(info); // we don't report on the "same" thread pool
-            } else {
-                // otherwise check we have the expected type
-                assertEquals(info.getThreadPoolType(), correctThreadPoolType);
-            }
+            assertEquals(info(threadPool, threadPoolName).getThreadPoolType(), correctThreadPoolType);
         } finally {
             terminateThreadPoolIfNeeded(threadPool);
         }

test/framework/src/main/java/org/elasticsearch/common/util/concurrent/DeterministicTaskQueue.java

@@ -379,7 +379,7 @@ public ExecutorService generic() {
 
             @Override
             public ExecutorService executor(String name) {
-                return Names.SAME.equals(name) ? EsExecutors.DIRECT_EXECUTOR_SERVICE : forkingExecutor;
+                return forkingExecutor;
             }
 
             @Override

test/framework/src/test/java/org/elasticsearch/common/util/concurrent/DeterministicTaskQueueTests.java

@@ -443,20 +443,4 @@ public void testThreadPoolSchedulesPeriodicFutureTasks() {
         assertThat(strings, contains("periodic-0", "periodic-1", "periodic-2"));
     }
 
-    public void testSameExecutor() {
-        final DeterministicTaskQueue taskQueue = new DeterministicTaskQueue();
-        final ThreadPool threadPool = taskQueue.getThreadPool();
-        final AtomicBoolean executed = new AtomicBoolean(false);
-        final AtomicBoolean executedNested = new AtomicBoolean(false);
-        threadPool.generic().execute(() -> {
-            final var executor = threadPool.executor(ThreadPool.Names.SAME);
-            assertSame(EsExecutors.DIRECT_EXECUTOR_SERVICE, executor);
-            executor.execute(() -> assertTrue(executedNested.compareAndSet(false, true)));
-            assertThat(executedNested.get(), is(true));
-            assertTrue(executed.compareAndSet(false, true));
-        });
-        taskQueue.runAllRunnableTasks();
-        assertThat(executed.get(), is(true));
-    }
-
 }

x-pack/plugin/ml/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/MlDailyMaintenanceServiceIT.java

@@ -9,7 +9,6 @@
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.cluster.service.ClusterService;
-import org.elasticsearch.common.util.concurrent.EsExecutors;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.threadpool.ThreadPool;
@@ -35,7 +34,6 @@
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.hamcrest.Matchers.is;
 import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.when;
 
 public class MlDailyMaintenanceServiceIT extends MlNativeAutodetectIntegTestCase {
 
@@ -46,7 +44,6 @@ public class MlDailyMaintenanceServiceIT extends MlNativeAutodetectIntegTestCase
     public void setUpMocks() {
         jobConfigProvider = new JobConfigProvider(client(), xContentRegistry());
         threadPool = mock(ThreadPool.class);
-        when(threadPool.executor(ThreadPool.Names.SAME)).thenReturn(EsExecutors.DIRECT_EXECUTOR_SERVICE);
     }
 
     public void testTriggerDeleteJobsInStateDeletingWithoutDeletionTask() throws InterruptedException {

x-pack/plugin/ml/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/MlInitializationServiceIT.java

@@ -39,13 +39,11 @@
 
 public class MlInitializationServiceIT extends MlNativeAutodetectIntegTestCase {
 
-    private ThreadPool threadPool;
     private MlInitializationService mlInitializationService;
 
     @Before
     public void setUpMocks() {
-        threadPool = mock(ThreadPool.class);
-        when(threadPool.executor(ThreadPool.Names.SAME)).thenReturn(EsExecutors.DIRECT_EXECUTOR_SERVICE);
+        final var threadPool = mock(ThreadPool.class);
         when(threadPool.executor(MachineLearning.UTILITY_THREAD_POOL_NAME)).thenReturn(EsExecutors.DIRECT_EXECUTOR_SERVICE);
         MlDailyMaintenanceService mlDailyMaintenanceService = mock(MlDailyMaintenanceService.class);
         ClusterService clusterService = mock(ClusterService.class);