Refactor: remove coreContainerWorkExecutor (#3405)

It's only used for one silly thing:
It's needless to create an executor just to asynchronously wait for another executor (coreLoadExecutor) to close.  We can instead call shutdownAndAwaitTermination on coreLoadExecutor when the container shuts down.

Author: dsmiley

solr/core/src/java/org/apache/solr/core/CoreContainer.java

@@ -245,9 +245,7 @@ public JerseyAppHandlerCache getJerseyAppHandlerCache() {
 
   private volatile HttpSolrClientProvider solrClientProvider;
 
-  private volatile ExecutorService coreContainerWorkExecutor =
-      ExecutorUtil.newMDCAwareCachedThreadPool(
-          new SolrNamedThreadFactory("coreContainerWorkExecutor"));
+  private volatile ExecutorService coreLoadExecutor;
 
   private final OrderedExecutor<BytesRef> replayUpdatesExecutor;
 
@@ -839,16 +837,6 @@ private void loadInternal() {
 
     tracer = TracerConfigurator.loadTracer(loader, cfg.getTracerConfiguratorPluginInfo());
 
-    coreContainerWorkExecutor =
-        MetricUtils.instrumentedExecutorService(
-            coreContainerWorkExecutor,
-            null,
-            metricManager.registry(SolrMetricManager.getRegistryName(SolrInfoBean.Group.node)),
-            SolrMetricManager.mkName(
-                "coreContainerWorkExecutor",
-                SolrInfoBean.Category.CONTAINER.toString(),
-                "threadPool"));
-
     shardHandlerFactory =
         ShardHandlerFactory.newInstance(cfg.getShardHandlerFactoryPluginInfo(), loader);
     if (shardHandlerFactory instanceof SolrMetricProducer metricProducer) {
@@ -1098,7 +1086,7 @@ private void loadInternal() {
     }
 
     // setup executor to load cores in parallel
-    ExecutorService coreLoadExecutor =
+    coreLoadExecutor =
         MetricUtils.instrumentedExecutorService(
             ExecutorUtil.newMDCAwareFixedThreadPool(
                 cfg.getCoreLoadThreadCount(isZooKeeperAware()),
@@ -1168,11 +1156,9 @@ private void loadInternal() {
       backgroundCloser.start();
 
     } finally {
-      if (asyncSolrCoreLoad) {
-        coreContainerWorkExecutor.execute(
-            () -> ExecutorUtil.shutdownAndAwaitTerminationForever(coreLoadExecutor));
-      } else {
-        ExecutorUtil.shutdownAndAwaitTerminationForever(coreLoadExecutor);
+      coreLoadExecutor.shutdown(); // doesn't block
+      if (!asyncSolrCoreLoad) {
+        ExecutorUtil.awaitTerminationForever(coreLoadExecutor);
       }
     }
 
@@ -1362,7 +1348,7 @@ public void shutdown() {
         zkSys.zkController.tryCancelAllElections();
       }
 
-      ExecutorUtil.shutdownAndAwaitTermination(coreContainerWorkExecutor);
+      ExecutorUtil.shutdownAndAwaitTermination(coreLoadExecutor); // actually already shutdown
 
       // First wake up the closer thread, it'll terminate almost immediately since it checks
       // isShutDown.

solr/core/src/test/org/apache/solr/handler/admin/MetricsHandlerTest.java

@@ -276,9 +276,8 @@ public void test() throws Exception {
     assertEquals(1, values.size());
     assertNotNull(values.get("solr.node"));
     values = (NamedList<?>) values.get("solr.node");
-    assertEquals(27, values.size());
+    assertEquals(15, values.size());
     assertNotNull(values.get("CONTAINER.cores.lazy")); // this is a gauge node
-    assertNotNull(values.get("CONTAINER.threadPool.coreContainerWorkExecutor.completed"));
     assertNotNull(values.get("CONTAINER.threadPool.coreLoadExecutor.completed"));
 
     resp = new SolrQueryResponse();
@@ -300,8 +299,7 @@ public void test() throws Exception {
     values = (NamedList<?>) values.get("metrics");
     assertNotNull(values.get("solr.node"));
     values = (NamedList<?>) values.get("solr.node");
-    assertEquals(7, values.size());
-    assertNotNull(values.get("CONTAINER.threadPool.coreContainerWorkExecutor.completed"));
+    assertEquals(5, values.size());
     assertNotNull(values.get("CONTAINER.threadPool.coreLoadExecutor.completed"));
 
     resp = new SolrQueryResponse();