YARN-10499. TestRouterWebServiceREST fails (#2490). Contributed by Akira Ajisaka

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/placement/CSMappingPlacementRule.java

@@ -181,6 +181,11 @@ public class CSMappingPlacementRule extends PlacementRule {
       return;
     }
     Set<String> groupsSet = groups.getGroupsSet(user);
+    if (groupsSet.isEmpty()) {
+      LOG.warn("There are no groups for user {}", user);
+      vctx.putExtraDataset("groups", groupsSet);
+      return;
+    }
     String secondaryGroup = null;
     Iterator<String> it = groupsSet.iterator();
     String primaryGroup = it.next();