/**
    *
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.hadoop.hbase.util;
  
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.PriorityBlockingQueue;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.locks.Condition;
  import java.util.concurrent.locks.Lock;
  import java.util.concurrent.locks.ReentrantLock;
  
  /**
   * This queue allows a ThreadPoolExecutor to steal jobs from another ThreadPoolExecutor.
   * This queue also acts as the factory for creating the PriorityBlockingQueue to be used in the
   * steal-from ThreadPoolExecutor. The behavior of this queue is the same as a normal
   * PriorityBlockingQueue except the take/poll(long,TimeUnit) methods would also check whether there
   * are jobs in the steal-from queue if this q ueue is empty.
   *
   * Note the workers in ThreadPoolExecutor must be pre-started so that they can steal job from the
   * other queue, otherwise the worker will only be started after there are jobs submitted to main
   * queue.
   */
  @InterfaceAudience.Private
  public class StealJobQueue<T> extends PriorityBlockingQueue<T> {
  
    private BlockingQueue<T> stealFromQueue;
  
    private final Lock lock = new ReentrantLock();
    private final Condition notEmpty = lock.newCondition();
  
    public StealJobQueue() {
      this.stealFromQueue = new PriorityBlockingQueue<T>() {
        @Override
        public boolean offer(T t) {
          lock.lock();
          try {
            notEmpty.signal();
            return super.offer(t);
          } finally {
            lock.unlock();
          }
        }
      };
    }
  
    public BlockingQueue<T> getStealFromQueue() {
      return stealFromQueue;
    }
  
    @Override
    public boolean offer(T t) {
      lock.lock();
      try {
        notEmpty.signal();
        return super.offer(t);
      } finally {
        lock.unlock();
      }
    }
  
  
    @Override
    public T take() throws InterruptedException {
      lock.lockInterruptibly();
      try {
        while (true) {
          T retVal = this.poll();
          if (retVal == null) {
            retVal = stealFromQueue.poll();
          }
          if (retVal == null) {
            notEmpty.await();
          } else {
            return retVal;
          }
        }
      } finally {
        lock.unlock();
      }
    }
  
   @Override
   public T poll(long timeout, TimeUnit unit) throws InterruptedException {
     long nanos = unit.toNanos(timeout);
     lock.lockInterruptibly();
     try {
       while (true) {
         T retVal = this.poll();
         if (retVal == null) {
           retVal = stealFromQueue.poll();
         }
         if (retVal == null) {
           if (nanos <= 0)
             return null;
           nanos = notEmpty.awaitNanos(nanos);
         } else {
           return retVal;
         }
       }
     } finally {
       lock.unlock();
     }
   }
 }