private E innerRemoveFirst() {
    E f = list.pollFirst();
    if (f == null)
      return null;


    notFull.signal();
    return f;
  }
Removes and returns first element, or null if empty.

@return The removed element.
llap-server/src/java/org/apache/hadoop/hive/llap/daemon/impl/PriorityBlockingDeque.java