/**
    * 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.ArrayList;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  import java.util.concurrent.ThreadPoolExecutor;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.classification.InterfaceAudience;
  import org.apache.hadoop.classification.InterfaceStability;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.fs.FSDataInputStream;
  import org.apache.hadoop.fs.FileStatus;
  import org.apache.hadoop.fs.FileSystem;
  import org.apache.hadoop.fs.Path;
  import org.apache.hadoop.hbase.HConstants;
  import org.apache.hadoop.hbase.HTableDescriptor;
  import org.apache.hadoop.hbase.Stoppable;
  import org.apache.hadoop.hbase.catalog.MetaReader;
  import org.apache.hadoop.hbase.errorhandling.ForeignException;
  import org.apache.hadoop.hbase.exceptions.HBaseSnapshotException;
  import org.apache.hadoop.hbase.exceptions.RestoreSnapshotException;
  import org.apache.hadoop.hbase.exceptions.SnapshotCreationException;
  import org.apache.hadoop.hbase.exceptions.SnapshotDoesNotExistException;
  import org.apache.hadoop.hbase.exceptions.SnapshotExistsException;
  import org.apache.hadoop.hbase.exceptions.TablePartiallyOpenException;
  import org.apache.hadoop.hbase.exceptions.UnknownSnapshotException;
  import org.apache.hadoop.hbase.executor.ExecutorService;
  import org.apache.hadoop.hbase.master.AssignmentManager;
  import org.apache.hadoop.hbase.master.MasterCoprocessorHost;
  import org.apache.hadoop.hbase.master.MasterFileSystem;
  import org.apache.hadoop.hbase.master.MasterServices;
  import org.apache.hadoop.hbase.master.MetricsMaster;
  import org.apache.hadoop.hbase.master.SnapshotSentinel;
  import org.apache.hadoop.hbase.master.cleaner.HFileCleaner;
  import org.apache.hadoop.hbase.master.cleaner.HFileLinkCleaner;
  import org.apache.hadoop.hbase.procedure.Procedure;
  import org.apache.hadoop.hbase.procedure.ProcedureCoordinator;
  import org.apache.hadoop.hbase.procedure.ProcedureCoordinatorRpcs;
  import org.apache.hadoop.hbase.procedure.ZKProcedureCoordinatorRpcs;
  import org.apache.hadoop.hbase.protobuf.generated.HBaseProtos.SnapshotDescription;
  import org.apache.hadoop.hbase.protobuf.generated.HBaseProtos.SnapshotDescription.Type;
  import org.apache.hadoop.hbase.snapshot.ClientSnapshotDescriptionUtils;
  import org.apache.hadoop.hbase.snapshot.RestoreSnapshotHelper;
  import org.apache.hadoop.hbase.snapshot.SnapshotDescriptionUtils;
  import org.apache.hadoop.hbase.util.Bytes;
  import org.apache.hadoop.hbase.util.EnvironmentEdgeManager;
  import org.apache.hadoop.hbase.util.FSTableDescriptors;
  import org.apache.hadoop.hbase.util.FSUtils;
  import org.apache.zookeeper.KeeperException;
  
  /**
   * This class manages the procedure of taking and restoring snapshots. There is only one
   * SnapshotManager for the master.
   * <p>
   * The class provides methods for monitoring in-progress snapshot actions.
   * <p>
   * Note: Currently there can only be one snapshot being taken at a time over the cluster. This is a
   * simplification in the current implementation.
   */
  @InterfaceAudience.Private
  @InterfaceStability.Unstable
  public class SnapshotManager implements Stoppable {
    private static final Log LOG = LogFactory.getLog(SnapshotManager.class);
  
    /** By default, check to see if the snapshot is complete every WAKE MILLIS (ms) */
    private static final int SNAPSHOT_WAKE_MILLIS_DEFAULT = 500;
  
    /**
     * Wait time before removing a finished sentinel from the in-progress map
     *
     * NOTE: This is used as a safety auto cleanup.
     * The snapshot and restore handlers map entries are removed when a user asks if a snapshot or
     * restore is completed. This operation is part of the HBaseAdmin snapshot/restore API flow.
    * In case something fails on the client side and the snapshot/restore state is not reclaimed
    * after a default timeout, the entry is removed from the in-progress map.
    * At this point, if the user asks for the snapshot/restore status, the result will be
    * snapshot done if exists or failed if it doesn't exists.
    */
   private static final int SNAPSHOT_SENTINELS_CLEANUP_TIMEOUT = 60 * 1000;
 
   /** Enable or disable snapshot support */
   public static final String HBASE_SNAPSHOT_ENABLED = "hbase.snapshot.enabled";
 
   /**
    * Conf key for # of ms elapsed between checks for snapshot errors while waiting for
    * completion.
    */
   private static final String SNAPSHOT_WAKE_MILLIS_KEY = "hbase.snapshot.master.wakeMillis";
 
   /** By default, check to see if the snapshot is complete (ms) */
   private static final int SNAPSHOT_TIMEOUT_MILLIS_DEFAULT = 5000;
 
   /**
    * Conf key for # of ms elapsed before injecting a snapshot timeout error when waiting for
    * completion.
    */
   private static final String SNAPSHOT_TIMEOUT_MILLIS_KEY = "hbase.snapshot.master.timeoutMillis";
 
   /** Name of the operation to use in the controller */
   public static final String ONLINE_SNAPSHOT_CONTROLLER_DESCRIPTION = "online-snapshot";
 
   /** Conf key for # of threads used by the SnapshotManager thread pool */
   private static final String SNAPSHOT_POOL_THREADS_KEY = "hbase.snapshot.master.threads";
 
   /** number of current operations running on the master */
   private static final int SNAPSHOT_POOL_THREADS_DEFAULT = 1;
 
   private boolean stopped;
   private final long wakeFrequency;
   private final MasterServices master;  // Needed by TableEventHandlers
   private final MetricsMaster metricsMaster;
   private final ProcedureCoordinator coordinator;
 
   // Is snapshot feature enabled?
   private boolean isSnapshotSupported = false;
 
   // Snapshot handlers map, with table name as key.
   // The map is always accessed and modified under the object lock using synchronized.
   // snapshotTable() will insert an Handler in the table.
   // isSnapshotDone() will remove the handler requested if the operation is finished.
   private Map<String, SnapshotSentinel> snapshotHandlers = new HashMap<String, SnapshotSentinel>();
 
   // Restore Sentinels map, with table name as key.
   // The map is always accessed and modified under the object lock using synchronized.
   // restoreSnapshot()/cloneSnapshot() will insert an Handler in the table.
   // isRestoreDone() will remove the handler requested if the operation is finished.
   private Map<String, SnapshotSentinel> restoreHandlers = new HashMap<String, SnapshotSentinel>();
 
   private final Path rootDir;
   private final ExecutorService executorService;
 
   /**
    * Construct a snapshot manager.
    * @param master
    */
   public SnapshotManager(final MasterServices master, final MetricsMaster metricsMaster)
       throws KeeperException, IOException, UnsupportedOperationException {
     this.master = master;
     this.metricsMaster = metricsMaster;
 
     this.rootDir = master.getMasterFileSystem().getRootDir();
     checkSnapshotSupport(master.getConfiguration(), master.getMasterFileSystem());
 
     // get the configuration for the coordinator
     Configuration conf = master.getConfiguration();
     this.wakeFrequency = conf.getInt(SNAPSHOT_WAKE_MILLIS_KEY, SNAPSHOT_WAKE_MILLIS_DEFAULT);
     long keepAliveTime = conf.getLong(SNAPSHOT_TIMEOUT_MILLIS_KEY, SNAPSHOT_TIMEOUT_MILLIS_DEFAULT);
     int opThreads = conf.getInt(SNAPSHOT_POOL_THREADS_KEY, SNAPSHOT_POOL_THREADS_DEFAULT);
 
     // setup the default procedure coordinator
     String name = master.getServerName().toString();
     ThreadPoolExecutor tpool = ProcedureCoordinator.defaultPool(name, keepAliveTime, opThreads, wakeFrequency);
     ProcedureCoordinatorRpcs comms = new ZKProcedureCoordinatorRpcs(
         master.getZooKeeper(), SnapshotManager.ONLINE_SNAPSHOT_CONTROLLER_DESCRIPTION, name);
     this.coordinator = new ProcedureCoordinator(comms, tpool);
     this.executorService = master.getExecutorService();
     resetTempDir();
   }
 
   /**
    * Fully specify all necessary components of a snapshot manager. Exposed for testing.
    * @param master services for the master where the manager is running
    * @param coordinator procedure coordinator instance.  exposed for testing.
    * @param pool HBase ExecutorServcie instance, exposed for testing.
    */
   public SnapshotManager(final MasterServices master, final MetricsMaster metricsMaster,
       ProcedureCoordinator coordinator, ExecutorService pool)
       throws IOException, UnsupportedOperationException {
     this.master = master;
     this.metricsMaster = metricsMaster;
 
     this.rootDir = master.getMasterFileSystem().getRootDir();
     checkSnapshotSupport(master.getConfiguration(), master.getMasterFileSystem());
 
     this.wakeFrequency = master.getConfiguration().getInt(SNAPSHOT_WAKE_MILLIS_KEY,
       SNAPSHOT_WAKE_MILLIS_DEFAULT);
     this.coordinator = coordinator;
     this.executorService = pool;
     resetTempDir();
   }
 
   /**
    * Gets the list of all completed snapshots.
    * @return list of SnapshotDescriptions
    * @throws IOException File system exception
    */
   public List<SnapshotDescription> getCompletedSnapshots() throws IOException {
     return getCompletedSnapshots(SnapshotDescriptionUtils.getSnapshotsDir(rootDir));
   }
 
   /**
    * Gets the list of all completed snapshots.
    * @param snapshotDir snapshot directory
    * @return list of SnapshotDescriptions
    * @throws IOException File system exception
    */
   private List<SnapshotDescription> getCompletedSnapshots(Path snapshotDir) throws IOException {
     List<SnapshotDescription> snapshotDescs = new ArrayList<SnapshotDescription>();
     // first create the snapshot root path and check to see if it exists
     FileSystem fs = master.getMasterFileSystem().getFileSystem();
     if (snapshotDir == null) snapshotDir = SnapshotDescriptionUtils.getSnapshotsDir(rootDir);
 
     // if there are no snapshots, return an empty list
     if (!fs.exists(snapshotDir)) {
       return snapshotDescs;
     }
 
     // ignore all the snapshots in progress
     FileStatus[] snapshots = fs.listStatus(snapshotDir,
       new SnapshotDescriptionUtils.CompletedSnaphotDirectoriesFilter(fs));
     // loop through all the completed snapshots
     for (FileStatus snapshot : snapshots) {
       Path info = new Path(snapshot.getPath(), SnapshotDescriptionUtils.SNAPSHOTINFO_FILE);
       // if the snapshot is bad
       if (!fs.exists(info)) {
         LOG.error("Snapshot information for " + snapshot.getPath() + " doesn't exist");
         continue;
       }
       FSDataInputStream in = null;
       try {
         in = fs.open(info);
         SnapshotDescription desc = SnapshotDescription.parseFrom(in);
         snapshotDescs.add(desc);
       } catch (IOException e) {
         LOG.warn("Found a corrupted snapshot " + snapshot.getPath(), e);
       } finally {
         if (in != null) {
           in.close();
         }
       }
     }
     return snapshotDescs;
   }
 
   /**
    * Cleans up any snapshots in the snapshot/.tmp directory that were left from failed
    * snapshot attempts.
    *
    * @throws IOException if we can't reach the filesystem
    */
   void resetTempDir() throws IOException {
     // cleanup any existing snapshots.
     Path tmpdir = SnapshotDescriptionUtils.getWorkingSnapshotDir(rootDir);
     if (!master.getMasterFileSystem().getFileSystem().delete(tmpdir, true)) {
       LOG.warn("Couldn't delete working snapshot directory: " + tmpdir);
     }
   }
 
   /**
    * Delete the specified snapshot
    * @param snapshot
    * @throws SnapshotDoesNotExistException If the specified snapshot does not exist.
    * @throws IOException For filesystem IOExceptions
    */
   public void deleteSnapshot(SnapshotDescription snapshot) throws SnapshotDoesNotExistException, IOException {
 
     // call coproc pre hook
     MasterCoprocessorHost cpHost = master.getCoprocessorHost();
     if (cpHost != null) {
       cpHost.preDeleteSnapshot(snapshot);
     }
 
     // check to see if it is completed
     if (!isSnapshotCompleted(snapshot)) {
       throw new SnapshotDoesNotExistException(snapshot);
     }
 
     String snapshotName = snapshot.getName();
     LOG.debug("Deleting snapshot: " + snapshotName);
     // first create the snapshot description and check to see if it exists
     MasterFileSystem fs = master.getMasterFileSystem();
     Path snapshotDir = SnapshotDescriptionUtils.getCompletedSnapshotDir(snapshotName, rootDir);
 
     // delete the existing snapshot
     if (!fs.getFileSystem().delete(snapshotDir, true)) {
       throw new HBaseSnapshotException("Failed to delete snapshot directory: " + snapshotDir);
     }
 
     // call coproc post hook
     if (cpHost != null) {
       cpHost.postDeleteSnapshot(snapshot);
     }
 
   }
 
   /**
    * Check if the specified snapshot is done
    *
    * @param expected
    * @return true if snapshot is ready to be restored, false if it is still being taken.
    * @throws IOException IOException if error from HDFS or RPC
    * @throws UnknownSnapshotException if snapshot is invalid or does not exist.
    */
   public boolean isSnapshotDone(SnapshotDescription expected) throws IOException {
     // check the request to make sure it has a snapshot
     if (expected == null) {
       throw new UnknownSnapshotException(
          "No snapshot name passed in request, can't figure out which snapshot you want to check.");
     }
 
     String ssString = ClientSnapshotDescriptionUtils.toString(expected);
 
     // check to see if the sentinel exists,
     // and if the task is complete removes it from the in-progress snapshots map.
     SnapshotSentinel handler = removeSentinelIfFinished(this.snapshotHandlers, expected);
 
     // stop tracking "abandoned" handlers
     cleanupSentinels();
 
     if (handler == null) {
       // If there's no handler in the in-progress map, it means one of the following:
       //   - someone has already requested the snapshot state
       //   - the requested snapshot was completed long time ago (cleanupSentinels() timeout)
       //   - the snapshot was never requested
       // In those cases returns to the user the "done state" if the snapshots exists on disk,
       // otherwise raise an exception saying that the snapshot is not running and doesn't exist.
       if (!isSnapshotCompleted(expected)) {
         throw new UnknownSnapshotException("Snapshot " + ssString
             + " is not currently running or one of the known completed snapshots.");
       }
       // was done, return true;
       return true;
     }
 
     // pass on any failure we find in the sentinel
     try {
       handler.rethrowExceptionIfFailed();
     } catch (ForeignException e) {
       // Give some procedure info on an exception.
       String status;
       Procedure p = coordinator.getProcedure(expected.getName());
       if (p != null) {
         status = p.getStatus();
       } else {
         status = expected.getName() + " not found in proclist " + coordinator.getProcedureNames();
       }
       throw new HBaseSnapshotException("Snapshot " + ssString +  " had an error.  " + status, e,
           expected);
     }
 
     // check to see if we are done
     if (handler.isFinished()) {
       LOG.debug("Snapshot '" + ssString + "' has completed, notifying client.");
       return true;
     } else if (LOG.isDebugEnabled()) {
       LOG.debug("Snapshoting '" + ssString + "' is still in progress!");
     }
     return false;
   }
 
   /**
    * Check to see if the specified table has a snapshot in progress.  Currently we have a
    * limitation only allowing a single snapshot per table at a time.
    * @param tableName name of the table being snapshotted.
    * @return <tt>true</tt> if there is a snapshot in progress on the specified table.
    */
   synchronized boolean isTakingSnapshot(final String tableName) {
     SnapshotSentinel handler = this.snapshotHandlers.get(tableName);
     return handler != null && !handler.isFinished();
   }
 
   /**
    * Check to make sure that we are OK to run the passed snapshot. Checks to make sure that we
    * aren't already running a snapshot or restore on the requested table.
    * @param snapshot description of the snapshot we want to start
    * @throws HBaseSnapshotException if the filesystem could not be prepared to start the snapshot
    */
   private synchronized void prepareToTakeSnapshot(SnapshotDescription snapshot)
       throws HBaseSnapshotException {
     FileSystem fs = master.getMasterFileSystem().getFileSystem();
     Path workingDir = SnapshotDescriptionUtils.getWorkingSnapshotDir(snapshot, rootDir);
 
     // make sure we aren't already running a snapshot
     if (isTakingSnapshot(snapshot.getTable())) {
       SnapshotSentinel handler = this.snapshotHandlers.get(snapshot.getTable());
       throw new SnapshotCreationException("Rejected taking "
           + ClientSnapshotDescriptionUtils.toString(snapshot)
           + " because we are already running another snapshot "
           + ClientSnapshotDescriptionUtils.toString(handler.getSnapshot()), snapshot);
     }
 
     // make sure we aren't running a restore on the same table
     if (isRestoringTable(snapshot.getTable())) {
       SnapshotSentinel handler = restoreHandlers.get(snapshot.getTable());
       throw new SnapshotCreationException("Rejected taking "
           + ClientSnapshotDescriptionUtils.toString(snapshot)
           + " because we are already have a restore in progress on the same snapshot "
           + ClientSnapshotDescriptionUtils.toString(handler.getSnapshot()), snapshot);
     }
 
     try {
       // delete the working directory, since we aren't running the snapshot. Likely leftovers
       // from a failed attempt.
       fs.delete(workingDir, true);
 
       // recreate the working directory for the snapshot
       if (!fs.mkdirs(workingDir)) {
         throw new SnapshotCreationException("Couldn't create working directory (" + workingDir
             + ") for snapshot" , snapshot);
       }
     } catch (HBaseSnapshotException e) {
       throw e;
     } catch (IOException e) {
       throw new SnapshotCreationException(
           "Exception while checking to see if snapshot could be started.", e, snapshot);
     }
   }
 
   /**
    * Take a snapshot of a disabled table.
    * @param snapshot description of the snapshot to take. Modified to be {@link Type#DISABLED}.
    * @throws HBaseSnapshotException if the snapshot could not be started
    */
   private synchronized void snapshotDisabledTable(SnapshotDescription snapshot)
       throws HBaseSnapshotException {
     // setup the snapshot
     prepareToTakeSnapshot(snapshot);
 
     // set the snapshot to be a disabled snapshot, since the client doesn't know about that
     snapshot = snapshot.toBuilder().setType(Type.DISABLED).build();
 
     // Take the snapshot of the disabled table
     DisabledTableSnapshotHandler handler =
         new DisabledTableSnapshotHandler(snapshot, master, metricsMaster);
     snapshotTable(snapshot, handler);
   }
 
   /**
    * Take a snapshot of an enabled table.
    * @param snapshot description of the snapshot to take.
    * @throws HBaseSnapshotException if the snapshot could not be started
    */
   private synchronized void snapshotEnabledTable(SnapshotDescription snapshot)
       throws HBaseSnapshotException {
     // setup the snapshot
     prepareToTakeSnapshot(snapshot);
 
     // Take the snapshot of the enabled table
     EnabledTableSnapshotHandler handler =
         new EnabledTableSnapshotHandler(snapshot, master, this, metricsMaster);
     snapshotTable(snapshot, handler);
   }
 
   /**
    * Take a snapshot using the specified handler.
    * On failure the snapshot temporary working directory is removed.
    * NOTE: prepareToTakeSnapshot() called before this one takes care of the rejecting the
    *       snapshot request if the table is busy with another snapshot/restore operation.
    * @param snapshot the snapshot description
    * @param handler the snapshot handler
    */
   private synchronized void snapshotTable(SnapshotDescription snapshot,
       final TakeSnapshotHandler handler) throws HBaseSnapshotException {
     try {
       handler.prepare();
       this.executorService.submit(handler);
       this.snapshotHandlers.put(snapshot.getTable(), handler);
     } catch (Exception e) {
       // cleanup the working directory by trying to delete it from the fs.
       Path workingDir = SnapshotDescriptionUtils.getWorkingSnapshotDir(snapshot, rootDir);
       try {
         if (!this.master.getMasterFileSystem().getFileSystem().delete(workingDir, true)) {
           LOG.error("Couldn't delete working directory (" + workingDir + " for snapshot:" +
               ClientSnapshotDescriptionUtils.toString(snapshot));
         }
       } catch (IOException e1) {
         LOG.error("Couldn't delete working directory (" + workingDir + " for snapshot:" +
             ClientSnapshotDescriptionUtils.toString(snapshot));
       }
       // fail the snapshot
       throw new SnapshotCreationException("Could not build snapshot handler", e, snapshot);
     }
   }
 
   /**
    * Take a snapshot based on the enabled/disabled state of the table.
    *
    * @param snapshot
    * @throws HBaseSnapshotException when a snapshot specific exception occurs.
    * @throws IOException when some sort of generic IO exception occurs.
    */
   public void takeSnapshot(SnapshotDescription snapshot) throws IOException {
     // check to see if we already completed the snapshot
     if (isSnapshotCompleted(snapshot)) {
       throw new SnapshotExistsException("Snapshot '" + snapshot.getName()
           + "' already stored on the filesystem.", snapshot);
     }
 
     LOG.debug("No existing snapshot, attempting snapshot...");
 
     // stop tracking "abandoned" handlers
     cleanupSentinels();
 
     // check to see if the table exists
     HTableDescriptor desc = null;
     try {
       desc = master.getTableDescriptors().get(snapshot.getTable());
     } catch (FileNotFoundException e) {
       String msg = "Table:" + snapshot.getTable() + " info doesn't exist!";
       LOG.error(msg);
       throw new SnapshotCreationException(msg, e, snapshot);
     } catch (IOException e) {
       throw new SnapshotCreationException("Error while geting table description for table "
           + snapshot.getTable(), e, snapshot);
     }
     if (desc == null) {
       throw new SnapshotCreationException("Table '" + snapshot.getTable()
           + "' doesn't exist, can't take snapshot.", snapshot);
     }
 
     // set the snapshot version, now that we are ready to take it
     snapshot = snapshot.toBuilder().setVersion(SnapshotDescriptionUtils.SNAPSHOT_LAYOUT_VERSION)
         .build();
 
     // call pre coproc hook
     MasterCoprocessorHost cpHost = master.getCoprocessorHost();
     if (cpHost != null) {
       cpHost.preSnapshot(snapshot, desc);
     }
 
     // if the table is enabled, then have the RS run actually the snapshot work
     AssignmentManager assignmentMgr = master.getAssignmentManager();
     if (assignmentMgr.getZKTable().isEnabledTable(snapshot.getTable())) {
       LOG.debug("Table enabled, starting distributed snapshot.");
       snapshotEnabledTable(snapshot);
       LOG.debug("Started snapshot: " + ClientSnapshotDescriptionUtils.toString(snapshot));
     }
     // For disabled table, snapshot is created by the master
     else if (assignmentMgr.getZKTable().isDisabledTable(snapshot.getTable())) {
       LOG.debug("Table is disabled, running snapshot entirely on master.");
       snapshotDisabledTable(snapshot);
       LOG.debug("Started snapshot: " + ClientSnapshotDescriptionUtils.toString(snapshot));
     } else {
       LOG.error("Can't snapshot table '" + snapshot.getTable()
           + "', isn't open or closed, we don't know what to do!");
       TablePartiallyOpenException tpoe = new TablePartiallyOpenException(snapshot.getTable()
           + " isn't fully open.");
       throw new SnapshotCreationException("Table is not entirely open or closed", tpoe, snapshot);
     }
 
     // call post coproc hook
     if (cpHost != null) {
       cpHost.postSnapshot(snapshot, desc);
     }
   }
 
   /**
    * Set the handler for the current snapshot
    * <p>
    * Exposed for TESTING
    * @param tableName
    * @param handler handler the master should use
    *
    * TODO get rid of this if possible, repackaging, modify tests.
    */
   public synchronized void setSnapshotHandlerForTesting(final String tableName,
       final SnapshotSentinel handler) {
     if (handler != null) {
       this.snapshotHandlers.put(tableName, handler);
     } else {
       this.snapshotHandlers.remove(tableName);
     }
   }
 
   /**
    * @return distributed commit coordinator for all running snapshots
    */
   ProcedureCoordinator getCoordinator() {
     return coordinator;
   }
 
   /**
    * Check to see if the snapshot is one of the currently completed snapshots
    * Returns true if the snapshot exists in the "completed snapshots folder".
    *
    * @param snapshot expected snapshot to check
    * @return <tt>true</tt> if the snapshot is stored on the {@link FileSystem}, <tt>false</tt> if is
    *         not stored
    * @throws IOException if the filesystem throws an unexpected exception,
    * @throws IllegalArgumentException if snapshot name is invalid.
    */
   private boolean isSnapshotCompleted(SnapshotDescription snapshot) throws IOException {
     try {
       final Path snapshotDir = SnapshotDescriptionUtils.getCompletedSnapshotDir(snapshot, rootDir);
       FileSystem fs = master.getMasterFileSystem().getFileSystem();
 
       // check to see if the snapshot already exists
       return fs.exists(snapshotDir);
     } catch (IllegalArgumentException iae) {
       throw new UnknownSnapshotException("Unexpected exception thrown", iae);
     }
   }
 
   /**
    * Clone the specified snapshot into a new table.
    * The operation will fail if the destination table has a snapshot or restore in progress.
    *
    * @param snapshot Snapshot Descriptor
    * @param hTableDescriptor Table Descriptor of the table to create
    */
   synchronized void cloneSnapshot(final SnapshotDescription snapshot,
       final HTableDescriptor hTableDescriptor) throws HBaseSnapshotException {
     String tableName = hTableDescriptor.getNameAsString();
 
     // make sure we aren't running a snapshot on the same table
     if (isTakingSnapshot(tableName)) {
       throw new RestoreSnapshotException("Snapshot in progress on the restore table=" + tableName);
     }
 
     // make sure we aren't running a restore on the same table
     if (isRestoringTable(tableName)) {
       throw new RestoreSnapshotException("Restore already in progress on the table=" + tableName);
     }
 
     try {
       CloneSnapshotHandler handler =
         new CloneSnapshotHandler(master, snapshot, hTableDescriptor, metricsMaster).prepare();
       this.executorService.submit(handler);
       this.restoreHandlers.put(tableName, handler);
     } catch (Exception e) {
       String msg = "Couldn't clone the snapshot=" + ClientSnapshotDescriptionUtils.toString(snapshot) +
         " on table=" + tableName;
       LOG.error(msg, e);
       throw new RestoreSnapshotException(msg, e);
     }
   }
 
   /**
    * Restore the specified snapshot
    * @param reqSnapshot
    * @throws IOException
    */
   public void restoreSnapshot(SnapshotDescription reqSnapshot) throws IOException {
     FileSystem fs = master.getMasterFileSystem().getFileSystem();
     Path snapshotDir = SnapshotDescriptionUtils.getCompletedSnapshotDir(reqSnapshot, rootDir);
     MasterCoprocessorHost cpHost = master.getCoprocessorHost();
 
     // check if the snapshot exists
     if (!fs.exists(snapshotDir)) {
       LOG.error("A Snapshot named '" + reqSnapshot.getName() + "' does not exist.");
       throw new SnapshotDoesNotExistException(reqSnapshot);
     }
 
     // read snapshot information
     SnapshotDescription fsSnapshot = SnapshotDescriptionUtils.readSnapshotInfo(fs, snapshotDir);
     HTableDescriptor snapshotTableDesc = FSTableDescriptors.getTableDescriptor(fs, snapshotDir);
     String tableName = reqSnapshot.getTable();
 
     // stop tracking "abandoned" handlers
     cleanupSentinels();
 
     // Execute the restore/clone operation
     if (MetaReader.tableExists(master.getCatalogTracker(), tableName)) {
       if (master.getAssignmentManager().getZKTable().isEnabledTable(fsSnapshot.getTable())) {
         throw new UnsupportedOperationException("Table '" +
           fsSnapshot.getTable() + "' must be disabled in order to perform a restore operation.");
       }
 
       // call coproc pre hook
       if (cpHost != null) {
         cpHost.preRestoreSnapshot(reqSnapshot, snapshotTableDesc);
       }
       restoreSnapshot(fsSnapshot, snapshotTableDesc);
       LOG.info("Restore snapshot=" + fsSnapshot.getName() + " as table=" + tableName);
 
       if (cpHost != null) {
         cpHost.postRestoreSnapshot(reqSnapshot, snapshotTableDesc);
       }
     } else {
       HTableDescriptor htd = RestoreSnapshotHelper.cloneTableSchema(snapshotTableDesc,
                                                          Bytes.toBytes(tableName));
       if (cpHost != null) {
         cpHost.preCloneSnapshot(reqSnapshot, htd);
       }
       cloneSnapshot(fsSnapshot, htd);
       LOG.info("Clone snapshot=" + fsSnapshot.getName() + " as table=" + tableName);
 
       if (cpHost != null) {
         cpHost.postCloneSnapshot(reqSnapshot, htd);
       }
     }
   }
 
   /**
    * Restore the specified snapshot.
    * The restore will fail if the destination table has a snapshot or restore in progress.
    *
    * @param snapshot Snapshot Descriptor
    * @param hTableDescriptor Table Descriptor
    */
   private synchronized void restoreSnapshot(final SnapshotDescription snapshot,
       final HTableDescriptor hTableDescriptor) throws HBaseSnapshotException {
     String tableName = hTableDescriptor.getNameAsString();
 
     // make sure we aren't running a snapshot on the same table
     if (isTakingSnapshot(tableName)) {
       throw new RestoreSnapshotException("Snapshot in progress on the restore table=" + tableName);
     }
 
     // make sure we aren't running a restore on the same table
     if (isRestoringTable(tableName)) {
       throw new RestoreSnapshotException("Restore already in progress on the table=" + tableName);
     }
 
     try {
       RestoreSnapshotHandler handler =
         new RestoreSnapshotHandler(master, snapshot, hTableDescriptor, metricsMaster).prepare();
       this.executorService.submit(handler);
       restoreHandlers.put(hTableDescriptor.getNameAsString(), handler);
     } catch (Exception e) {
       String msg = "Couldn't restore the snapshot=" + ClientSnapshotDescriptionUtils.toString(
           snapshot)  +
           " on table=" + tableName;
       LOG.error(msg, e);
       throw new RestoreSnapshotException(msg, e);
     }
   }
 
   /**
    * Verify if the restore of the specified table is in progress.
    *
    * @param tableName table under restore
    * @return <tt>true</tt> if there is a restore in progress of the specified table.
    */
   private synchronized boolean isRestoringTable(final String tableName) {
     SnapshotSentinel sentinel = this.restoreHandlers.get(tableName);
     return(sentinel != null && !sentinel.isFinished());
   }
 
   /**
    * Returns the status of a restore operation.
    * If the in-progress restore is failed throws the exception that caused the failure.
    *
    * @param snapshot
    * @return false if in progress, true if restore is completed or not requested.
    * @throws IOException if there was a failure during the restore
    */
   public boolean isRestoreDone(final SnapshotDescription snapshot) throws IOException {
     // check to see if the sentinel exists,
     // and if the task is complete removes it from the in-progress restore map.
     SnapshotSentinel sentinel = removeSentinelIfFinished(this.restoreHandlers, snapshot);
 
     // stop tracking "abandoned" handlers
     cleanupSentinels();
 
     if (sentinel == null) {
       // there is no sentinel so restore is not in progress.
       return true;
     }
 
     LOG.debug("Verify snapshot=" + snapshot.getName() + " against="
         + sentinel.getSnapshot().getName() + " table=" + snapshot.getTable());
 
     // If the restore is failed, rethrow the exception
     sentinel.rethrowExceptionIfFailed();
 
     // check to see if we are done
     if (sentinel.isFinished()) {
       LOG.debug("Restore snapshot=" + ClientSnapshotDescriptionUtils.toString(snapshot) +
           " has completed. Notifying the client.");
       return true;
     }
 
     if (LOG.isDebugEnabled()) {
       LOG.debug("Sentinel is not yet finished with restoring snapshot=" +
           ClientSnapshotDescriptionUtils.toString(snapshot));
     }
     return false;
   }
 
   /**
    * Return the handler if it is currently live and has the same snapshot target name.
    * The handler is removed from the sentinels map if completed.
    * @param sentinels live handlers
    * @param snapshot snapshot description
    * @return null if doesn't match, else a live handler.
    */
   private synchronized SnapshotSentinel removeSentinelIfFinished(
       final Map<String, SnapshotSentinel> sentinels, final SnapshotDescription snapshot) {
     SnapshotSentinel h = sentinels.get(snapshot.getTable());
     if (h == null) {
       return null;
     }
 
     if (!h.getSnapshot().getName().equals(snapshot.getName())) {
       // specified snapshot is to the one currently running
       return null;
     }
 
     // Remove from the "in-progress" list once completed
     if (h.isFinished()) {
       sentinels.remove(snapshot.getTable());
     }
 
     return h;
   }
 
   /**
    * Removes "abandoned" snapshot/restore requests.
    * As part of the HBaseAdmin snapshot/restore API the operation status is checked until completed,
    * and the in-progress maps are cleaned up when the status of a completed task is requested.
    * To avoid having sentinels staying around for long time if something client side is failed,
    * each operation tries to clean up the in-progress maps sentinels finished from a long time.
    */
   private void cleanupSentinels() {
     cleanupSentinels(this.snapshotHandlers);
     cleanupSentinels(this.restoreHandlers);
   }
 
   /**
    * Remove the sentinels that are marked as finished and the completion time
    * has exceeded the removal timeout.
    * @param sentinels map of sentinels to clean
    */
   private synchronized void cleanupSentinels(final Map<String, SnapshotSentinel> sentinels) {
     long currentTime = EnvironmentEdgeManager.currentTimeMillis();
     Iterator<Map.Entry<String, SnapshotSentinel>> it = sentinels.entrySet().iterator();
     while (it.hasNext()) {
       Map.Entry<String, SnapshotSentinel> entry = it.next();
       SnapshotSentinel sentinel = entry.getValue();
       if (sentinel.isFinished() &&
           (currentTime - sentinel.getCompletionTimestamp()) > SNAPSHOT_SENTINELS_CLEANUP_TIMEOUT)
       {
         it.remove();
       }
     }
   }
 
   //
   // Implementing Stoppable interface
   //
 
   @Override
   public void stop(String why) {
     // short circuit
     if (this.stopped) return;
     // make sure we get stop
     this.stopped = true;
     // pass the stop onto take snapshot handlers
     for (SnapshotSentinel snapshotHandler: this.snapshotHandlers.values()) {
       snapshotHandler.cancel(why);
     }
 
     // pass the stop onto all the restore handlers
     for (SnapshotSentinel restoreHandler: this.restoreHandlers.values()) {
       restoreHandler.cancel(why);
     }
   }
 
   @Override
   public boolean isStopped() {
     return this.stopped;
   }
 
   /**
    * Throws an exception if snapshot operations (take a snapshot, restore, clone) are not supported.
    * Called at the beginning of snapshot() and restoreSnapshot() methods.
    * @throws UnsupportedOperationException if snapshot are not supported
    */
   public void checkSnapshotSupport() throws UnsupportedOperationException {
     if (!this.isSnapshotSupported) {
       throw new UnsupportedOperationException(
         "To use snapshots, You must add to the hbase-site.xml of the HBase Master: '" +
           HBASE_SNAPSHOT_ENABLED + "' property with value 'true'.");
     }
   }
 
   /**
    * Called at startup, to verify if snapshot operation is supported, and to avoid
    * starting the master if there're snapshots present but the cleaners needed are missing.
    * Otherwise we can end up with snapshot data loss.
    * @param conf The {@link Configuration} object to use
    * @param mfs The MasterFileSystem to use
    * @throws IOException in case of file-system operation failure
    * @throws UnsupportedOperationException in case cleaners are missing and
    *         there're snapshot in the system
    */
   private void checkSnapshotSupport(final Configuration conf, final MasterFileSystem mfs)
       throws IOException, UnsupportedOperationException {
     // Verify if snapshot is disabled by the user
     String enabled = conf.get(HBASE_SNAPSHOT_ENABLED);
     boolean snapshotEnabled = conf.getBoolean(HBASE_SNAPSHOT_ENABLED, false);
     boolean userDisabled = (enabled != null && enabled.trim().length() > 0 && !snapshotEnabled);
 
     // Extract cleaners from conf
     Set<String> hfileCleaners = new HashSet<String>();
     String[] cleaners = conf.getStrings(HFileCleaner.MASTER_HFILE_CLEANER_PLUGINS);
     if (cleaners != null) Collections.addAll(hfileCleaners, cleaners);
 
     Set<String> logCleaners = new HashSet<String>();
     cleaners = conf.getStrings(HConstants.HBASE_MASTER_LOGCLEANER_PLUGINS);
     if (cleaners != null) Collections.addAll(logCleaners, cleaners);
 
     // check if an older version of snapshot directory was present
     Path oldSnapshotDir = new Path(mfs.getRootDir(), HConstants.OLD_SNAPSHOT_DIR_NAME);
     FileSystem fs = mfs.getFileSystem();
     List<SnapshotDescription> ss = getCompletedSnapshots(new Path(rootDir, oldSnapshotDir));
     if (ss != null && !ss.isEmpty()) {
       LOG.error("Snapshots from an earlier release were found under: " + oldSnapshotDir);
       LOG.error("Please rename the directory as " + HConstants.SNAPSHOT_DIR_NAME);
     }
 
     // If the user has enabled the snapshot, we force the cleaners to be present
     // otherwise we still need to check if cleaners are enabled or not and verify
     // that there're no snapshot in the .snapshot folder.
     if (snapshotEnabled) {
       // Inject snapshot cleaners, if snapshot.enable is true
       hfileCleaners.add(SnapshotHFileCleaner.class.getName());
       hfileCleaners.add(HFileLinkCleaner.class.getName());
       logCleaners.add(SnapshotLogCleaner.class.getName());
 
       // Set cleaners conf
       conf.setStrings(HFileCleaner.MASTER_HFILE_CLEANER_PLUGINS,
         hfileCleaners.toArray(new String[hfileCleaners.size()]));
       conf.setStrings(HConstants.HBASE_MASTER_LOGCLEANER_PLUGINS,
         logCleaners.toArray(new String[logCleaners.size()]));
     } else {
       // Verify if cleaners are present
       snapshotEnabled = logCleaners.contains(SnapshotLogCleaner.class.getName()) &&
         hfileCleaners.contains(SnapshotHFileCleaner.class.getName()) &&
         hfileCleaners.contains(HFileLinkCleaner.class.getName());
 
       // Warn if the cleaners are enabled but the snapshot.enabled property is false/not set.
       if (snapshotEnabled) {
         LOG.warn("Snapshot log and hfile cleaners are present in the configuration, " +
           "but the '" + HBASE_SNAPSHOT_ENABLED + "' property " +
           (userDisabled ? "is set to 'false'." : "is not set."));
       }
     }
 
     // Mark snapshot feature as enabled if cleaners are present and user has not disabled it.
     this.isSnapshotSupported = snapshotEnabled && !userDisabled;
 
     // If cleaners are not enabled, verify that there're no snapshot in the .snapshot folder
     // otherwise we end up with snapshot data loss.
     if (!snapshotEnabled) {
       LOG.info("Snapshot feature is not enabled, missing log and hfile cleaners.");
       Path snapshotDir = SnapshotDescriptionUtils.getSnapshotsDir(mfs.getRootDir());
       if (fs.exists(snapshotDir)) {
         FileStatus[] snapshots = FSUtils.listStatus(fs, snapshotDir,
           new SnapshotDescriptionUtils.CompletedSnaphotDirectoriesFilter(fs));
         if (snapshots != null) {
           LOG.error("Snapshots are present, but cleaners are not enabled.");
           checkSnapshotSupport();
         }
       }
     }
   }
 }