/**
 * 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.util;

import com.google.common.util.concurrent.ThreadFactoryBuilder;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * The <code>ShutdownHookManager</code> enables running shutdownHook
 * in a deterministic order, higher priority first.
 * <p/>
 * The JVM runs ShutdownHooks in a non-deterministic order or in parallel.
 * This class registers a single JVM shutdownHook and run all the
 * shutdownHooks registered to it (to this class) in order based on their
 * priority.
 */
public class ShutdownHookManager {

  private static final ShutdownHookManager MGR = new ShutdownHookManager();

  private static final Log LOG = LogFactory.getLog(ShutdownHookManager.class);
  private static final long TIMEOUT_DEFAULT = 10;
  private static final TimeUnit TIME_UNIT_DEFAULT = TimeUnit.SECONDS;

  private static final ExecutorService EXECUTOR =
      Executors.newSingleThreadExecutor(new ThreadFactoryBuilder()
          .setDaemon(true).build());
  static {
    try {
      Runtime.getRuntime().addShutdownHook(
        new Thread() {
          @Override
          public void run() {
            MGR.shutdownInProgress.set(true);
            for (HookEntry entry: MGR.getShutdownHooksInOrder()) {
              Future<?> future = EXECUTOR.submit(entry.getHook());
              try {
                future.get(entry.getTimeout(), entry.getTimeUnit());
              } catch (TimeoutException ex) {
                future.cancel(true);
                LOG.warn("ShutdownHook '" + entry.getHook().getClass().
                    getSimpleName() + "' timeout, " + ex.toString(), ex);
              } catch (Throwable ex) {
                LOG.warn("ShutdownHook '" + entry.getHook().getClass().
                    getSimpleName() + "' failed, " + ex.toString(), ex);
              }
            }
            try {
              EXECUTOR.shutdown();
              if (!EXECUTOR.awaitTermination(TIMEOUT_DEFAULT,
                  TIME_UNIT_DEFAULT)) {
                LOG.error("ShutdownHookManger shutdown forcefully.");
                EXECUTOR.shutdownNow();
              }
              LOG.debug("ShutdownHookManger complete shutdown.");
            } catch (InterruptedException ex) {
              LOG.error("ShutdownHookManger interrupted while waiting for " +
                  "termination.", ex);
              EXECUTOR.shutdownNow();
              Thread.currentThread().interrupt();
            }
          }
        }
      );
    } catch (IllegalStateException ex) {
      // JVM is being shut down. Ignore
      LOG.warn("Failed to add the ShutdownHook", ex);
    }
  }

  /**
   * Return <code>ShutdownHookManager</code> singleton.
   *
   * @return <code>ShutdownHookManager</code> singleton.
   */
  public static ShutdownHookManager get() {
    return MGR;
  }

  /**
   * Private structure to store ShutdownHook, its priority and timeout
   * settings.
   */
  static class HookEntry {
    private final Runnable hook;
    private final int priority;
    private final long timeout;
    private final TimeUnit unit;

    HookEntry(Runnable hook, int priority) {
      this(hook, priority, TIMEOUT_DEFAULT, TIME_UNIT_DEFAULT);
    }

    HookEntry(Runnable hook, int priority, long timeout, TimeUnit unit) {
      this.hook = hook;
      this.priority = priority;
      this.timeout = timeout;
      this.unit = unit;
    }

    @Override
    public int hashCode() {
      return hook.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
      boolean eq = false;
      if (obj != null) {
        if (obj instanceof HookEntry) {
          eq = (hook == ((HookEntry)obj).hook);
        }
      }
      return eq;
    }

    Runnable getHook() {
      return hook;
    }

    int getPriority() {
      return priority;
    }

    long getTimeout() {
      return timeout;
    }

    TimeUnit getTimeUnit() {
      return unit;
    }
  }

  private final Set<HookEntry> hooks =
      Collections.synchronizedSet(new HashSet<HookEntry>());

  private AtomicBoolean shutdownInProgress = new AtomicBoolean(false);

  //private to constructor to ensure singularity
  private ShutdownHookManager() {
  }

  /**
   * Returns the list of shutdownHooks in order of execution,
   * Highest priority first.
   *
   * @return the list of shutdownHooks in order of execution.
   */
  List<HookEntry> getShutdownHooksInOrder() {
    List<HookEntry> list;
    synchronized (MGR.hooks) {
      list = new ArrayList<HookEntry>(MGR.hooks);
    }
    Collections.sort(list, new Comparator<HookEntry>() {

      //reversing comparison so highest priority hooks are first
      @Override
      public int compare(HookEntry o1, HookEntry o2) {
        return o2.priority - o1.priority;
      }
    });
    return list;
  }

  /**
   * Adds a shutdownHook with a priority, the higher the priority
   * the earlier will run. ShutdownHooks with same priority run
   * in a non-deterministic order.
   *
   * @param shutdownHook shutdownHook <code>Runnable</code>
   * @param priority priority of the shutdownHook.
   */
  public void addShutdownHook(Runnable shutdownHook, int priority) {
    if (shutdownHook == null) {
      throw new IllegalArgumentException("shutdownHook cannot be NULL");
    }
    if (shutdownInProgress.get()) {
      throw new IllegalStateException("Shutdown in progress, cannot add a " +
          "shutdownHook");
    }
    hooks.add(new HookEntry(shutdownHook, priority));
  }

  /**
   *
   * Adds a shutdownHook with a priority and timeout the higher the priority
   * the earlier will run. ShutdownHooks with same priority run
   * in a non-deterministic order. The shutdown hook will be terminated if it
   * has not been finished in the specified period of time.
   *
   * @param shutdownHook shutdownHook <code>Runnable</code>
   * @param priority priority of the shutdownHook
   * @param timeout timeout of the shutdownHook
   * @param unit unit of the timeout <code>TimeUnit</code>
   */
  public void addShutdownHook(Runnable shutdownHook, int priority, long timeout,
      TimeUnit unit) {
    if (shutdownHook == null) {
      throw new IllegalArgumentException("shutdownHook cannot be NULL");
    }
    if (shutdownInProgress.get()) {
      throw new IllegalStateException("Shutdown in progress, cannot add a " +
          "shutdownHook");
    }
    hooks.add(new HookEntry(shutdownHook, priority, timeout, unit));
  }

  /**
   * Removes a shutdownHook.
   *
   * @param shutdownHook shutdownHook to remove.
   * @return TRUE if the shutdownHook was registered and removed,
   * FALSE otherwise.
   */
  public boolean removeShutdownHook(Runnable shutdownHook) {
    if (shutdownInProgress.get()) {
      throw new IllegalStateException("Shutdown in progress, cannot remove a " +
          "shutdownHook");
    }
    return hooks.remove(new HookEntry(shutdownHook, 0));
  }

  /**
   * Indicates if a shutdownHook is registered or not.
   *
   * @param shutdownHook shutdownHook to check if registered.
   * @return TRUE/FALSE depending if the shutdownHook is is registered.
   */
  public boolean hasShutdownHook(Runnable shutdownHook) {
    return hooks.contains(new HookEntry(shutdownHook, 0));
  }
  
  /**
   * Indicates if shutdown is in progress or not.
   * 
   * @return TRUE if the shutdown is in progress, otherwise FALSE.
   */
  public boolean isShutdownInProgress() {
    return shutdownInProgress.get();
  }

  /**
   * clear all registered shutdownHooks.
   */
  public void clearShutdownHooks() {
    hooks.clear();
  }
}