/**
    * 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.master.snapshot;
  
  import java.io.FileNotFoundException;
  import java.io.IOException;
  import java.util.Collection;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Map;
  import java.util.Set;
  import java.util.Timer;
  import java.util.TimerTask;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.classification.InterfaceStability;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.fs.FileStatus;
  import org.apache.hadoop.fs.FileSystem;
  import org.apache.hadoop.fs.Path;
  import org.apache.hadoop.hbase.Stoppable;
  import org.apache.hadoop.hbase.snapshot.SnapshotDescriptionUtils;
  import org.apache.hadoop.hbase.util.FSUtils;
  
  /**
   * Intelligently keep track of all the files for all the snapshots.
   * <p>
   * A cache of files is kept to avoid querying the {@link FileSystem} frequently. If there is a cache
   * miss the directory modification time is used to ensure that we don't rescan directories that we
   * already have in cache. We only check the modification times of the snapshot directories
   * (/hbase/.snapshot/[snapshot_name]) to determine if the files need to be loaded into the cache.
   * <p>
   * New snapshots will be added to the cache and deleted snapshots will be removed when we refresh
   * the cache. If the files underneath a snapshot directory are changed, but not the snapshot itself,
   * we will ignore updates to that snapshot's files.
   * <p>
   * This is sufficient because each snapshot has its own directory and is added via an atomic rename
   * <i>once</i>, when the snapshot is created. We don't need to worry about the data in the snapshot
   * being run.
   * <p>
   * Further, the cache is periodically refreshed ensure that files in snapshots that were deleted are
   * also removed from the cache.
   * <p>
   * A {@link SnapshotFileCache.SnapshotFileInspector} must be passed when creating <tt>this</tt> to
   * allow extraction of files under /hbase/.snapshot/[snapshot name] directory, for each snapshot.
   * This allows you to only cache files under, for instance, all the logs in the .logs directory or
   * all the files under all the regions.
   * <p>
   * <tt>this</tt> also considers all running snapshots (those under /hbase/.snapshot/.tmp) as valid
   * snapshots and will attempt to cache files from those snapshots as well.
   * <p>
   * Queries about a given file are thread-safe with respect to multiple queries and cache refreshes.
   */
  @InterfaceAudience.Private
  @InterfaceStability.Evolving
  public class SnapshotFileCache implements Stoppable {
    interface SnapshotFileInspector {
      /**
       * Returns a collection of file names needed by the snapshot.
       * @param snapshotDir {@link Path} to the snapshot directory to scan.
       * @return the collection of file names needed by the snapshot.
       */
      Collection<String> filesUnderSnapshot(final Path snapshotDir) throws IOException;
    }
  
    private static final Log LOG = LogFactory.getLog(SnapshotFileCache.class);
    private volatile boolean stop = false;
    private final FileSystem fs;
    private final SnapshotFileInspector fileInspector;
    private final Path snapshotDir;
    private final Set<String> cache = new HashSet<String>();
    /**
     * This is a helper map of information about the snapshot directories so we don't need to rescan
     * them if they haven't changed since the last time we looked.
     */
    private final Map<String, SnapshotDirectoryInfo> snapshots =
        new HashMap<String, SnapshotDirectoryInfo>();
    private final Timer refreshTimer;
  
    private long lastModifiedTime = Long.MIN_VALUE;
  
    /**
    * Create a snapshot file cache for all snapshots under the specified [root]/.snapshot on the
    * filesystem.
    * <p>
    * Immediately loads the file cache.
    * @param conf to extract the configured {@link FileSystem} where the snapshots are stored and
    *          hbase root directory
    * @param cacheRefreshPeriod frequency (ms) with which the cache should be refreshed
    * @param refreshThreadName name of the cache refresh thread
    * @param inspectSnapshotFiles Filter to apply to each snapshot to extract the files.
    * @throws IOException if the {@link FileSystem} or root directory cannot be loaded
    */
   public SnapshotFileCache(Configuration conf, long cacheRefreshPeriod, String refreshThreadName,
       SnapshotFileInspector inspectSnapshotFiles) throws IOException {
     this(FSUtils.getCurrentFileSystem(conf), FSUtils.getRootDir(conf), 0, cacheRefreshPeriod,
         refreshThreadName, inspectSnapshotFiles);
   }
 
   /**
    * Create a snapshot file cache for all snapshots under the specified [root]/.snapshot on the
    * filesystem
    * @param fs {@link FileSystem} where the snapshots are stored
    * @param rootDir hbase root directory
    * @param cacheRefreshPeriod period (ms) with which the cache should be refreshed
    * @param cacheRefreshDelay amount of time to wait for the cache to be refreshed
    * @param refreshThreadName name of the cache refresh thread
    * @param inspectSnapshotFiles Filter to apply to each snapshot to extract the files.
    */
   public SnapshotFileCache(FileSystem fs, Path rootDir, long cacheRefreshPeriod,
       long cacheRefreshDelay, String refreshThreadName, SnapshotFileInspector inspectSnapshotFiles) {
     this.fs = fs;
     this.fileInspector = inspectSnapshotFiles;
     this.snapshotDir = SnapshotDescriptionUtils.getSnapshotsDir(rootDir);
     // periodically refresh the file cache to make sure we aren't superfluously saving files.
     this.refreshTimer = new Timer(refreshThreadName, true);
     this.refreshTimer.scheduleAtFixedRate(new RefreshCacheTask(), cacheRefreshDelay,
       cacheRefreshPeriod);
   }
 
   /**
    * Trigger a cache refresh, even if its before the next cache refresh. Does not affect pending
    * cache refreshes.
    * <p>
    * Blocks until the cache is refreshed.
    * <p>
    * Exposed for TESTING.
    */
   public void triggerCacheRefreshForTesting() {
     try {
       SnapshotFileCache.this.refreshCache();
     } catch (IOException e) {
       LOG.warn("Failed to refresh snapshot hfile cache!", e);
     }
     LOG.debug("Current cache:" + cache);
   }
 
   /**
    * Check to see if the passed file name is contained in any of the snapshots. First checks an
    * in-memory cache of the files to keep. If its not in the cache, then the cache is refreshed and
    * the cache checked again for that file. This ensures that we always return <tt>true</tt> for a
    * files that exists.
    * <p>
    * Note this may lead to periodic false positives for the file being referenced. Periodically, the
    * cache is refreshed even if there are no requests to ensure that the false negatives get removed
    * eventually. For instance, suppose you have a file in the snapshot and it gets loaded into the
    * cache. Then at some point later that snapshot is deleted. If the cache has not been refreshed
    * at that point, cache will still think the file system contains that file and return
    * <tt>true</tt>, even if it is no longer present (false positive). However, if the file never was
    * on the filesystem, we will never find it and always return <tt>false</tt>.
    * @param fileName file to check
    * @return <tt>false</tt> if the file is not referenced in any current or running snapshot,
    *         <tt>true</tt> if the file is in the cache.
    * @throws IOException if there is an unexpected error reaching the filesystem.
    */
   // XXX this is inefficient to synchronize on the method, when what we really need to guard against
   // is an illegal access to the cache. Really we could do a mutex-guarded pointer swap on the
   // cache, but that seems overkill at the moment and isn't necessarily a bottleneck.
   public synchronized boolean contains(String fileName) throws IOException {
     boolean hasFile = this.cache.contains(fileName);
     if (!hasFile) {
       refreshCache();
       // then check again
       hasFile = this.cache.contains(fileName);
     }
     return hasFile;
   }
 
   private synchronized void refreshCache() throws IOException {
     long lastTimestamp = Long.MAX_VALUE;
     boolean hasChanges = false;
 
     // get the status of the snapshots directory and check if it is has changes
     try {
       FileStatus dirStatus = fs.getFileStatus(snapshotDir);
       lastTimestamp = dirStatus.getModificationTime();
       hasChanges |= (lastTimestamp >= lastModifiedTime);
     } catch (FileNotFoundException e) {
       if (this.cache.size() > 0) {
         LOG.error("Snapshot directory: " + snapshotDir + " doesn't exist");
       }
       return;
     }
 
     // get the status of the snapshots temporary directory and check if it has changes
     // The top-level directory timestamp is not updated, so we have to check the inner-level.
     try {
       Path snapshotTmpDir = new Path(snapshotDir, SnapshotDescriptionUtils.SNAPSHOT_TMP_DIR_NAME);
       FileStatus tempDirStatus = fs.getFileStatus(snapshotTmpDir);
       lastTimestamp = Math.min(lastTimestamp, tempDirStatus.getModificationTime());
       hasChanges |= (lastTimestamp >= lastModifiedTime);
       if (!hasChanges) {
         FileStatus[] tmpSnapshots = FSUtils.listStatus(fs, snapshotDir);
         if (tmpSnapshots != null) {
           for (FileStatus dirStatus: tmpSnapshots) {
             lastTimestamp = Math.min(lastTimestamp, dirStatus.getModificationTime());
           }
           hasChanges |= (lastTimestamp >= lastModifiedTime);
         }
       }
     } catch (FileNotFoundException e) {
       // Nothing todo, if the tmp dir is empty
     }
 
     // if the snapshot directory wasn't modified since we last check, we are done
     if (!hasChanges) {
       return;
     }
 
     // directory was modified, so we need to reload our cache
     // there could be a slight race here where we miss the cache, check the directory modification
     // time, then someone updates the directory, causing us to not scan the directory again.
     // However, snapshot directories are only created once, so this isn't an issue.
 
     // 1. update the modified time
     this.lastModifiedTime = lastTimestamp;
 
     // 2.clear the cache
     this.cache.clear();
     Map<String, SnapshotDirectoryInfo> known = new HashMap<String, SnapshotDirectoryInfo>();
 
     // 3. check each of the snapshot directories
     FileStatus[] snapshots = FSUtils.listStatus(fs, snapshotDir);
     if (snapshots == null) {
       // remove all the remembered snapshots because we don't have any left
       if (LOG.isDebugEnabled() && this.snapshots.size() > 0) {
         LOG.debug("No snapshots on-disk, cache empty");
       }
       this.snapshots.clear();
       return;
     }
 
     // 3.1 iterate through the on-disk snapshots
     for (FileStatus snapshot : snapshots) {
       String name = snapshot.getPath().getName();
       // its the tmp dir
       if (name.equals(SnapshotDescriptionUtils.SNAPSHOT_TMP_DIR_NAME)) {
         // only add those files to the cache, but not to the known snapshots
         FileStatus[] running = FSUtils.listStatus(fs, snapshot.getPath());
         if (running == null) continue;
         for (FileStatus run : running) {
           this.cache.addAll(fileInspector.filesUnderSnapshot(run.getPath()));
         }
       } else {
         SnapshotDirectoryInfo files = this.snapshots.remove(name);
         // 3.1.1 if we don't know about the snapshot or its been modified, we need to update the files
         // the latter could occur where I create a snapshot, then delete it, and then make a new
         // snapshot with the same name. We will need to update the cache the information from that new
         // snapshot, even though it has the same name as the files referenced have probably changed.
         if (files == null || files.hasBeenModified(snapshot.getModificationTime())) {
           // get all files for the snapshot and create a new info
           Collection<String> storedFiles = fileInspector.filesUnderSnapshot(snapshot.getPath());
           files = new SnapshotDirectoryInfo(snapshot.getModificationTime(), storedFiles);
         }
         // 3.2 add all the files to cache
         this.cache.addAll(files.getFiles());
         known.put(name, files);
       }
     }
 
     // 4. set the snapshots we are tracking
     this.snapshots.clear();
     this.snapshots.putAll(known);
   }
 
   /**
    * Simple helper task that just periodically attempts to refresh the cache
    */
   public class RefreshCacheTask extends TimerTask {
     @Override
     public void run() {
       try {
         SnapshotFileCache.this.refreshCache();
       } catch (IOException e) {
         LOG.warn("Failed to refresh snapshot hfile cache!", e);
       }
     }
   }
 
   @Override
   public void stop(String why) {
     if (!this.stop) {
       this.stop = true;
       this.refreshTimer.cancel();
     }
 
   }
 
   @Override
   public boolean isStopped() {
     return this.stop;
   }
 
   /**
    * Information about a snapshot directory
    */
   private static class SnapshotDirectoryInfo {
     long lastModified;
     Collection<String> files;
 
     public SnapshotDirectoryInfo(long mtime, Collection<String> files) {
       this.lastModified = mtime;
       this.files = files;
     }
 
     /**
      * @return the hfiles in the snapshot when <tt>this</tt> was made.
      */
     public Collection<String> getFiles() {
       return this.files;
     }
 
     /**
      * Check if the snapshot directory has been modified
      * @param mtime current modification time of the directory
      * @return <tt>true</tt> if it the modification time of the directory is newer time when we
      *         created <tt>this</tt>
      */
     public boolean hasBeenModified(long mtime) {
       return this.lastModified < mtime;
     }
   }
 }