YARN-3957. FairScheduler NPE In FairSchedulerQueueInfo causing scheduler page to return 500. (Anubhav Dhoot via kasha)

(cherry picked from commit d19d187753)

hadoop-yarn-project/CHANGES.txt

@@ -620,6 +620,9 @@ Release 2.8.0 - UNRELEASED
     YARN-3845. Scheduler page does not render RGBA color combinations in IE11. 
     (Contributed by Mohammad Shahid Khan)
 
+    YARN-3957. FairScheduler NPE In FairSchedulerQueueInfo causing scheduler page to 
+    return 500. (Anubhav Dhoot via kasha)
+
 Release 2.7.2 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/webapp/dao/FairSchedulerQueueInfo.java

@@ -19,6 +19,7 @@
 package org.apache.hadoop.yarn.server.resourcemanager.webapp.dao;
 
 
+import java.util.ArrayList;
 import java.util.Collection;
 
 import javax.xml.bind.annotation.XmlAccessType;
@@ -204,6 +205,7 @@ public String getSchedulingPolicy() {
   }
 
   public Collection<FairSchedulerQueueInfo> getChildQueues() {
-    return childQueues.getQueueInfoList();
+    return childQueues != null ? childQueues.getQueueInfoList() :
+        new ArrayList<FairSchedulerQueueInfo>();
   }
 }

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/webapp/dao/TestFairSchedulerQueueInfo.java

@@ -0,0 +1,59 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF 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.apache.hadoop.yarn.server.resourcemanager.webapp.dao;
+
+import org.apache.hadoop.yarn.api.records.Resource;
+import org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.AllocationConfiguration;
+import org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FSQueue;
+import org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FairScheduler;
+import org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.FairSchedulerConfiguration;
+import org.apache.hadoop.yarn.server.resourcemanager.scheduler.fair.QueueManager;
+import org.apache.hadoop.yarn.util.SystemClock;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.Collection;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class TestFairSchedulerQueueInfo {
+
+  @Test
+  public void testEmptyChildQueues() throws Exception {
+    FairSchedulerConfiguration conf = new FairSchedulerConfiguration();
+    FairScheduler scheduler = mock(FairScheduler.class);
+    AllocationConfiguration allocConf = new AllocationConfiguration(conf);
+    when(scheduler.getAllocationConfiguration()).thenReturn(allocConf);
+    when(scheduler.getConf()).thenReturn(conf);
+    when(scheduler.getClusterResource()).thenReturn(Resource.newInstance(1, 1));
+    SystemClock clock = new SystemClock();
+    when(scheduler.getClock()).thenReturn(clock);
+    QueueManager queueManager = new QueueManager(scheduler);
+    queueManager.initialize(conf);
+
+    FSQueue testQueue = queueManager.getLeafQueue("test", true);
+    FairSchedulerQueueInfo queueInfo =
+        new FairSchedulerQueueInfo(testQueue, scheduler);
+    Collection<FairSchedulerQueueInfo> childQueues =
+        queueInfo.getChildQueues();
+    Assert.assertNotNull(childQueues);
+    Assert.assertEquals("Child QueueInfo was not empty", 0, childQueues.size());
+  }
+}