Introduce deterministic task queue (#32197)

The cluster coordination layer relies on timeouts to ensure that a cluster can
successfully form, and must also deal with concurrent activity in the cluster.
This commit introduces some infrastructure that will help us to
deterministically test components that use concurrency and/or timeouts.

Author: DaveCTurner

server/src/main/java/org/elasticsearch/cluster/coordination/FutureExecutor.java

@@ -0,0 +1,33 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.cluster.coordination;
+
+import org.elasticsearch.common.unit.TimeValue;
+
+/**
+ * Device which supports running a task after some delay has elapsed.
+ */
+public interface FutureExecutor {
+    /**
+     * Schedule the given task for execution after the given delay has elapsed.
+     */
+    void schedule(Runnable task, TimeValue delay);
+}
+

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

@@ -362,9 +362,13 @@ public Runnable preserveContext(Runnable command) {
         return getThreadContext().preserveContext(command);
     }
 
-    public void shutdown() {
+    protected final void stopCachedTimeThread() {
         cachedTimeThread.running = false;
         cachedTimeThread.interrupt();
+    }
+
+    public void shutdown() {
+        stopCachedTimeThread();
         scheduler.shutdown();
         for (ExecutorHolder executor : executors.values()) {
             if (executor.executor() instanceof ThreadPoolExecutor) {
@@ -374,8 +378,7 @@ public void shutdown() {
     }
 
     public void shutdownNow() {
-        cachedTimeThread.running = false;
-        cachedTimeThread.interrupt();
+        stopCachedTimeThread();
         scheduler.shutdownNow();
         for (ExecutorHolder executor : executors.values()) {
             if (executor.executor() instanceof ThreadPoolExecutor) {