/**
    * 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;
  
  import java.io.BufferedReader;
  import java.io.BufferedWriter;
  import java.io.File;
  import java.io.FileNotFoundException;
  import java.io.FileReader;
  import java.io.FileWriter;
  import java.io.IOException;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.zookeeper.MasterAddressTracker;
  import org.apache.hadoop.hbase.zookeeper.ZooKeeperWatcher;
  
  /**
   * <p>Contains a set of methods for the collaboration between the start/stop scripts and the
   * servers. It allows to delete immediately the znode when the master or the regions server crashes.
   * The region server / master writes a specific file when it starts / becomes main master. When they
   * end properly, they delete the file.</p>
   * <p>In the script, we check for the existence of these files when the program ends. If they still
   * exist we conclude that the server crashed, likely without deleting their znode. To have a faster
   * recovery we delete immediately the znode.</p>
   * <p>The strategy depends on the server type. For a region server we store the znode path in the
   * file, and use it to delete it. for a master, as the znode path constant whatever the server, we
   * check its content to make sure that the backup server is not now in charge.</p>
   */
  public class ZNodeClearer {
    public static final Log LOG = LogFactory.getLog(ZNodeClearer.class);
  
    private ZNodeClearer() {}
  
    /**
     * Logs the errors without failing on exception.
     */
    public static void writeMyEphemeralNodeOnDisk(String fileContent) {
      String fileName = ZNodeClearer.getMyEphemeralNodeFileName();
      if (fileName == null) {
        LOG.warn("Environment variable HBASE_ZNODE_FILE not set; znodes will not be cleared " +
          "on crash by start scripts (Longer MTTR!)");
        return;
      }
  
      FileWriter fstream;
      try {
        fstream = new FileWriter(fileName);
      } catch (IOException e) {
        LOG.warn("Can't write znode file "+fileName, e);
        return;
      }
  
      BufferedWriter out = new BufferedWriter(fstream);
  
      try {
        try {
          out.write(fileContent + "\n");
        } finally {
          try {
            out.close();
          } finally {
            fstream.close();
          }
        }
      } catch (IOException e) {
        LOG.warn("Can't write znode file "+fileName, e);
      }
    }
  
    /**
     * read the content of znode file, expects a single line.
     */
    public static String readMyEphemeralNodeOnDisk() throws IOException {
      String fileName = getMyEphemeralNodeFileName();
      if (fileName == null){
        throw new FileNotFoundException("No filename; set environment variable HBASE_ZNODE_FILE");
      }
      FileReader znodeFile = new FileReader(fileName);
      BufferedReader br = null;
      try {
        br = new BufferedReader(znodeFile);
       String file_content = br.readLine();
       return file_content;
     } finally {
       if (br != null) br.close();
     }
   }
 
   /**
    * Get the name of the file used to store the znode contents
    */
   public static String getMyEphemeralNodeFileName() {
     return System.getenv().get("HBASE_ZNODE_FILE");
   }
 
   /**
    *  delete the znode file
    */
   public static void deleteMyEphemeralNodeOnDisk() {
     String fileName = getMyEphemeralNodeFileName();
 
     if (fileName != null) {
       new File(fileName).delete();
     }
   }
 
   /**
    * Delete the master znode if its content (ServerName string) is the same
    *  as the one in the znode file. (env: HBASE_ZNODE_FILE).
    * @return true on successful deletion, false otherwise.
    */
   public static boolean clear(Configuration conf) {
     Configuration tempConf = new Configuration(conf);
     tempConf.setInt("zookeeper.recovery.retry", 0);
 
     ZooKeeperWatcher zkw;
     try {
       zkw = new ZooKeeperWatcher(tempConf, "clean znode for master",
           new Abortable() {
             @Override public void abort(String why, Throwable e) {}
             @Override public boolean isAborted() { return false; }
           });
     } catch (IOException e) {
       LOG.warn("Can't connect to zookeeper to read the master znode", e);
       return false;
     }
 
     String znodeFileContent;
     try {
       znodeFileContent = ZNodeClearer.readMyEphemeralNodeOnDisk();
       return MasterAddressTracker.deleteIfEquals(zkw, znodeFileContent);
     } catch (FileNotFoundException fnfe) {
       // If no file, just keep going -- return success.
       LOG.warn("Can't find the znode file; presume non-fatal", fnfe);
       return true;
     } catch (IOException e) {
       LOG.warn("Can't read the content of the znode file", e);
       return false;
     } finally {
       zkw.close();
     }
   }
 }