/**
    *
    * 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.monitoring;
  
  import java.io.PrintWriter;
  import java.lang.ref.WeakReference;
  import java.lang.reflect.InvocationHandler;
  import java.lang.reflect.Method;
  import java.lang.reflect.Proxy;
  import java.util.ArrayList;
  import java.util.Iterator;
  import java.util.List;
  
  import org.apache.commons.collections.buffer.CircularFifoBuffer;
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  
  import com.google.common.annotations.VisibleForTesting;
  import com.google.common.collect.Lists;
  
  /**
   * Singleton which keeps track of tasks going on in this VM.
   * A Task here is anything which takes more than a few seconds
   * and the user might want to inquire about the status
   */
  @InterfaceAudience.Private
  public class TaskMonitor {
    private static final Log LOG = LogFactory.getLog(TaskMonitor.class);
  
    // Don't keep around any tasks that have completed more than
    // 60 seconds ago
    private static final long EXPIRATION_TIME = 60*1000;
  
    @VisibleForTesting
    static final int MAX_TASKS = 1000;
    
    private static TaskMonitor instance;
    private CircularFifoBuffer tasks = new CircularFifoBuffer(MAX_TASKS);
  
    /**
     * Get singleton instance.
     * TODO this would be better off scoped to a single daemon
     */
    public static synchronized TaskMonitor get() {
      if (instance == null) {
        instance = new TaskMonitor();
      }
      return instance;
    }
    
    public synchronized MonitoredTask createStatus(String description) {
      MonitoredTask stat = new MonitoredTaskImpl();
      stat.setDescription(description);
      MonitoredTask proxy = (MonitoredTask) Proxy.newProxyInstance(
          stat.getClass().getClassLoader(),
          new Class<?>[] { MonitoredTask.class },
          new PassthroughInvocationHandler<MonitoredTask>(stat));
      TaskAndWeakRefPair pair = new TaskAndWeakRefPair(stat, proxy);
      tasks.add(pair);
      return proxy;
    }
  
    public synchronized MonitoredRPCHandler createRPCStatus(String description) {
      MonitoredRPCHandler stat = new MonitoredRPCHandlerImpl();
      stat.setDescription(description);
      MonitoredRPCHandler proxy = (MonitoredRPCHandler) Proxy.newProxyInstance(
          stat.getClass().getClassLoader(),
          new Class<?>[] { MonitoredRPCHandler.class },
          new PassthroughInvocationHandler<MonitoredRPCHandler>(stat));
      TaskAndWeakRefPair pair = new TaskAndWeakRefPair(stat, proxy);
      tasks.add(pair);
      return proxy;
    }
  
    private synchronized void purgeExpiredTasks() {
      for (Iterator<TaskAndWeakRefPair> it = tasks.iterator();
           it.hasNext();) {
        TaskAndWeakRefPair pair = it.next();
        MonitoredTask stat = pair.get();
        
        if (pair.isDead()) {
          // The class who constructed this leaked it. So we can
         // assume it's done.
         if (stat.getState() == MonitoredTaskImpl.State.RUNNING) {
           LOG.warn("Status " + stat + " appears to have been leaked");
           stat.cleanup();
         }
       }
       
       if (canPurge(stat)) {
         it.remove();
       }
     }
   }
 
   /**
    * Produces a list containing copies of the current state of all non-expired 
    * MonitoredTasks handled by this TaskMonitor.
    * @return A complete list of MonitoredTasks.
    */
   public synchronized List<MonitoredTask> getTasks() {
     purgeExpiredTasks();
     ArrayList<MonitoredTask> ret = Lists.newArrayListWithCapacity(tasks.size());
     for (Iterator<TaskAndWeakRefPair> it = tasks.iterator();
          it.hasNext();) {
       TaskAndWeakRefPair pair = it.next();
       MonitoredTask t = pair.get();
       ret.add(t.clone());
     }
     return ret;
   }
 
   private boolean canPurge(MonitoredTask stat) {
     long cts = stat.getCompletionTimestamp();
     return (cts > 0 && System.currentTimeMillis() - cts > EXPIRATION_TIME);
   }
   
 
   public void dumpAsText(PrintWriter out) {
     long now = System.currentTimeMillis();
     
     List<MonitoredTask> tasks = getTasks();
     for (MonitoredTask task : tasks) {
       out.println("Task: " + task.getDescription());
       out.println("Status: " + task.getState() + ":" + task.getStatus());
       long running = (now - task.getStartTime())/1000;
       if (task.getCompletionTimestamp() != -1) {
         long completed = (now - task.getCompletionTimestamp()) / 1000;
         out.println("Completed " + completed + "s ago");
         out.println("Ran for " +
             (task.getCompletionTimestamp() - task.getStartTime())/1000
             + "s");
       } else {
         out.println("Running for " + running + "s");
       }
       out.println();
     }
   }
 
   /**
    * This class encapsulates an object as well as a weak reference to a proxy
    * that passes through calls to that object. In art form:
    * <code>
    *     Proxy  <------------------
    *       |                       \
    *       v                        \
    * PassthroughInvocationHandler   |  weak reference
    *       |                       /
    * MonitoredTaskImpl            / 
    *       |                     /
    * StatAndWeakRefProxy  ------/
    *
    * Since we only return the Proxy to the creator of the MonitorableStatus,
    * this means that they can leak that object, and we'll detect it
    * since our weak reference will go null. But, we still have the actual
    * object, so we can log it and display it as a leaked (incomplete) action.
    */
   private static class TaskAndWeakRefPair {
     private MonitoredTask impl;
     private WeakReference<MonitoredTask> weakProxy;
     
     public TaskAndWeakRefPair(MonitoredTask stat,
         MonitoredTask proxy) {
       this.impl = stat;
       this.weakProxy = new WeakReference<MonitoredTask>(proxy);
     }
     
     public MonitoredTask get() {
       return impl;
     }
     
     public boolean isDead() {
       return weakProxy.get() == null;
     }
   }
   
   /**
    * An InvocationHandler that simply passes through calls to the original 
    * object.
    */
   private static class PassthroughInvocationHandler<T> implements InvocationHandler {
     private T delegatee;
     
     public PassthroughInvocationHandler(T delegatee) {
       this.delegatee = delegatee;
     }
 
     @Override
     public Object invoke(Object proxy, Method method, Object[] args)
         throws Throwable {
       return method.invoke(delegatee, args);
     }    
   }
 }