@Override
      public int compare(TezSessionPoolSession o1, TezSessionPoolSession o2) {
        assert o1.getExpirationNs() != null && o2.getExpirationNs() != null;
        return o1.getExpirationNs().compareTo(o2.getExpirationNs());
      }
The background restart queue that is populated when expiration is triggered by a foreground
thread (i.e. getting or returning a session), to avoid delaying it.
ql/src/java/org/apache/hadoop/hive/ql/exec/tez/SessionExpirationTracker.java