Wakeupable getPendingTask() {
        return pendingTask.get();
    }
If there is no pending task, set the pending task active.
    If wakeup was called before setting an active task, the current task will complete exceptionally with
    WakeupException right
    away.
    if there is an active task, throw exception.
@param currentTask
@param <T>
@return
clients/src/main/java/org/apache/kafka/clients/consumer/internals/WakeupTrigger.java