HDDS-11350. NullPointerException thrown on checking container balancer status

hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/container/balancer/ContainerBalancerTask.java

@@ -158,6 +158,14 @@ public ContainerBalancerTask(StorageContainerManager scm,
     this.selectedSources = new HashSet<>();
     this.selectedTargets = new HashSet<>();
     findSourceStrategy = new FindSourceGreedy(nodeManager);
+    if (config.getNetworkTopologyEnable()) {
+      findTargetStrategy = new FindTargetGreedyByNetworkTopology(
+          containerManager, placementPolicyValidateProxy,
+          nodeManager, networkTopology);
+    } else {
+      findTargetStrategy = new FindTargetGreedyByUsageInfo(containerManager,
+          placementPolicyValidateProxy, nodeManager);
+    }
     this.iterationsStatistic = new ArrayList<>();
   }
 
@@ -432,14 +440,7 @@ private boolean initializeIteration() {
     this.maxDatanodesRatioToInvolvePerIteration =
         config.getMaxDatanodesRatioToInvolvePerIteration();
     this.maxSizeToMovePerIteration = config.getMaxSizeToMovePerIteration();
-    if (config.getNetworkTopologyEnable()) {
-      findTargetStrategy = new FindTargetGreedyByNetworkTopology(
-          containerManager, placementPolicyValidateProxy,
-          nodeManager, networkTopology);
-    } else {
-      findTargetStrategy = new FindTargetGreedyByUsageInfo(containerManager,
-          placementPolicyValidateProxy, nodeManager);
-    }
+
     this.excludeNodes = config.getExcludeNodes();
     this.includeNodes = config.getIncludeNodes();
     // include/exclude nodes from balancing according to configs

hadoop-hdds/server-scm/src/test/java/org/apache/hadoop/hdds/scm/container/balancer/MockedSCM.java

@@ -86,7 +86,7 @@ public MockedSCM(@Nonnull TestableCluster testableCluster) {
     }
   }
 
-  private void init(@Nonnull ContainerBalancerConfiguration balancerConfig, @Nonnull OzoneConfiguration ozoneCfg) {
+  void init(@Nonnull ContainerBalancerConfiguration balancerConfig, @Nonnull OzoneConfiguration ozoneCfg) {
     ozoneCfg.setFromObject(balancerConfig);
     try {
       doMock(balancerConfig, ozoneCfg);

hadoop-hdds/server-scm/src/test/java/org/apache/hadoop/hdds/scm/container/balancer/TestContainerBalancerStatusInfo.java

@@ -19,7 +19,9 @@
 package org.apache.hadoop.hdds.scm.container.balancer;
 
 import org.apache.hadoop.hdds.conf.OzoneConfiguration;
+import org.apache.hadoop.hdds.scm.server.StorageContainerManager;
 import org.apache.hadoop.ozone.OzoneConsts;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
 import java.util.List;
@@ -55,4 +57,30 @@ void testGetIterationStatistics() {
     });
 
   }
+
+  /**
+   * @see <a href="https://issues.apache.org/jira/browse/HDDS-11350">HDDS-11350</a>
+   */
+  @Test
+  void testGetCurrentIterationsStatisticDoesNotThrowNullPointerExceptionWhenBalancingThreadIsSleeping() {
+    MockedSCM mockedScm = new MockedSCM(new TestableCluster(10, OzoneConsts.GB));
+    OzoneConfiguration ozoneConfig = new OzoneConfiguration();
+    ContainerBalancerConfiguration config = ozoneConfig.getObject(ContainerBalancerConfiguration.class);
+
+    config.setIterations(2);
+    // the following config makes the balancing thread go to sleep while waiting for DU to be triggered in DNs and
+    // updated storage reports to arrive via DN heartbeats - of course, this is a unit test and NodeManager, DNs etc.
+    // are all mocked
+    config.setTriggerDuEnable(true);
+    mockedScm.init(config, ozoneConfig);
+
+    // run ContainerBalancerTask in a new thread and have the current thread call getCurrentIterationsStatistic
+    StorageContainerManager scm = mockedScm.getStorageContainerManager();
+    ContainerBalancer cb = new ContainerBalancer(scm);
+    ContainerBalancerTask task = new ContainerBalancerTask(scm, 0, cb, cb.getMetrics(), config, false);
+    Thread thread = new Thread(task);
+    thread.setDaemon(true);
+    thread.start();
+    Assertions.assertDoesNotThrow(task::getCurrentIterationsStatistic);
+  }
 }