SOLR-15485: discourage Collections.singleton.forEach use (apache#179)

Resolved Conflicts:
	gradle/validation/validate-source-patterns.gradle
	solr/core/src/java/org/apache/solr/core/CoreContainer.java

Author: cpoerschke

Diff for: solr/core/src/java/org/apache/solr/core/CoreContainer.java

@@ -1138,9 +1138,7 @@ public void shutdown() {
       } finally {
         try {
           if (updateShardHandler != null) {
-            customThreadPool.submit(() -> Collections.singleton(shardHandlerFactory).parallelStream().forEach(c -> {
-              updateShardHandler.close();
-            }));
+            customThreadPool.submit(() -> updateShardHandler.close());
           }
         } finally {
           try {

Diff for: solr/test-framework/src/java/org/apache/solr/BaseDistributedSearchTestCase.java

@@ -414,9 +414,7 @@ protected String getShardsString() {
   protected void destroyServers() throws Exception {
     ExecutorService customThreadPool = ExecutorUtil.newMDCAwareCachedThreadPool(new SolrNamedThreadFactory("closeThreadPool"));
 
-    customThreadPool.submit(() -> Collections.singleton(controlClient).parallelStream().forEach(c -> {
-      IOUtils.closeQuietly(c);
-    }));
+    customThreadPool.submit(() -> IOUtils.closeQuietly(controlClient));
 
     customThreadPool.submit(() -> {
       try {

Diff for: solr/test-framework/src/java/org/apache/solr/cloud/AbstractFullDistribZkTestBase.java

@@ -1732,25 +1732,17 @@ public void distribTearDown() throws Exception {
   protected void destroyServers() throws Exception {
     ExecutorService customThreadPool = ExecutorUtil.newMDCAwareCachedThreadPool(new SolrNamedThreadFactory("closeThreadPool"));
 
-    customThreadPool.submit(() -> Collections.singleton(commonCloudSolrClient).parallelStream().forEach(c -> {
-      IOUtils.closeQuietly(c);
-    }));
+    customThreadPool.submit(() -> IOUtils.closeQuietly(commonCloudSolrClient));
 
-    customThreadPool.submit(() -> Collections.singleton(controlClient).parallelStream().forEach(c -> {
-      IOUtils.closeQuietly(c);
-    }));
+    customThreadPool.submit(() -> IOUtils.closeQuietly(controlClient));
 
     customThreadPool.submit(() -> coreClients.parallelStream().forEach(c -> {
       IOUtils.closeQuietly(c);
     }));
 
-    customThreadPool.submit(() -> Collections.singletonList(controlClientCloud).parallelStream().forEach(c -> {
-      IOUtils.closeQuietly(c);
-    }));
+    customThreadPool.submit(() -> IOUtils.closeQuietly(controlClientCloud));
 
-    customThreadPool.submit(() -> Collections.singletonList(cloudClient).parallelStream().forEach(c -> {
-      IOUtils.closeQuietly(c);
-    }));
+    customThreadPool.submit(() -> IOUtils.closeQuietly(cloudClient));
 
     ExecutorUtil.shutdownAndAwaitTermination(customThreadPool);