Varun Saxena created YARN-2983:

             Summary: NPE possible in ClientRMService#getQueueInfo
                 Key: YARN-2983
             Project: Hadoop YARN
          Issue Type: Bug
          Components: resourcemanager
    Affects Versions: 2.5.1
            Reporter: Varun Saxena
            Assignee: Varun Saxena

While going through code for checking YARN-2978 , found one issue. 
During construction GetQueueInfoResponse in {{ClientRMService#getQueueInfo}}, 
we first collect application attempts from scheduler and then get apps from a 
ConcurrentHashMap in {{RMContext}}. Although the operation(get/put/remove,etc) 
itself on a ConcurrentHashMap is thread-safe, but a series of multiple 
ConcurrentHashMap#get (say, in a for loop) is not. 

For instance, in code below, we are calling rmContext.getRMApps()#get in a 
loop. Now a ConcurrentHashMap#get can return null if the key doesnt exist. But 
there is no null check inside this for loop before dereferencing the value 
returned i.e. rmApp. Although all the applicationattempts  have been fetched 
for the queue just above the for loop, but as this block of code is not 
synchronized, there is a possibility that another thread may delete RMApp from 
the ConcurrentHashMap at the same time. This can happen when an app 
finishes/completes and number of completed apps exceed the config  
I think there should be a null check inside this for loop, otherwise a NPE can 

public GetQueueInfoResponse getQueueInfo(GetQueueInfoRequest request)
      throws YarnException {
  if (request.getIncludeApplications()) {
    List<ApplicationAttemptId> apps =
    appReports = new ArrayList<ApplicationReport>(apps.size());
    for (ApplicationAttemptId app : apps) {
      RMApp rmApp = rmContext.getRMApps().get(app.getApplicationId());
      appReports.add(rmApp.createAndGetApplicationReport(null, true));

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