YARN-8025. UsersManangers#getComputedResourceLimitForActiveUsers throws NPE due to preComputedActiveUserLimit is empty. Contributed by Tao Yang.

(Cherry picked from commit 67f239c42f676237290d18ddbbc9aec369267692)

(cherry picked from commit a0b7abf278c147d3ae7821c6783a85cb972c6ec2)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/UsersManager.java

@@ -491,11 +491,12 @@ public class UsersManager implements AbstractUsersManager {
       Resource clusterResource, String nodePartition,
       SchedulingMode schedulingMode) {
 
-    Map<SchedulingMode, Resource> userLimitPerSchedulingMode = preComputedActiveUserLimit
-        .get(nodePartition);
+    Map<SchedulingMode, Resource> userLimitPerSchedulingMode;
 
     try {
       writeLock.lock();
+      userLimitPerSchedulingMode =
+          preComputedActiveUserLimit.get(nodePartition);
       if (isRecomputeNeeded(schedulingMode, nodePartition, true)) {
         // recompute
         userLimitPerSchedulingMode = reComputeUserLimits(userName,
@@ -549,11 +550,11 @@ public class UsersManager implements AbstractUsersManager {
       Resource clusterResource, String nodePartition,
       SchedulingMode schedulingMode) {
 
-    Map<SchedulingMode, Resource> userLimitPerSchedulingMode = preComputedAllUserLimit
-        .get(nodePartition);
+    Map<SchedulingMode, Resource> userLimitPerSchedulingMode;
 
     try {
       writeLock.lock();
+      userLimitPerSchedulingMode = preComputedAllUserLimit.get(nodePartition);
       if (isRecomputeNeeded(schedulingMode, nodePartition, false)) {
         // recompute
         userLimitPerSchedulingMode = reComputeUserLimits(userName,