YARN-8464. Async scheduling thread could be interrupted when there are no NodeManagers in cluster. (Sunil G via wangda)

Change-Id: I4f5f856373378685713e77752ba6cf0988a66065
(cherry picked from commit bedc4fe079)

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

@@ -519,7 +519,14 @@ public class CapacityScheduler extends
     // First randomize the start point
     int current = 0;
     Collection<FiCaSchedulerNode> nodes = cs.nodeTracker.getAllNodes();
-    int start = random.nextInt(nodes.size());
+
+    // If nodes size is 0 (when there are no node managers registered,
+    // we can return from here itself.
+    int nodeSize = nodes.size();
+    if(nodeSize == 0) {
+      return;
+    }
+    int start = random.nextInt(nodeSize);
 
     // To avoid too verbose DEBUG logging, only print debug log once for
     // every 10 secs.
@@ -572,6 +579,7 @@ public class CapacityScheduler extends
 
     @Override
     public void run() {
+      int debuggingLogCounter = 0;
       while (!Thread.currentThread().isInterrupted()) {
         try {
           if (!runSchedules.get()) {
@@ -583,6 +591,14 @@ public class CapacityScheduler extends
               Thread.sleep(1);
             } else{
               schedule(cs);
+              if(LOG.isDebugEnabled()) {
+                // Adding a debug log here to ensure that the thread is alive
+                // and running fine.
+                if (debuggingLogCounter++ > 10000) {
+                  debuggingLogCounter = 0;
+                  LOG.debug("AsyncScheduleThread[" + getName() + "] is running!");
+                }
+              }
             }
           }
         } catch (InterruptedException ie) {