YARN-7489. ConcurrentModificationException in RMAppImpl#getRMAppMetrics. Contributed by Tao Yang.

(cherry picked from commit 5c2986e58dfc86fada8554a3f48cd417a4458abc)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/rmapp/RMAppImpl.java

@@ -1616,24 +1616,29 @@ public class RMAppImpl implements RMApp, Recoverable {
     long vcoreSeconds = 0;
     long preemptedMemorySeconds = 0;
     long preemptedVcoreSeconds = 0;
-    for (RMAppAttempt attempt : attempts.values()) {
-      if (null != attempt) {
-        RMAppAttemptMetrics attemptMetrics =
-            attempt.getRMAppAttemptMetrics();
-        Resources.addTo(resourcePreempted,
-            attemptMetrics.getResourcePreempted());
-        numAMContainerPreempted += attemptMetrics.getIsPreempted() ? 1 : 0;
-        numNonAMContainerPreempted +=
-            attemptMetrics.getNumNonAMContainersPreempted();
-        // getAggregateAppResourceUsage() will calculate resource usage stats
-        // for both running and finished containers.
-        AggregateAppResourceUsage resUsage =
-            attempt.getRMAppAttemptMetrics().getAggregateAppResourceUsage();
-        memorySeconds += resUsage.getMemorySeconds();
-        vcoreSeconds += resUsage.getVcoreSeconds();
-        preemptedMemorySeconds += attemptMetrics.getPreemptedMemory();
-        preemptedVcoreSeconds += attemptMetrics.getPreemptedVcore();
+    this.readLock.lock();
+    try {
+      for (RMAppAttempt attempt : attempts.values()) {
+        if (null != attempt) {
+          RMAppAttemptMetrics attemptMetrics =
+              attempt.getRMAppAttemptMetrics();
+          Resources.addTo(resourcePreempted,
+              attemptMetrics.getResourcePreempted());
+          numAMContainerPreempted += attemptMetrics.getIsPreempted() ? 1 : 0;
+          numNonAMContainerPreempted +=
+              attemptMetrics.getNumNonAMContainersPreempted();
+          // getAggregateAppResourceUsage() will calculate resource usage stats
+          // for both running and finished containers.
+          AggregateAppResourceUsage resUsage =
+              attempt.getRMAppAttemptMetrics().getAggregateAppResourceUsage();
+          memorySeconds += resUsage.getMemorySeconds();
+          vcoreSeconds += resUsage.getVcoreSeconds();
+          preemptedMemorySeconds += attemptMetrics.getPreemptedMemory();
+          preemptedVcoreSeconds += attemptMetrics.getPreemptedVcore();
+        }
       }
+    } finally {
+      this.readLock.unlock();
     }
 
     return new RMAppMetrics(resourcePreempted,