/**
    * 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.regionserver.wal;
  
  import static org.apache.hadoop.hbase.wal.DefaultWALProvider.WAL_FILE_NAME_DELIMITER;
  
  import java.io.FileNotFoundException;
  import java.io.IOException;
  import java.io.InterruptedIOException;
  import java.io.OutputStream;
  import java.lang.management.MemoryUsage;
  import java.lang.reflect.InvocationTargetException;
  import java.net.URLEncoder;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Comparator;
  import java.util.List;
  import java.util.Map;
  import java.util.NavigableMap;
  import java.util.Set;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ConcurrentSkipListMap;
  import java.util.concurrent.CopyOnWriteArrayList;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.Executors;
  import java.util.concurrent.LinkedBlockingQueue;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.locks.ReentrantLock;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.fs.FSDataOutputStream;
  import org.apache.hadoop.fs.FileStatus;
  import org.apache.hadoop.fs.FileSystem;
  import org.apache.hadoop.fs.Path;
  import org.apache.hadoop.fs.PathFilter;
  import org.apache.hadoop.hbase.Cell;
  import org.apache.hadoop.hbase.CellUtil;
  import org.apache.hadoop.hbase.HBaseConfiguration;
  import org.apache.hadoop.hbase.HConstants;
  import org.apache.hadoop.hbase.HRegionInfo;
  import org.apache.hadoop.hbase.HTableDescriptor;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.exceptions.TimeoutIOException;
  import org.apache.hadoop.hbase.io.util.HeapMemorySizeUtil;
  import org.apache.hadoop.hbase.util.Bytes;
  import org.apache.hadoop.hbase.util.ClassSize;
  import org.apache.hadoop.hbase.util.DrainBarrier;
  import org.apache.hadoop.hbase.util.EnvironmentEdgeManager;
  import org.apache.hadoop.hbase.util.FSUtils;
  import org.apache.hadoop.hbase.util.HasThread;
  import org.apache.hadoop.hbase.util.Threads;
  import org.apache.hadoop.hbase.wal.DefaultWALProvider;
  import org.apache.hadoop.hbase.wal.WAL;
  import org.apache.hadoop.hbase.wal.WALFactory;
  import org.apache.hadoop.hbase.wal.WALKey;
  import org.apache.hadoop.hbase.wal.WALPrettyPrinter;
  import org.apache.hadoop.hbase.wal.WALProvider.Writer;
  import org.apache.hadoop.hbase.wal.WALSplitter;
  import org.apache.hadoop.hdfs.DFSOutputStream;
  import org.apache.hadoop.hdfs.client.HdfsDataOutputStream;
  import org.apache.hadoop.hdfs.protocol.DatanodeInfo;
  import org.apache.hadoop.util.StringUtils;
  import org.apache.htrace.NullScope;
  import org.apache.htrace.Span;
  import org.apache.htrace.Trace;
  import org.apache.htrace.TraceScope;
  
  import com.google.common.annotations.VisibleForTesting;
  import com.lmax.disruptor.BlockingWaitStrategy;
  import com.lmax.disruptor.EventHandler;
  import com.lmax.disruptor.ExceptionHandler;
  import com.lmax.disruptor.LifecycleAware;
  import com.lmax.disruptor.TimeoutException;
  import com.lmax.disruptor.dsl.Disruptor;
  import com.lmax.disruptor.dsl.ProducerType;
  
 /**
  * Implementation of {@link WAL} to go against {@link FileSystem}; i.e. keep WALs in HDFS.
  * Only one WAL is ever being written at a time.  When a WAL hits a configured maximum size,
  * it is rolled.  This is done internal to the implementation.
  *
  * <p>As data is flushed from the MemStore to other on-disk structures (files sorted by
  * key, hfiles), a WAL becomes obsolete. We can let go of all the log edits/entries for a given
  * HRegion-sequence id.  A bunch of work in the below is done keeping account of these region
  * sequence ids -- what is flushed out to hfiles, and what is yet in WAL and in memory only.
  *
  * <p>It is only practical to delete entire files. Thus, we delete an entire on-disk file
  * <code>F</code> when all of the edits in <code>F</code> have a log-sequence-id that's older
  * (smaller) than the most-recent flush.
  *
  * <p>To read an WAL, call {@link WALFactory#createReader(org.apache.hadoop.fs.FileSystem,
  * org.apache.hadoop.fs.Path)}.
  *
  * <h2>Failure Semantic</h2>
  * If an exception on append or sync, roll the WAL because the current WAL is now a lame duck;
  * any more appends or syncs will fail also with the same original exception. If we have made
  * successful appends to the WAL and we then are unable to sync them, our current semantic is to
  * return error to the client that the appends failed but also to abort the current context,
  * usually the hosting server. We need to replay the WALs. TODO: Change this semantic. A roll of
  * WAL may be sufficient as long as we have flagged client that the append failed. TODO:
  * replication may pick up these last edits though they have been marked as failed append (Need to
  * keep our own file lengths, not rely on HDFS).
  */
 @InterfaceAudience.Private
 public class FSHLog implements WAL {
   // IMPLEMENTATION NOTES:
   //
   // At the core is a ring buffer.  Our ring buffer is the LMAX Disruptor.  It tries to
   // minimize synchronizations and volatile writes when multiple contending threads as is the case
   // here appending and syncing on a single WAL.  The Disruptor is configured to handle multiple
   // producers but it has one consumer only (the producers in HBase are IPC Handlers calling append
   // and then sync).  The single consumer/writer pulls the appends and syncs off the ring buffer.
   // When a handler calls sync, it is given back a future. The producer 'blocks' on the future so
   // it does not return until the sync completes.  The future is passed over the ring buffer from
   // the producer/handler to the consumer thread where it does its best to batch up the producer
   // syncs so one WAL sync actually spans multiple producer sync invocations.  How well the
   // batching works depends on the write rate; i.e. we tend to batch more in times of
   // high writes/syncs.
   //
   // Calls to append now also wait until the append has been done on the consumer side of the
   // disruptor.  We used to not wait but it makes the implemenation easier to grok if we have
   // the region edit/sequence id after the append returns.
   //
   // TODO: Handlers need to coordinate appending AND syncing.  Can we have the threads contend
   // once only?  Probably hard given syncs take way longer than an append.
   //
   // The consumer threads pass the syncs off to multiple syncing threads in a round robin fashion
   // to ensure we keep up back-to-back FS sync calls (FS sync calls are the long poll writing the
   // WAL).  The consumer thread passes the futures to the sync threads for it to complete
   // the futures when done.
   //
   // The 'sequence' in the below is the sequence of the append/sync on the ringbuffer.  It
   // acts as a sort-of transaction id.  It is always incrementing.
   //
   // The RingBufferEventHandler class hosts the ring buffer consuming code.  The threads that
   // do the actual FS sync are implementations of SyncRunner.  SafePointZigZagLatch is a
   // synchronization class used to halt the consumer at a safe point --  just after all outstanding
   // syncs and appends have completed -- so the log roller can swap the WAL out under it.
 
   private static final Log LOG = LogFactory.getLog(FSHLog.class);
 
   private static final int DEFAULT_SLOW_SYNC_TIME_MS = 100; // in ms
 
   private static final int DEFAULT_WAL_SYNC_TIMEOUT_MS = 5 * 60 * 1000; // in ms, 5min
 
   /**
    * The nexus at which all incoming handlers meet.  Does appends and sync with an ordering.
    * Appends and syncs are each put on the ring which means handlers need to
    * smash up against the ring twice (can we make it once only? ... maybe not since time to append
    * is so different from time to sync and sometimes we don't want to sync or we want to async
    * the sync).  The ring is where we make sure of our ordering and it is also where we do
    * batching up of handler sync calls.
    */
   private final Disruptor<RingBufferTruck> disruptor;
 
   /**
    * An executorservice that runs the disruptor AppendEventHandler append executor.
    */
   private final ExecutorService appendExecutor;
 
   /**
    * This fellow is run by the above appendExecutor service but it is all about batching up appends
    * and syncs; it may shutdown without cleaning out the last few appends or syncs.  To guard
    * against this, keep a reference to this handler and do explicit close on way out to make sure
    * all flushed out before we exit.
    */
   private final RingBufferEventHandler ringBufferEventHandler;
 
   /**
    * Map of {@link SyncFuture}s owned by Thread objects. Used so we reuse SyncFutures.
    * Thread local is used so JVM can GC the terminated thread for us. See HBASE-21228
    * <p>
    */
   private final ThreadLocal<SyncFuture> cachedSyncFutures;
 
   /**
    * The highest known outstanding unsync'd WALEdit sequence number where sequence number is the
    * ring buffer sequence.  Maintained by the ring buffer consumer.
    */
   private volatile long highestUnsyncedSequence = -1;
 
   /**
    * Updated to the ring buffer sequence of the last successful sync call.  This can be less than
    * {@link #highestUnsyncedSequence} for case where we have an append where a sync has not yet
    * come in for it.  Maintained by the syncing threads.
    */
   private final AtomicLong highestSyncedSequence = new AtomicLong(0);
 
   /**
    * file system instance
    */
   protected final FileSystem fs;
 
   /**
    * WAL directory, where all WAL files would be placed.
    */
   private final Path fullPathLogDir;
 
   /**
    * dir path where old logs are kept.
    */
   private final Path fullPathArchiveDir;
 
   /**
    * Matches just those wal files that belong to this wal instance.
    */
   private final PathFilter ourFiles;
 
   /**
    * Prefix of a WAL file, usually the region server name it is hosted on.
    */
   private final String logFilePrefix;
 
   /**
    * Suffix included on generated wal file names
    */
   private final String logFileSuffix;
 
   /**
    * Prefix used when checking for wal membership.
    */
   private final String prefixPathStr;
 
   private final WALCoprocessorHost coprocessorHost;
 
   /**
    * conf object
    */
   protected final Configuration conf;
 
   /** Listeners that are called on WAL events. */
   private final List<WALActionsListener> listeners =
     new CopyOnWriteArrayList<WALActionsListener>();
 
   @Override
   public void registerWALActionsListener(final WALActionsListener listener) {
     this.listeners.add(listener);
   }
 
   @Override
   public boolean unregisterWALActionsListener(final WALActionsListener listener) {
     return this.listeners.remove(listener);
   }
 
   @Override
   public WALCoprocessorHost getCoprocessorHost() {
     return coprocessorHost;
   }
 
   /**
    * FSDataOutputStream associated with the current SequenceFile.writer
    */
   private FSDataOutputStream hdfs_out;
 
   // All about log rolling if not enough replicas outstanding.
 
   // Minimum tolerable replicas, if the actual value is lower than it, rollWriter will be triggered
   private final int minTolerableReplication;
 
   private final int slowSyncNs;
 
   private final long walSyncTimeout;
 
   // If live datanode count is lower than the default replicas value,
   // RollWriter will be triggered in each sync(So the RollWriter will be
   // triggered one by one in a short time). Using it as a workaround to slow
   // down the roll frequency triggered by checkLowReplication().
   private final AtomicInteger consecutiveLogRolls = new AtomicInteger(0);
 
   private final int lowReplicationRollLimit;
 
   // If consecutiveLogRolls is larger than lowReplicationRollLimit,
   // then disable the rolling in checkLowReplication().
   // Enable it if the replications recover.
   private volatile boolean lowReplicationRollEnabled = true;
 
   /**
    * Class that does accounting of sequenceids in WAL subsystem. Holds oldest outstanding
    * sequence id as yet not flushed as well as the most recent edit sequence id appended to the
    * WAL. Has facility for answering questions such as "Is it safe to GC a WAL?".
    */
   private SequenceIdAccounting sequenceIdAccounting = new SequenceIdAccounting();
 
   /**
    * Current log file.
    */
   volatile Writer writer;
 
   /** The barrier used to ensure that close() waits for all log rolls and flushes to finish. */
   private final DrainBarrier closeBarrier = new DrainBarrier();
 
   /**
    * This lock makes sure only one log roll runs at a time. Should not be taken while any other
    * lock is held. We don't just use synchronized because that results in bogus and tedious
    * findbugs warning when it thinks synchronized controls writer thread safety.  It is held when
    * we are actually rolling the log.  It is checked when we are looking to see if we should roll
    * the log or not.
    */
   private final ReentrantLock rollWriterLock = new ReentrantLock(true);
 
   private volatile boolean closed = false;
   private final AtomicBoolean shutdown = new AtomicBoolean(false);
 
   // The timestamp (in ms) when the log file was created.
   private final AtomicLong filenum = new AtomicLong(-1);
 
   // Number of transactions in the current Wal.
   private final AtomicInteger numEntries = new AtomicInteger(0);
 
   // If > than this size, roll the log.
   private final long logrollsize;
 
   /**
    * The total size of wal
    */
   private AtomicLong totalLogSize = new AtomicLong(0);
 
   /*
    * If more than this many logs, force flush of oldest region to oldest edit
    * goes to disk.  If too many and we crash, then will take forever replaying.
    * Keep the number of logs tidy.
    */
   private final int maxLogs;
 
   /** Number of log close errors tolerated before we abort */
   private final int closeErrorsTolerated;
 
   private final AtomicInteger closeErrorCount = new AtomicInteger();
 
   // Last time to check low replication on hlog's pipeline
   private volatile long lastTimeCheckLowReplication = EnvironmentEdgeManager.currentTime();
 
 
   /**
    * WAL Comparator; it compares the timestamp (log filenum), present in the log file name.
    * Throws an IllegalArgumentException if used to compare paths from different wals.
    */
   final Comparator<Path> LOG_NAME_COMPARATOR = new Comparator<Path>() {
     @Override
     public int compare(Path o1, Path o2) {
       long t1 = getFileNumFromFileName(o1);
       long t2 = getFileNumFromFileName(o2);
       if (t1 == t2) return 0;
       return (t1 > t2) ? 1 : -1;
     }
   };
 
   /**
    * Map of WAL log file to the latest sequence ids of all regions it has entries of.
    * The map is sorted by the log file creation timestamp (contained in the log file name).
    */
   private NavigableMap<Path, Map<byte[], Long>> byWalRegionSequenceIds =
     new ConcurrentSkipListMap<Path, Map<byte[], Long>>(LOG_NAME_COMPARATOR);
 
   /**
    * Exception handler to pass the disruptor ringbuffer.  Same as native implementation only it
    * logs using our logger instead of java native logger.
    */
   static class RingBufferExceptionHandler implements ExceptionHandler {
     @Override
     public void handleEventException(Throwable ex, long sequence, Object event) {
       LOG.error("Sequence=" + sequence + ", event=" + event, ex);
       throw new RuntimeException(ex);
     }
 
     @Override
     public void handleOnStartException(Throwable ex) {
       LOG.error(ex);
       throw new RuntimeException(ex);
     }
 
     @Override
     public void handleOnShutdownException(Throwable ex) {
       LOG.error(ex);
       throw new RuntimeException(ex);
     }
   }
 
   /**
    * Constructor.
    *
    * @param fs filesystem handle
    * @param root path for stored and archived wals
    * @param logDir dir where wals are stored
    * @param conf configuration to use
    * @throws IOException
    */
   public FSHLog(final FileSystem fs, final Path root, final String logDir, final Configuration conf)
       throws IOException {
     this(fs, root, logDir, HConstants.HREGION_OLDLOGDIR_NAME, conf, null, true, null, null);
   }
 
   /**
    * Create an edit log at the given <code>dir</code> location.
    *
    * You should never have to load an existing log. If there is a log at
    * startup, it should have already been processed and deleted by the time the
    * WAL object is started up.
    *
    * @param fs filesystem handle
    * @param rootDir path to where logs and oldlogs
    * @param logDir dir where wals are stored
    * @param archiveDir dir where wals are archived
    * @param conf configuration to use
    * @param listeners Listeners on WAL events. Listeners passed here will
    * be registered before we do anything else; e.g. the
    * Constructor {@link #rollWriter()}.
    * @param failIfWALExists If true IOException will be thrown if files related to this wal
    *        already exist.
    * @param prefix should always be hostname and port in distributed env and
    *        it will be URL encoded before being used.
    *        If prefix is null, "wal" will be used
    * @param suffix will be url encoded. null is treated as empty. non-empty must start with
    *        {@link DefaultWALProvider#WAL_FILE_NAME_DELIMITER}
    * @throws IOException
    */
   public FSHLog(final FileSystem fs, final Path rootDir, final String logDir,
       final String archiveDir, final Configuration conf,
       final List<WALActionsListener> listeners,
       final boolean failIfWALExists, final String prefix, final String suffix)
       throws IOException {
     this.fs = fs;
     this.fullPathLogDir = new Path(rootDir, logDir);
     this.fullPathArchiveDir = new Path(rootDir, archiveDir);
     this.conf = conf;
 
     if (!fs.exists(fullPathLogDir) && !fs.mkdirs(fullPathLogDir)) {
       throw new IOException("Unable to mkdir " + fullPathLogDir);
     }
 
     if (!fs.exists(this.fullPathArchiveDir)) {
       if (!fs.mkdirs(this.fullPathArchiveDir)) {
         throw new IOException("Unable to mkdir " + this.fullPathArchiveDir);
       }
     }
 
     // If prefix is null||empty then just name it wal
     this.logFilePrefix =
       prefix == null || prefix.isEmpty() ? "wal" : URLEncoder.encode(prefix, "UTF8");
     // we only correctly differentiate suffices when numeric ones start with '.'
     if (suffix != null && !(suffix.isEmpty()) && !(suffix.startsWith(WAL_FILE_NAME_DELIMITER))) {
       throw new IllegalArgumentException("WAL suffix must start with '" + WAL_FILE_NAME_DELIMITER +
           "' but instead was '" + suffix + "'");
     }
     // Now that it exists, set the storage policy for the entire directory of wal files related to
     // this FSHLog instance
     FSUtils.setStoragePolicy(fs, conf, this.fullPathLogDir, HConstants.WAL_STORAGE_POLICY,
       HConstants.DEFAULT_WAL_STORAGE_POLICY);
     this.logFileSuffix = (suffix == null) ? "" : URLEncoder.encode(suffix, "UTF8");
     this.prefixPathStr = new Path(fullPathLogDir,
         logFilePrefix + WAL_FILE_NAME_DELIMITER).toString();
 
     this.ourFiles = new PathFilter() {
       @Override
       public boolean accept(final Path fileName) {
         // The path should start with dir/<prefix> and end with our suffix
         final String fileNameString = fileName.toString();
         if (!fileNameString.startsWith(prefixPathStr)) {
           return false;
         }
         if (logFileSuffix.isEmpty()) {
           // in the case of the null suffix, we need to ensure the filename ends with a timestamp.
           return org.apache.commons.lang.StringUtils.isNumeric(
               fileNameString.substring(prefixPathStr.length()));
         } else if (!fileNameString.endsWith(logFileSuffix)) {
           return false;
         }
         return true;
       }
     };
 
     if (failIfWALExists) {
       final FileStatus[] walFiles = FSUtils.listStatus(fs, fullPathLogDir, ourFiles);
       if (null != walFiles && 0 != walFiles.length) {
         throw new IOException("Target WAL already exists within directory " + fullPathLogDir);
       }
     }
 
     // Register listeners.  TODO: Should this exist anymore?  We have CPs?
     if (listeners != null) {
       for (WALActionsListener i: listeners) {
         registerWALActionsListener(i);
       }
     }
     this.coprocessorHost = new WALCoprocessorHost(this, conf);
 
     // Get size to roll log at. Roll at 95% of HDFS block size so we avoid crossing HDFS blocks
     // (it costs a little x'ing bocks)
     final long blocksize = this.conf.getLong("hbase.regionserver.hlog.blocksize",
         FSUtils.getDefaultBlockSize(this.fs, this.fullPathLogDir));
     this.logrollsize =
       (long)(blocksize * conf.getFloat("hbase.regionserver.logroll.multiplier", 0.95f));
 
     float memstoreRatio = conf.getFloat(HeapMemorySizeUtil.MEMSTORE_SIZE_KEY,
       conf.getFloat(HeapMemorySizeUtil.MEMSTORE_SIZE_OLD_KEY,
         HeapMemorySizeUtil.DEFAULT_MEMSTORE_SIZE));
     boolean maxLogsDefined = conf.get("hbase.regionserver.maxlogs") != null;
     if(maxLogsDefined){
       LOG.warn("'hbase.regionserver.maxlogs' was deprecated.");
     }
     this.maxLogs = conf.getInt("hbase.regionserver.maxlogs",
         Math.max(32, calculateMaxLogFiles(memstoreRatio, logrollsize)));
     this.minTolerableReplication = conf.getInt("hbase.regionserver.hlog.tolerable.lowreplication",
         FSUtils.getDefaultReplication(fs, this.fullPathLogDir));
     this.lowReplicationRollLimit =
       conf.getInt("hbase.regionserver.hlog.lowreplication.rolllimit", 5);
     this.closeErrorsTolerated = conf.getInt("hbase.regionserver.logroll.errors.tolerated", 0);
     int maxHandlersCount = conf.getInt(HConstants.REGION_SERVER_HANDLER_COUNT, 200);
 
     LOG.info("WAL configuration: blocksize=" + StringUtils.byteDesc(blocksize) +
       ", rollsize=" + StringUtils.byteDesc(this.logrollsize) +
       ", prefix=" + this.logFilePrefix + ", suffix=" + logFileSuffix + ", logDir=" +
       this.fullPathLogDir + ", archiveDir=" + this.fullPathArchiveDir);
 
     // rollWriter sets this.hdfs_out if it can.
     rollWriter();
 
     this.slowSyncNs =
         1000000 * conf.getInt("hbase.regionserver.hlog.slowsync.ms",
           DEFAULT_SLOW_SYNC_TIME_MS);
     this.walSyncTimeout = conf.getLong("hbase.regionserver.hlog.sync.timeout",
         DEFAULT_WAL_SYNC_TIMEOUT_MS);
 
     // This is the 'writer' -- a single threaded executor.  This single thread 'consumes' what is
     // put on the ring buffer.
     String hostingThreadName = Thread.currentThread().getName();
     this.appendExecutor = Executors.
       newSingleThreadExecutor(Threads.getNamedThreadFactory(hostingThreadName + ".append"));
     // Preallocate objects to use on the ring buffer.  The way that appends and syncs work, we will
     // be stuck and make no progress if the buffer is filled with appends only and there is no
     // sync. If no sync, then the handlers will be outstanding just waiting on sync completion
     // before they return.
     final int preallocatedEventCount =
       this.conf.getInt("hbase.regionserver.wal.disruptor.event.count", 1024 * 16);
     // Using BlockingWaitStrategy.  Stuff that is going on here takes so long it makes no sense
     // spinning as other strategies do.
     this.disruptor =
       new Disruptor<RingBufferTruck>(RingBufferTruck.EVENT_FACTORY, preallocatedEventCount,
         this.appendExecutor, ProducerType.MULTI, new BlockingWaitStrategy());
     // Advance the ring buffer sequence so that it starts from 1 instead of 0,
     // because SyncFuture.NOT_DONE = 0.
     this.disruptor.getRingBuffer().next();
     this.ringBufferEventHandler =
       new RingBufferEventHandler(conf.getInt("hbase.regionserver.hlog.syncer.count", 5),
         maxHandlersCount);
     this.disruptor.handleExceptionsWith(new RingBufferExceptionHandler());
     this.disruptor.handleEventsWith(new RingBufferEventHandler [] {this.ringBufferEventHandler});
     this.cachedSyncFutures = new ThreadLocal<SyncFuture>() {
       @Override
       protected SyncFuture initialValue() {
         return new SyncFuture();
       }
     };
     // Starting up threads in constructor is a no no; Interface should have an init call.
     this.disruptor.start();
   }
 
   private int calculateMaxLogFiles(float memstoreSizeRatio, long logRollSize) {
     long max = -1L;
     final MemoryUsage usage = HeapMemorySizeUtil.safeGetHeapMemoryUsage();
     if (usage != null) {
       max = usage.getMax();
     }
     int maxLogs = Math.round(max * memstoreSizeRatio * 2 / logRollSize);
     return maxLogs;
   }
 
   /**
    * Get the backing files associated with this WAL.
    * @return may be null if there are no files.
    */
   protected FileStatus[] getFiles() throws IOException {
     return FSUtils.listStatus(fs, fullPathLogDir, ourFiles);
   }
 
   /**
    * Currently, we need to expose the writer's OutputStream to tests so that they can manipulate
    * the default behavior (such as setting the maxRecoveryErrorCount value for example (see
    * {@link TestWALReplay#testReplayEditsWrittenIntoWAL()}). This is done using reflection on the
    * underlying HDFS OutputStream.
    * NOTE: This could be removed once Hadoop1 support is removed.
    * @return null if underlying stream is not ready.
    */
   @VisibleForTesting
   OutputStream getOutputStream() {
     FSDataOutputStream fsdos = this.hdfs_out;
     if (fsdos == null) return null;
     return fsdos.getWrappedStream();
   }
 
   @Override
   public byte [][] rollWriter() throws FailedLogCloseException, IOException {
     return rollWriter(false);
   }
 
   /**
    * retrieve the next path to use for writing.
    * Increments the internal filenum.
    */
   private Path getNewPath() throws IOException {
     this.filenum.set(System.currentTimeMillis());
     Path newPath = getCurrentFileName();
     while (fs.exists(newPath)) {
       this.filenum.incrementAndGet();
       newPath = getCurrentFileName();
     }
     return newPath;
   }
 
   Path getOldPath() {
     long currentFilenum = this.filenum.get();
     Path oldPath = null;
     if (currentFilenum > 0) {
       // ComputeFilename  will take care of meta wal filename
       oldPath = computeFilename(currentFilenum);
     } // I presume if currentFilenum is <= 0, this is first file and null for oldPath if fine?
     return oldPath;
   }
 
   /**
    * Tell listeners about pre log roll.
    * @throws IOException
    */
   private void tellListenersAboutPreLogRoll(final Path oldPath, final Path newPath)
   throws IOException {
     coprocessorHost.preWALRoll(oldPath, newPath);
 
     if (!this.listeners.isEmpty()) {
       for (WALActionsListener i : this.listeners) {
         i.preLogRoll(oldPath, newPath);
       }
     }
   }
 
   /**
    * Tell listeners about post log roll.
    * @throws IOException
    */
   private void tellListenersAboutPostLogRoll(final Path oldPath, final Path newPath)
   throws IOException {
     if (!this.listeners.isEmpty()) {
       for (WALActionsListener i : this.listeners) {
         i.postLogRoll(oldPath, newPath);
       }
     }
 
     coprocessorHost.postWALRoll(oldPath, newPath);
   }
 
   /**
    * Run a sync after opening to set up the pipeline.
    * @param nextWriter
    * @param startTimeNanos
    */
   private void preemptiveSync(final ProtobufLogWriter nextWriter) {
     long startTimeNanos = System.nanoTime();
     try {
       nextWriter.sync();
       postSync(System.nanoTime() - startTimeNanos, 0);
     } catch (IOException e) {
       // optimization failed, no need to abort here.
       LOG.warn("pre-sync failed but an optimization so keep going", e);
     }
   }
 
   @Override
   public byte [][] rollWriter(boolean force) throws FailedLogCloseException, IOException {
     rollWriterLock.lock();
     try {
       // Return if nothing to flush.
       if (!force && (this.writer != null && this.numEntries.get() <= 0)) return null;
       byte [][] regionsToFlush = null;
       if (this.closed) {
         LOG.debug("WAL closed. Skipping rolling of writer");
         return regionsToFlush;
       }
       if (!closeBarrier.beginOp()) {
         LOG.debug("WAL closing. Skipping rolling of writer");
         return regionsToFlush;
       }
       TraceScope scope = Trace.startSpan("FSHLog.rollWriter");
       try {
         Path oldPath = getOldPath();
         Path newPath = getNewPath();
         // Any exception from here on is catastrophic, non-recoverable so we currently abort.
         Writer nextWriter = this.createWriterInstance(newPath);
         FSDataOutputStream nextHdfsOut = null;
         if (nextWriter instanceof ProtobufLogWriter) {
           nextHdfsOut = ((ProtobufLogWriter)nextWriter).getStream();
           // If a ProtobufLogWriter, go ahead and try and sync to force setup of pipeline.
           // If this fails, we just keep going.... it is an optimization, not the end of the world.
           preemptiveSync((ProtobufLogWriter)nextWriter);
         }
         tellListenersAboutPreLogRoll(oldPath, newPath);
         // NewPath could be equal to oldPath if replaceWriter fails.
         newPath = replaceWriter(oldPath, newPath, nextWriter, nextHdfsOut);
         tellListenersAboutPostLogRoll(oldPath, newPath);
         // Can we delete any of the old log files?
         if (getNumRolledLogFiles() > 0) {
           cleanOldLogs();
           regionsToFlush = findRegionsToForceFlush();
         }
       } finally {
         closeBarrier.endOp();
         assert scope == NullScope.INSTANCE || !scope.isDetached();
         scope.close();
       }
       return regionsToFlush;
     } finally {
       rollWriterLock.unlock();
     }
   }
 
   /**
    * This method allows subclasses to inject different writers without having to
    * extend other methods like rollWriter().
    *
    * @return Writer instance
    */
   protected Writer createWriterInstance(final Path path) throws IOException {
     return DefaultWALProvider.createWriter(conf, fs, path, false);
   }
 
   /**
    * Archive old logs. A WAL is eligible for archiving if all its WALEdits have been flushed.
    * @throws IOException
    */
   private void cleanOldLogs() throws IOException {
     List<Path> logsToArchive = null;
     // For each log file, look at its Map of regions to highest sequence id; if all sequence ids
     // are older than what is currently in memory, the WAL can be GC'd.
     for (Map.Entry<Path, Map<byte[], Long>> e : this.byWalRegionSequenceIds.entrySet()) {
       Path log = e.getKey();
       Map<byte[], Long> sequenceNums = e.getValue();
       if (this.sequenceIdAccounting.areAllLower(sequenceNums)) {
         if (logsToArchive == null) logsToArchive = new ArrayList<Path>();
         logsToArchive.add(log);
         if (LOG.isTraceEnabled()) LOG.trace("WAL file ready for archiving " + log);
       }
     }
     if (logsToArchive != null) {
       for (Path p : logsToArchive) {
         this.totalLogSize.addAndGet(-this.fs.getFileStatus(p).getLen());
         archiveLogFile(p);
         this.byWalRegionSequenceIds.remove(p);
       }
     }
   }
 
   /**
    * If the number of un-archived WAL files is greater than maximum allowed, check the first
    * (oldest) WAL file, and returns those regions which should be flushed so that it can
    * be archived.
    * @return regions (encodedRegionNames) to flush in order to archive oldest WAL file.
    * @throws IOException
    */
   byte[][] findRegionsToForceFlush() throws IOException {
     byte [][] regions = null;
     int logCount = getNumRolledLogFiles();
     if (logCount > this.maxLogs && logCount > 0) {
       Map.Entry<Path, Map<byte[], Long>> firstWALEntry =
         this.byWalRegionSequenceIds.firstEntry();
       regions = this.sequenceIdAccounting.findLower(firstWALEntry.getValue());
     }
     if (regions != null) {
       StringBuilder sb = new StringBuilder();
       for (int i = 0; i < regions.length; i++) {
         if (i > 0) sb.append(", ");
         sb.append(Bytes.toStringBinary(regions[i]));
       }
       LOG.info("Too many WALs; count=" + logCount + ", max=" + this.maxLogs +
         "; forcing flush of " + regions.length + " regions(s): " + sb.toString());
     }
     return regions;
   }
 
   /**
    * Used to manufacture race condition reliably. For testing only.
    * @see #beforeWaitOnSafePoint()
    */
   @VisibleForTesting
   protected void afterCreatingZigZagLatch() {}
 
   /**
    * @see #afterCreatingZigZagLatch()
    */
   @VisibleForTesting
   protected void beforeWaitOnSafePoint() {};
 
   /**
    * Cleans up current writer closing it and then puts in place the passed in
    * <code>nextWriter</code>.
    *
    * In the case of creating a new WAL, oldPath will be null.
    *
    * In the case of rolling over from one file to the next, none of the params will be null.
    *
    * In the case of closing out this FSHLog with no further use newPath, nextWriter, and
    * nextHdfsOut will be null.
    *
    * @param oldPath may be null
    * @param newPath may be null
    * @param nextWriter may be null
    * @param nextHdfsOut may be null
    * @return the passed in <code>newPath</code>
    * @throws IOException if there is a problem flushing or closing the underlying FS
    */
   Path replaceWriter(final Path oldPath, final Path newPath, Writer nextWriter,
       final FSDataOutputStream nextHdfsOut)
   throws IOException {
     // Ask the ring buffer writer to pause at a safe point.  Once we do this, the writer
     // thread will eventually pause. An error hereafter needs to release the writer thread
     // regardless -- hence the finally block below.  Note, this method is called from the FSHLog
     // constructor BEFORE the ring buffer is set running so it is null on first time through
     // here; allow for that.
     SyncFuture syncFuture = null;
     SafePointZigZagLatch zigzagLatch = null;
     long sequence = -1L;
     if (this.ringBufferEventHandler != null) {
       // Get sequence first to avoid dead lock when ring buffer is full
       // Considering below sequence
       // 1. replaceWriter is called and zigzagLatch is initialized
       // 2. ringBufferEventHandler#onEvent is called and arrives at #attainSafePoint(long) then wait
       // on safePointReleasedLatch
       // 3. Since ring buffer is full, if we get sequence when publish sync, the replaceWriter
       // thread will wait for the ring buffer to be consumed, but the only consumer is waiting
       // replaceWriter thread to release safePointReleasedLatch, which causes a deadlock
       sequence = getSequenceOnRingBuffer();
       zigzagLatch = this.ringBufferEventHandler.attainSafePoint();
     }
     afterCreatingZigZagLatch();
     TraceScope scope = Trace.startSpan("FSHFile.replaceWriter");
     try {
       // Wait on the safe point to be achieved.  Send in a sync in case nothing has hit the
       // ring buffer between the above notification of writer that we want it to go to
       // 'safe point' and then here where we are waiting on it to attain safe point.  Use
       // 'sendSync' instead of 'sync' because we do not want this thread to block waiting on it
       // to come back.  Cleanup this syncFuture down below after we are ready to run again.
       try {
         if (zigzagLatch != null) {
           // use assert to make sure no change breaks the logic that
           // sequence and zigzagLatch will be set together
           assert sequence > 0L : "Failed to get sequence from ring buffer";
           Trace.addTimelineAnnotation("awaiting safepoint");
           syncFuture = zigzagLatch.waitSafePoint(publishSyncOnRingBuffer(sequence));
         }
       } catch (FailedSyncBeforeLogCloseException e) {
         // If unflushed/unsynced entries on close, it is reason to abort.
         if (isUnflushedEntries()) throw e;
         LOG.warn("Failed sync-before-close but no outstanding appends; closing WAL: " +
           e.getMessage());
       }
 
       // It is at the safe point.  Swap out writer from under the blocked writer thread.
       // TODO: This is close is inline with critical section.  Should happen in background?
       try {
         if (this.writer != null) {
           Trace.addTimelineAnnotation("closing writer");
           this.writer.close();
           Trace.addTimelineAnnotation("writer closed");
         }
         this.closeErrorCount.set(0);
       } catch (IOException ioe) {
         int errors = closeErrorCount.incrementAndGet();
         if (!isUnflushedEntries() && (errors <= this.closeErrorsTolerated)) {
           LOG.warn("Riding over failed WAL close of " + oldPath + ", cause=\"" +
             ioe.getMessage() + "\", errors=" + errors +
             "; THIS FILE WAS NOT CLOSED BUT ALL EDITS SYNCED SO SHOULD BE OK");
         } else {
           throw ioe;
         }
       }
       this.writer = nextWriter;
       this.hdfs_out = nextHdfsOut;
       int oldNumEntries = this.numEntries.get();
       this.numEntries.set(0);
       final String newPathString = (null == newPath ? null : FSUtils.getPath(newPath));
       if (oldPath != null) {
         this.byWalRegionSequenceIds.put(oldPath, this.sequenceIdAccounting.resetHighest());
         long oldFileLen = this.fs.getFileStatus(oldPath).getLen();
         this.totalLogSize.addAndGet(oldFileLen);
         LOG.info("Rolled WAL " + FSUtils.getPath(oldPath) + " with entries=" + oldNumEntries +
           ", filesize=" + StringUtils.byteDesc(oldFileLen) + "; new WAL " +
           newPathString);
       } else {
         LOG.info("New WAL " + newPathString);
       }
     } catch (InterruptedException ie) {
       // Perpetuate the interrupt
       Thread.currentThread().interrupt();
     } catch (IOException e) {
       long count = getUnflushedEntriesCount();
       LOG.error("Failed close of WAL writer " + oldPath + ", unflushedEntries=" + count, e);
       throw new FailedLogCloseException(oldPath + ", unflushedEntries=" + count, e);
     } finally {
       try {
         // Let the writer thread go regardless, whether error or not.
         if (zigzagLatch != null) {
           zigzagLatch.releaseSafePoint();
           // syncFuture will be null if we failed our wait on safe point above. Otherwise, if
           // latch was obtained successfully, the sync we threw in either trigger the latch or it
           // got stamped with an exception because the WAL was damaged and we could not sync. Now
           // the write pipeline has been opened up again by releasing the safe point, process the
           // syncFuture we got above. This is probably a noop but it may be stale exception from
           // when old WAL was in place. Catch it if so.
           if (syncFuture != null) {
             try {
               blockOnSync(syncFuture);
             } catch (IOException ioe) {
               if (LOG.isTraceEnabled()) LOG.trace("Stale sync exception", ioe);
             }
           }
         }
       } finally {
         scope.close();
       }
     }
     return newPath;
   }
 
   long getUnflushedEntriesCount() {
     long highestSynced = this.highestSyncedSequence.get();
     return highestSynced > this.highestUnsyncedSequence?
       0: this.highestUnsyncedSequence - highestSynced;
   }
 
   boolean isUnflushedEntries() {
     return getUnflushedEntriesCount() > 0;
   }
 
   /*
    * only public so WALSplitter can use.
    * @return archived location of a WAL file with the given path p
    */
   public static Path getWALArchivePath(Path archiveDir, Path p) {
     return new Path(archiveDir, p.getName());
   }
 
   private void archiveLogFile(final Path p) throws IOException {
     Path newPath = getWALArchivePath(this.fullPathArchiveDir, p);
     // Tell our listeners that a log is going to be archived.
     if (!this.listeners.isEmpty()) {
       for (WALActionsListener i : this.listeners) {
         i.preLogArchive(p, newPath);
       }
     }
     LOG.info("Archiving " + p + " to " + newPath);
     if (!FSUtils.renameAndSetModifyTime(this.fs, p, newPath)) {
       throw new IOException("Unable to rename " + p + " to " + newPath);
     }
     // Tell our listeners that a log has been archived.
     if (!this.listeners.isEmpty()) {
       for (WALActionsListener i : this.listeners) {
         i.postLogArchive(p, newPath);
       }
     }
   }
 
   /**
    * This is a convenience method that computes a new filename with a given
    * file-number.
    * @param filenum to use
    * @return Path
    */
   protected Path computeFilename(final long filenum) {
     if (filenum < 0) {
       throw new RuntimeException("WAL file number can't be < 0");
     }
     String child = logFilePrefix + WAL_FILE_NAME_DELIMITER + filenum + logFileSuffix;
     return new Path(fullPathLogDir, child);
   }
 
   /**
    * This is a convenience method that computes a new filename with a given
    * using the current WAL file-number
    * @return Path
    */
   public Path getCurrentFileName() {
     return computeFilename(this.filenum.get());
   }
 
   @Override
   public String toString() {
     return "FSHLog " + logFilePrefix + ":" + logFileSuffix + "(num " + filenum + ")";
   }
 
 /**
  * A log file has a creation timestamp (in ms) in its file name ({@link #filenum}.
  * This helper method returns the creation timestamp from a given log file.
  * It extracts the timestamp assuming the filename is created with the
  * {@link #computeFilename(long filenum)} method.
  * @param fileName
  * @return timestamp, as in the log file name.
  */
   protected long getFileNumFromFileName(Path fileName) {
     if (fileName == null) throw new IllegalArgumentException("file name can't be null");
     if (!ourFiles.accept(fileName)) {
       throw new IllegalArgumentException("The log file " + fileName +
           " doesn't belong to this WAL. (" + toString() + ")");
     }
     final String fileNameString = fileName.toString();
     String chompedPath = fileNameString.substring(prefixPathStr.length(),
         (fileNameString.length() - logFileSuffix.length()));
     return Long.parseLong(chompedPath);
   }
 
   @Override
   public void close() throws IOException {
     shutdown();
     final FileStatus[] files = getFiles();
     if (null != files && 0 != files.length) {
       for (FileStatus file : files) {
         Path p = getWALArchivePath(this.fullPathArchiveDir, file.getPath());
         // Tell our listeners that a log is going to be archived.
         if (!this.listeners.isEmpty()) {
           for (WALActionsListener i : this.listeners) {
             i.preLogArchive(file.getPath(), p);
           }
         }
 
         if (!FSUtils.renameAndSetModifyTime(fs, file.getPath(), p)) {
           throw new IOException("Unable to rename " + file.getPath() + " to " + p);
         }
         // Tell our listeners that a log was archived.
         if (!this.listeners.isEmpty()) {
           for (WALActionsListener i : this.listeners) {
             i.postLogArchive(file.getPath(), p);
           }
         }
       }
       LOG.debug("Moved " + files.length + " WAL file(s) to " +
         FSUtils.getPath(this.fullPathArchiveDir));
     }
     LOG.info("Closed WAL: " + toString());
   }
 
   @Override
   public void shutdown() throws IOException {
     if (shutdown.compareAndSet(false, true)) {
       try {
         // Prevent all further flushing and rolling.
         closeBarrier.stopAndDrainOps();
       } catch (InterruptedException e) {
         LOG.error("Exception while waiting for cache flushes and log rolls", e);
         Thread.currentThread().interrupt();
       }
 
       // Shutdown the disruptor.  Will stop after all entries have been processed.  Make sure we
       // have stopped incoming appends before calling this else it will not shutdown.  We are
       // conservative below waiting a long time and if not elapsed, then halting.
       if (this.disruptor != null) {
         long timeoutms = conf.getLong("hbase.wal.disruptor.shutdown.timeout.ms", 60000);
         try {
           this.disruptor.shutdown(timeoutms, TimeUnit.MILLISECONDS);
         } catch (TimeoutException e) {
           LOG.warn("Timed out bringing down disruptor after " + timeoutms + "ms; forcing halt " +
             "(It is a problem if this is NOT an ABORT! -- DATALOSS!!!!)");
           this.disruptor.halt();
           this.disruptor.shutdown();
         }
       }
       // With disruptor down, this is safe to let go.
       if (this.appendExecutor !=  null) this.appendExecutor.shutdown();
 
       // Tell our listeners that the log is closing
       if (!this.listeners.isEmpty()) {
         for (WALActionsListener i : this.listeners) {
           i.logCloseRequested();
         }
       }
       this.closed = true;
       if (LOG.isDebugEnabled()) {
         LOG.debug("Closing WAL writer in " + FSUtils.getPath(fullPathLogDir));
       }
       if (this.writer != null) {
         this.writer.close();
         this.writer = null;
       }
     }
   }
 
   @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="NP_NULL_ON_SOME_PATH_EXCEPTION",
       justification="Will never be null")
   @Override
   public long append(final HTableDescriptor htd, final HRegionInfo hri, final WALKey key,
       final WALEdit edits, final boolean inMemstore) throws IOException {
     if (this.closed) throw new IOException("Cannot append; log is closed");
     // Make a trace scope for the append.  It is closed on other side of the ring buffer by the
     // single consuming thread.  Don't have to worry about it.
     TraceScope scope = Trace.startSpan("FSHLog.append");
 
     // This is crazy how much it takes to make an edit.  Do we need all this stuff!!!!????  We need
     // all this to make a key and then below to append the edit, we need to carry htd, info,
     // etc. all over the ring buffer.
     FSWALEntry entry = null;
     long sequence = this.disruptor.getRingBuffer().next();
     try {
       RingBufferTruck truck = this.disruptor.getRingBuffer().get(sequence);
       // TODO: reuse FSWALEntry as we do SyncFuture rather create per append.
       entry = new FSWALEntry(sequence, key, edits, htd, hri, inMemstore);
       truck.loadPayload(entry, scope.detach());
     } finally {
       this.disruptor.getRingBuffer().publish(sequence);
     }
     return sequence;
   }
 
   /**
    * Thread to runs the hdfs sync call. This call takes a while to complete.  This is the longest
    * pole adding edits to the WAL and this must complete to be sure all edits persisted.  We run
    * multiple threads sync'ng rather than one that just syncs in series so we have better
    * latencies; otherwise, an edit that arrived just after a sync started, might have to wait
    * almost the length of two sync invocations before it is marked done.
    * <p>When the sync completes, it marks all the passed in futures done.  On the other end of the
    * sync future is a blocked thread, usually a regionserver Handler.  There may be more than one
    * future passed in the case where a few threads arrive at about the same time and all invoke
    * 'sync'.  In this case we'll batch up the invocations and run one filesystem sync only for a
    * batch of Handler sync invocations.  Do not confuse these Handler SyncFutures with the futures
    * an ExecutorService returns when you call submit. We have no use for these in this model. These
    * SyncFutures are 'artificial', something to hold the Handler until the filesystem sync
    * completes.
    */
   private class SyncRunner extends HasThread {
     private volatile long sequence;
     // Keep around last exception thrown. Clear on successful sync.
     private final BlockingQueue<SyncFuture> syncFutures;
     private volatile SyncFuture takeSyncFuture = null;
 
     /**
      * UPDATE!
      * @param syncs the batch of calls to sync that arrived as this thread was starting; when done,
      * we will put the result of the actual hdfs sync call as the result.
      * @param sequence The sequence number on the ring buffer when this thread was set running.
      * If this actual writer sync completes then all appends up this point have been
      * flushed/synced/pushed to datanodes.  If we fail, then the passed in <code>syncs</code>
      * futures will return the exception to their clients; some of the edits may have made it out
      * to data nodes but we will report all that were part of this session as failed.
      */
     SyncRunner(final String name, final int maxHandlersCount) {
       super(name);
       // LinkedBlockingQueue because of
       // http://www.javacodegeeks.com/2010/09/java-best-practices-queue-battle-and.html
       // Could use other blockingqueues here or concurrent queues.
       //
       // We could let the capacity be 'open' but bound it so we get alerted in pathological case
       // where we cannot sync and we have a bunch of threads all backed up waiting on their syncs
       // to come in.  LinkedBlockingQueue actually shrinks when you remove elements so Q should
       // stay neat and tidy in usual case.  Let the max size be three times the maximum handlers.
       // The passed in maxHandlerCount is the user-level handlers which is what we put up most of
       // but HBase has other handlers running too -- opening region handlers which want to write
       // the meta table when succesful (i.e. sync), closing handlers -- etc.  These are usually
       // much fewer in number than the user-space handlers so Q-size should be user handlers plus
       // some space for these other handlers.  Lets multiply by 3 for good-measure.
       this.syncFutures = new LinkedBlockingQueue<SyncFuture>(maxHandlersCount * 3);
     }
 
     void offer(final long sequence, final SyncFuture [] syncFutures, final int syncFutureCount) {
       // Set sequence first because the add to the queue will wake the thread if sleeping.
       this.sequence = sequence;
       for (int i = 0; i < syncFutureCount; ++i) {
         this.syncFutures.add(syncFutures[i]);
       }
     }
 
     /**
      * Release the passed <code>syncFuture</code>
      * @param syncFuture
      * @param currentSequence
      * @param t
      * @return Returns 1.
      */
     private int releaseSyncFuture(final SyncFuture syncFuture, final long currentSequence,
         final Throwable t) {
       if (!syncFuture.done(currentSequence, t)) throw new IllegalStateException();
       // This function releases one sync future only.
       return 1;
     }
 
     /**
      * Release all SyncFutures whose sequence is <= <code>currentSequence</code>.
      * @param currentSequence
      * @param t May be non-null if we are processing SyncFutures because an exception was thrown.
      * @return Count of SyncFutures we let go.
      */
     private int releaseSyncFutures(final long currentSequence, final Throwable t) {
       int syncCount = 0;
       for (SyncFuture syncFuture; (syncFuture = this.syncFutures.peek()) != null;) {
         if (syncFuture.getRingBufferSequence() > currentSequence) break;
         releaseSyncFuture(syncFuture, currentSequence, t);
         if (!this.syncFutures.remove(syncFuture)) {
           throw new IllegalStateException(syncFuture.toString());
         }
         syncCount++;
       }
       return syncCount;
     }
 
     /**
      * @param sequence The sequence we ran the filesystem sync against.
      * @return Current highest synced sequence.
      */
     private long updateHighestSyncedSequence(long sequence) {
       long currentHighestSyncedSequence;
       // Set the highestSyncedSequence IFF our current sequence id is the 'highest'.
       do {
         currentHighestSyncedSequence = highestSyncedSequence.get();
         if (currentHighestSyncedSequence >= sequence) {
           // Set the sync number to current highwater mark; might be able to let go more
           // queued sync futures
           sequence = currentHighestSyncedSequence;
           break;
         }
       } while (!highestSyncedSequence.compareAndSet(currentHighestSyncedSequence, sequence));
       return sequence;
     }
 
     boolean areSyncFuturesReleased() {
       // check whether there is no sync futures offered, and no in-flight sync futures that is being
       // processed.
       return syncFutures.size() <= 0
           && takeSyncFuture == null;
     }
 
     public void run() {
       long currentSequence;
       while (!isInterrupted()) {
         int syncCount = 0;
 
         try {
           while (true) {
             takeSyncFuture = null;
             // We have to process what we 'take' from the queue
             takeSyncFuture = this.syncFutures.take();
             currentSequence = this.sequence;
             long syncFutureSequence = takeSyncFuture.getRingBufferSequence();
             if (syncFutureSequence > currentSequence) {
               throw new IllegalStateException("currentSequence=" + syncFutureSequence +
                 ", syncFutureSequence=" + syncFutureSequence);
             }
             // See if we can process any syncfutures BEFORE we go sync.
             long currentHighestSyncedSequence = highestSyncedSequence.get();
             if (currentSequence < currentHighestSyncedSequence) {
               syncCount += releaseSyncFuture(takeSyncFuture, currentHighestSyncedSequence, null);
               // Done with the 'take'.  Go around again and do a new 'take'.
               continue;
             }
             break;
           }
           // I got something.  Lets run.  Save off current sequence number in case it changes
           // while we run.
           TraceScope scope = Trace.continueSpan(takeSyncFuture.getSpan());
           long start = System.nanoTime();
           Throwable lastException = null;
           try {
             Trace.addTimelineAnnotation("syncing writer");
             writer.sync();
             Trace.addTimelineAnnotation("writer synced");
             currentSequence = updateHighestSyncedSequence(currentSequence);
           } catch (IOException e) {
             LOG.error("Error syncing, request close of WAL", e);
             lastException = e;
           } catch (Exception e) {
             LOG.warn("UNEXPECTED", e);
             lastException = e;
           } finally {
             // reattach the span to the future before releasing.
             takeSyncFuture.setSpan(scope.detach());
             // First release what we 'took' from the queue.
             syncCount += releaseSyncFuture(takeSyncFuture, currentSequence, lastException);
             // Can we release other syncs?
             syncCount += releaseSyncFutures(currentSequence, lastException);
             if (lastException != null) requestLogRoll();
             else checkLogRoll();
           }
           postSync(System.nanoTime() - start, syncCount);
         } catch (InterruptedException e) {
           // Presume legit interrupt.
           Thread.currentThread().interrupt();
         } catch (Throwable t) {
           LOG.warn("UNEXPECTED, continuing", t);
         }
       }
     }
   }
 
   /**
    * Schedule a log roll if needed.
    */
   public void checkLogRoll() {
     // Will return immediately if we are in the middle of a WAL log roll currently.
     if (!rollWriterLock.tryLock()) return;
     boolean lowReplication;
     try {
       lowReplication = checkLowReplication();
     } finally {
       rollWriterLock.unlock();
     }
     try {
       if (lowReplication || writer != null && writer.getLength() > logrollsize) {
         requestLogRoll(lowReplication);
       }
     } catch (IOException e) {
       LOG.warn("Writer.getLength() failed; continuing", e);
     }
   }
 
   /*
    * @return true if number of replicas for the WAL is lower than threshold
    */
   private boolean checkLowReplication() {
     boolean logRollNeeded = false;
     this.lastTimeCheckLowReplication = EnvironmentEdgeManager.currentTime();
     // if the number of replicas in HDFS has fallen below the configured
     // value, then roll logs.
     try {
       int numCurrentReplicas = getLogReplication();
       if (numCurrentReplicas != 0 && numCurrentReplicas < this.minTolerableReplication) {
         if (this.lowReplicationRollEnabled) {
           if (this.consecutiveLogRolls.get() < this.lowReplicationRollLimit) {
             LOG.warn("HDFS pipeline error detected. " + "Found "
                 + numCurrentReplicas + " replicas but expecting no less than "
                 + this.minTolerableReplication + " replicas. "
                 + " Requesting close of WAL. current pipeline: "
                 + Arrays.toString(getPipeLine()));
             logRollNeeded = true;
             // If rollWriter is requested, increase consecutiveLogRolls. Once it
             // is larger than lowReplicationRollLimit, disable the
             // LowReplication-Roller
             this.consecutiveLogRolls.getAndIncrement();
           } else {
             LOG.warn("Too many consecutive RollWriter requests, it's a sign of "
                 + "the total number of live datanodes is lower than the tolerable replicas.");
             this.consecutiveLogRolls.set(0);
             this.lowReplicationRollEnabled = false;
           }
         }
       } else if (numCurrentReplicas >= this.minTolerableReplication) {
         if (!this.lowReplicationRollEnabled) {
           // The new writer's log replicas is always the default value.
           // So we should not enable LowReplication-Roller. If numEntries
           // is lower than or equals 1, we consider it as a new writer.
           if (this.numEntries.get() <= 1) {
             return logRollNeeded;
           }
           // Once the live datanode number and the replicas return to normal,
           // enable the LowReplication-Roller.
           this.lowReplicationRollEnabled = true;
           LOG.info("LowReplication-Roller was enabled.");
         }
       }
     } catch (Exception e) {
       LOG.warn("DFSOutputStream.getNumCurrentReplicas failed because of " + e +
         ", continuing...");
     }
     return logRollNeeded;
   }
 
   private SyncFuture publishSyncOnRingBuffer(long sequence) {
     return publishSyncOnRingBuffer(sequence, null);
   }
 
   private long getSequenceOnRingBuffer() {
     return this.disruptor.getRingBuffer().next();
   }
 
   private SyncFuture publishSyncOnRingBuffer(Span span) {
     long sequence = this.disruptor.getRingBuffer().next();
     return publishSyncOnRingBuffer(sequence, span);
   }
 
   private SyncFuture publishSyncOnRingBuffer(long sequence, Span span) {
     SyncFuture syncFuture = getSyncFuture(sequence, span);
     try {
       RingBufferTruck truck = this.disruptor.getRingBuffer().get(sequence);
       truck.loadPayload(syncFuture);
     } finally {
       this.disruptor.getRingBuffer().publish(sequence);
     }
     return syncFuture;
   }
 
   // Sync all known transactions
   private Span publishSyncThenBlockOnCompletion(Span span) throws IOException {
     return blockOnSync(publishSyncOnRingBuffer(span));
   }
 
   private Span blockOnSync(final SyncFuture syncFuture) throws IOException {
     // Now we have published the ringbuffer, halt the current thread until we get an answer back.
     try {
       syncFuture.get(walSyncTimeout);
       return syncFuture.getSpan();
     } catch (TimeoutIOException tioe) {
       // SyncFuture reuse by thread, if TimeoutIOException happens, ringbuffer
       // still refer to it, so if this thread use it next time may get a wrong
       // result.
       this.cachedSyncFutures.remove();
       throw tioe;
     } catch (InterruptedException ie) {
       LOG.warn("Interrupted", ie);
       throw convertInterruptedExceptionToIOException(ie);
     } catch (ExecutionException e) {
       throw ensureIOException(e.getCause());
     }
   }
 
   private IOException convertInterruptedExceptionToIOException(final InterruptedException ie) {
     Thread.currentThread().interrupt();
     IOException ioe = new InterruptedIOException();
     ioe.initCause(ie);
     return ioe;
   }
 
   private SyncFuture getSyncFuture(final long sequence, Span span) {
     return cachedSyncFutures.get().reset(sequence);
   }
 
   private void postSync(final long timeInNanos, final int handlerSyncs) {
     if (timeInNanos > this.slowSyncNs) {
       String msg =
           new StringBuilder().append("Slow sync cost: ")
               .append(timeInNanos / 1000000).append(" ms, current pipeline: ")
               .append(Arrays.toString(getPipeLine())).toString();
       Trace.addTimelineAnnotation(msg);
       LOG.info(msg);
     }
     if (!listeners.isEmpty()) {
       for (WALActionsListener listener : listeners) {
         listener.postSync(timeInNanos, handlerSyncs);
       }
     }
   }
 
   private long postAppend(final Entry e, final long elapsedTime) throws IOException {
     long len = 0;
     if (!listeners.isEmpty()) {
       for (Cell cell : e.getEdit().getCells()) {
         len += CellUtil.estimatedSerializedSizeOf(cell);
       }
       for (WALActionsListener listener : listeners) {
         listener.postAppend(len, elapsedTime, e.getKey(), e.getEdit());
       }
     }
     return len;
   }
 
 
   /**
    * This method gets the datanode replication count for the current WAL.
    *
    * If the pipeline isn't started yet or is empty, you will get the default
    * replication factor.  Therefore, if this function returns 0, it means you
    * are not properly running with the HDFS-826 patch.
    * @throws InvocationTargetException
    * @throws IllegalAccessException
    * @throws IllegalArgumentException
    *
    * @throws Exception
    */
   @VisibleForTesting
   int getLogReplication() {
     try {
       //in standalone mode, it will return 0
       if (this.hdfs_out instanceof HdfsDataOutputStream) {
         return ((HdfsDataOutputStream) this.hdfs_out).getCurrentBlockReplication();
       }
     } catch (IOException e) {
       LOG.info("", e);
     }
     return 0;
   }
 
   @Override
   public void sync() throws IOException {
     TraceScope scope = Trace.startSpan("FSHLog.sync");
     try {
       scope = Trace.continueSpan(publishSyncThenBlockOnCompletion(scope.detach()));
     } finally {
       assert scope == NullScope.INSTANCE || !scope.isDetached();
       scope.close();
     }
   }
 
   @Override
   public void sync(long txid) throws IOException {
     if (this.highestSyncedSequence.get() >= txid){
       // Already sync'd.
       return;
     }
     TraceScope scope = Trace.startSpan("FSHLog.sync");
     try {
       scope = Trace.continueSpan(publishSyncThenBlockOnCompletion(scope.detach()));
     } finally {
       assert scope == NullScope.INSTANCE || !scope.isDetached();
       scope.close();
     }
   }
 
   // public only until class moves to o.a.h.h.wal
   public void requestLogRoll() {
     requestLogRoll(false);
   }
 
   private void requestLogRoll(boolean tooFewReplicas) {
     if (!this.listeners.isEmpty()) {
       for (WALActionsListener i: this.listeners) {
         i.logRollRequested(tooFewReplicas);
       }
     }
   }
 
   // public only until class moves to o.a.h.h.wal
   /** @return the number of rolled log files */
   public int getNumRolledLogFiles() {
     return byWalRegionSequenceIds.size();
   }
 
   // public only until class moves to o.a.h.h.wal
   /** @return the number of log files in use */
   public int getNumLogFiles() {
     // +1 for current use log
     return getNumRolledLogFiles() + 1;
   }
 
   // public only until class moves to o.a.h.h.wal
   /** @return the size of log files in use */
   public long getLogFileSize() {
     return this.totalLogSize.get();
   }
 
   @Override
   public Long startCacheFlush(final byte[] encodedRegionName, Set<byte[]> families) {
     if (!closeBarrier.beginOp()) {
       LOG.info("Flush not started for " + Bytes.toString(encodedRegionName) + "; server closing.");
       return null;
     }
     return this.sequenceIdAccounting.startCacheFlush(encodedRegionName, families);
   }
 
   @Override
   public void completeCacheFlush(final byte [] encodedRegionName) {
     this.sequenceIdAccounting.completeCacheFlush(encodedRegionName);
     closeBarrier.endOp();
   }
 
   @Override
   public void abortCacheFlush(byte[] encodedRegionName) {
     this.sequenceIdAccounting.abortCacheFlush(encodedRegionName);
     closeBarrier.endOp();
   }
 
   @VisibleForTesting
   boolean isLowReplicationRollEnabled() {
       return lowReplicationRollEnabled;
   }
 
   public static final long FIXED_OVERHEAD = ClassSize.align(
     ClassSize.OBJECT + (5 * ClassSize.REFERENCE) +
     ClassSize.ATOMIC_INTEGER + Bytes.SIZEOF_INT + (3 * Bytes.SIZEOF_LONG));
 
 
   private static void split(final Configuration conf, final Path p) throws IOException {
     FileSystem fs = FSUtils.getWALFileSystem(conf);
     if (!fs.exists(p)) {
       throw new FileNotFoundException(p.toString());
     }
     if (!fs.getFileStatus(p).isDirectory()) {
       throw new IOException(p + " is not a directory");
     }
 
     final Path baseDir = FSUtils.getWALRootDir(conf);
     final Path archiveDir = new Path(baseDir, HConstants.HREGION_OLDLOGDIR_NAME);
     WALSplitter.split(baseDir, p, archiveDir, fs, conf, WALFactory.getInstance(conf));
   }
 
 
   @Override
   public long getEarliestMemstoreSeqNum(byte[] encodedRegionName) {
     // Used by tests. Deprecated as too subtle for general usage.
     return this.sequenceIdAccounting.getLowestSequenceId(encodedRegionName);
   }
 
   @Override
   public long getEarliestMemstoreSeqNum(byte[] encodedRegionName, byte[] familyName) {
     // This method is used by tests and for figuring if we should flush or not because our
     // sequenceids are too old. It is also used reporting the master our oldest sequenceid for use
     // figuring what edits can be skipped during log recovery. getEarliestMemStoreSequenceId
     // from this.sequenceIdAccounting is looking first in flushingOldestStoreSequenceIds, the
     // currently flushing sequence ids, and if anything found there, it is returning these. This is
     // the right thing to do for the reporting oldest sequenceids to master; we won't skip edits if
     // we crash during the flush. For figuring what to flush, we might get requeued if our sequence
     // id is old even though we are currently flushing. This may mean we do too much flushing.
     return this.sequenceIdAccounting.getLowestSequenceId(encodedRegionName, familyName);
   }
 
   /**
    * This class is used coordinating two threads holding one thread at a
    * 'safe point' while the orchestrating thread does some work that requires the first thread
    * paused: e.g. holding the WAL writer while its WAL is swapped out from under it by another
    * thread.
    *
    * <p>Thread A signals Thread B to hold when it gets to a 'safe point'.  Thread A wait until
    * Thread B gets there. When the 'safe point' has been attained, Thread B signals Thread A.
    * Thread B then holds at the 'safe point'.  Thread A on notification that Thread B is paused,
    * goes ahead and does the work it needs to do while Thread B is holding.  When Thread A is done,
    * it flags B and then Thread A and Thread B continue along on their merry way.  Pause and
    * signalling 'zigzags' between the two participating threads.  We use two latches -- one the
    * inverse of the other -- pausing and signaling when states are achieved.
    *
    * <p>To start up the drama, Thread A creates an instance of this class each time it would do
    * this zigzag dance and passes it to Thread B (these classes use Latches so it is one shot
    * only). Thread B notices the new instance (via reading a volatile reference or how ever) and it
    * starts to work toward the 'safe point'.  Thread A calls {@link #waitSafePoint()} when it
    * cannot proceed until the Thread B 'safe point' is attained. Thread A will be held inside in
    * {@link #waitSafePoint()} until Thread B reaches the 'safe point'.  Once there, Thread B
    * frees Thread A by calling {@link #safePointAttained()}.  Thread A now knows Thread B
    * is at the 'safe point' and that it is holding there (When Thread B calls
    * {@link #safePointAttained()} it blocks here until Thread A calls {@link #releaseSafePoint()}).
    * Thread A proceeds to do what it needs to do while Thread B is paused.  When finished,
    * it lets Thread B lose by calling {@link #releaseSafePoint()} and away go both Threads again.
    */
   static class SafePointZigZagLatch {
     /**
      * Count down this latch when safe point attained.
      */
     private volatile CountDownLatch safePointAttainedLatch = new CountDownLatch(1);
     /**
      * Latch to wait on.  Will be released when we can proceed.
      */
     private volatile CountDownLatch safePointReleasedLatch = new CountDownLatch(1);
 
     private void checkIfSyncFailed(SyncFuture syncFuture) throws FailedSyncBeforeLogCloseException {
       if (syncFuture.isThrowable()) {
         throw new FailedSyncBeforeLogCloseException(syncFuture.getThrowable());
       }
     }
 
     /**
      * For Thread A to call when it is ready to wait on the 'safe point' to be attained. Thread A
      * will be held in here until Thread B calls {@link #safePointAttained()}
      * @param syncFuture We need this as barometer on outstanding syncs.  If it comes home with an
      *          exception, then something is up w/ our syncing.
      * @return The passed <code>syncFuture</code>
      */
     SyncFuture waitSafePoint(SyncFuture syncFuture) throws InterruptedException,
         FailedSyncBeforeLogCloseException {
       while (!this.safePointAttainedLatch.await(1, TimeUnit.MILLISECONDS)) {
         checkIfSyncFailed(syncFuture);
       }
       checkIfSyncFailed(syncFuture);
       return syncFuture;
     }
 
     /**
      * Called by Thread B when it attains the 'safe point'.  In this method, Thread B signals
      * Thread A it can proceed. Thread B will be held in here until {@link #releaseSafePoint()}
      * is called by Thread A.
      * @throws InterruptedException
      */
     void safePointAttained() throws InterruptedException {
       this.safePointAttainedLatch.countDown();
       this.safePointReleasedLatch.await();
     }
 
     /**
      * Called by Thread A when it is done with the work it needs to do while Thread B is
      * halted.  This will release the Thread B held in a call to {@link #safePointAttained()}
      */
     void releaseSafePoint() {
       this.safePointReleasedLatch.countDown();
     }
 
     /**
      * @return True is this is a 'cocked', fresh instance, and not one that has already fired.
      */
     boolean isCocked() {
       return this.safePointAttainedLatch.getCount() > 0 &&
         this.safePointReleasedLatch.getCount() > 0;
     }
   }
 
   /**
    * Handler that is run by the disruptor ringbuffer consumer. Consumer is a SINGLE
    * 'writer/appender' thread.  Appends edits and starts up sync runs.  Tries its best to batch up
    * syncs.  There is no discernible benefit batching appends so we just append as they come in
    * because it simplifies the below implementation.  See metrics for batching effectiveness
    * (In measurement, at 100 concurrent handlers writing 1k, we are batching > 10 appends and 10
    * handler sync invocations for every actual dfsclient sync call; at 10 concurrent handlers,
    * YMMV).
    * <p>Herein, we have an array into which we store the sync futures as they come in.  When we
    * have a 'batch', we'll then pass what we have collected to a SyncRunner thread to do the
    * filesystem sync.  When it completes, it will then call
    * {@link SyncFuture#done(long, Throwable)} on each of SyncFutures in the batch to release
    * blocked Handler threads.
    * <p>I've tried various effects to try and make latencies low while keeping throughput high.
    * I've tried keeping a single Queue of SyncFutures in this class appending to its tail as the
    * syncs coming and having sync runner threads poll off the head to 'finish' completed
    * SyncFutures.  I've tried linkedlist, and various from concurrent utils whether
    * LinkedBlockingQueue or ArrayBlockingQueue, etc.  The more points of synchronization, the
    * more 'work' (according to 'perf stats') that has to be done; small increases in stall
    * percentages seem to have a big impact on throughput/latencies.  The below model where we have
    * an array into which we stash the syncs and then hand them off to the sync thread seemed like
    * a decent compromise.  See HBASE-8755 for more detail.
    */
   class RingBufferEventHandler implements EventHandler<RingBufferTruck>, LifecycleAware {
     private final SyncRunner [] syncRunners;
     private final SyncFuture [] syncFutures;
     // Had 'interesting' issues when this was non-volatile.  On occasion, we'd not pass all
     // syncFutures to the next sync'ing thread.
     private volatile int syncFuturesCount = 0;
     private volatile SafePointZigZagLatch zigzagLatch;
     /**
      * Set if we get an exception appending or syncing so that all subsequence appends and syncs
      * on this WAL fail until WAL is replaced.
      */
     private Exception exception = null;
     /**
      * Object to block on while waiting on safe point.
      */
     private final Object safePointWaiter = new Object();
     private volatile boolean shutdown = false;
 
     /**
      * Which syncrunner to use next.
      */
     private int syncRunnerIndex;
 
     RingBufferEventHandler(final int syncRunnerCount, final int maxHandlersCount) {
       this.syncFutures = new SyncFuture[maxHandlersCount];
       this.syncRunners = new SyncRunner[syncRunnerCount];
       for (int i = 0; i < syncRunnerCount; i++) {
         this.syncRunners[i] = new SyncRunner("sync." + i, maxHandlersCount);
       }
     }
 
     private void cleanupOutstandingSyncsOnException(final long sequence, final Exception e) {
       // There could be handler-count syncFutures outstanding.
       for (int i = 0; i < this.syncFuturesCount; i++) this.syncFutures[i].done(sequence, e);
       this.syncFuturesCount = 0;
     }
 
     /**
      * @return True if outstanding sync futures still
      */
     private boolean isOutstandingSyncs() {
       // Look at SyncFutures in the EventHandler
       for (int i = 0; i < this.syncFuturesCount; i++) {
         if (!this.syncFutures[i].isDone()) return true;
       }
 
       return false;
     }
 
     private boolean isOutstandingSyncsFromRunners() {
       // Look at SyncFutures in the SyncRunners
       for (SyncRunner syncRunner: syncRunners) {
         if(syncRunner.isAlive() && !syncRunner.areSyncFuturesReleased()) {
           return true;
         }
       }
       return false;
     }
 
     @Override
     // We can set endOfBatch in the below method if at end of our this.syncFutures array
     public void onEvent(final RingBufferTruck truck, final long sequence, boolean endOfBatch)
     throws Exception {
       // Appends and syncs are coming in order off the ringbuffer.  We depend on this fact.  We'll
       // add appends to dfsclient as they come in.  Batching appends doesn't give any significant
       // benefit on measurement.  Handler sync calls we will batch up. If we get an exception
       // appending an edit, we fail all subsequent appends and syncs with the same exception until
       // the WAL is reset. It is important that we not short-circuit and exit early this method.
       // It is important that we always go through the attainSafePoint on the end. Another thread,
       // the log roller may be waiting on a signal from us here and will just hang without it.
 
       try {
         if (truck.hasSyncFuturePayload()) {
           this.syncFutures[this.syncFuturesCount++] = truck.unloadSyncFuturePayload();
           // Force flush of syncs if we are carrying a full complement of syncFutures.
           if (this.syncFuturesCount == this.syncFutures.length) endOfBatch = true;
         } else if (truck.hasFSWALEntryPayload()) {
           TraceScope scope = Trace.continueSpan(truck.unloadSpanPayload());
           try {
             FSWALEntry entry = truck.unloadFSWALEntryPayload();
             if (this.exception != null) {
               // We got an exception on an earlier attempt at append. Do not let this append
               // go through. Fail it but stamp the sequenceid into this append though failed.
               // We need to do this to close the latch held down deep in WALKey...that is waiting
               // on sequenceid assignment otherwise it will just hang out (The #append method
               // called below does this also internally).
               entry.stampRegionSequenceId();
               // Return to keep processing events coming off the ringbuffer
               return;
             }
             append(entry);
           } catch (Exception e) {
             // Failed append. Record the exception.
             this.exception = e;
             // invoking cleanupOutstandingSyncsOnException when append failed with exception,
             // it will cleanup existing sync requests recorded in syncFutures but not offered to SyncRunner yet,
             // so there won't be any sync future left over if no further truck published to disruptor.
             cleanupOutstandingSyncsOnException(sequence,
                 this.exception instanceof DamagedWALException ? this.exception
                     : new DamagedWALException("On sync", this.exception));
             // Return to keep processing events coming off the ringbuffer
             return;
           } finally {
             assert scope == NullScope.INSTANCE || !scope.isDetached();
             scope.close(); // append scope is complete
           }
         } else {
           // What is this if not an append or sync. Fail all up to this!!!
           cleanupOutstandingSyncsOnException(sequence,
             new IllegalStateException("Neither append nor sync"));
           // Return to keep processing.
           return;
         }
 
         // TODO: Check size and if big go ahead and call a sync if we have enough data.
         // This is a sync. If existing exception, fall through. Else look to see if batch.
         if (this.exception == null) {
           // If not a batch, return to consume more events from the ring buffer before proceeding;
           // we want to get up a batch of syncs and appends before we go do a filesystem sync.
           if (!endOfBatch || this.syncFuturesCount <= 0) return;
           // syncRunnerIndex is bound to the range [0, Integer.MAX_INT - 1] as follows:
           //   * The maximum value possible for syncRunners.length is Integer.MAX_INT
           //   * syncRunnerIndex starts at 0 and is incremented only here
           //   * after the increment, the value is bounded by the '%' operator to [0, syncRunners.length),
           //     presuming the value was positive prior to the '%' operator.
           //   * after being bound to [0, Integer.MAX_INT - 1], the new value is stored in syncRunnerIndex
           //     ensuring that it can't grow without bound and overflow.
           //   * note that the value after the increment must be positive, because the most it could have
           //     been prior was Integer.MAX_INT - 1 and we only increment by 1.
           this.syncRunnerIndex = (this.syncRunnerIndex + 1) % this.syncRunners.length;
           try {
             // Below expects that the offer 'transfers' responsibility for the outstanding syncs to
             // the syncRunner. We should never get an exception in here.
             this.syncRunners[this.syncRunnerIndex].offer(sequence, this.syncFutures,
               this.syncFuturesCount);
           } catch (Exception e) {
             // Should NEVER get here.
             requestLogRoll();
             this.exception = new DamagedWALException("Failed offering sync", e);
           }
         }
         // We may have picked up an exception above trying to offer sync
         if (this.exception != null) {
           cleanupOutstandingSyncsOnException(sequence,
             this.exception instanceof DamagedWALException?
               this.exception:
               new DamagedWALException("On sync", this.exception));
         }
         attainSafePoint(sequence);
         this.syncFuturesCount = 0;
       } catch (Throwable t) {
         LOG.error("UNEXPECTED!!! syncFutures.length=" + this.syncFutures.length, t);
       }
     }
 
     SafePointZigZagLatch attainSafePoint() {
       this.zigzagLatch = new SafePointZigZagLatch();
       return this.zigzagLatch;
     }
 
     /**
      * Check if we should attain safe point.  If so, go there and then wait till signalled before
      * we proceeding.
      */
     private void attainSafePoint(final long currentSequence) {
       if (this.zigzagLatch == null || !this.zigzagLatch.isCocked()) return;
       // If here, another thread is waiting on us to get to safe point.  Don't leave it hanging.
       beforeWaitOnSafePoint();
       try {
         // Wait on outstanding syncers; wait for them to finish syncing (unless we've been
         // shutdown or unless our latch has been thrown because we have been aborted or unless
         // this WAL is broken and we can't get a sync/append to complete).
         while ((!this.shutdown && this.zigzagLatch.isCocked()
             && highestSyncedSequence.get() < currentSequence &&
             // We could be in here and all syncs are failing or failed. Check for this. Otherwise
             // we'll just be stuck here for ever. In other words, ensure there syncs running.
             isOutstandingSyncs())
             // Wait for all SyncRunners to finish their work so that we can replace the writer
             || isOutstandingSyncsFromRunners()) {
           synchronized (this.safePointWaiter) {
             this.safePointWaiter.wait(0, 1);
           }
         }
         // Tell waiting thread we've attained safe point. Can clear this.throwable if set here
         // because we know that next event through the ringbuffer will be going to a new WAL
         // after we do the zigzaglatch dance.
         this.exception = null;
         this.zigzagLatch.safePointAttained();
       } catch (InterruptedException e) {
         LOG.warn("Interrupted ", e);
         Thread.currentThread().interrupt();
       }
     }
 
     /**
      * Append to the WAL.  Does all CP and WAL listener calls.
      * @param entry
      * @throws Exception
      */
     void append(final FSWALEntry entry) throws Exception {
       // TODO: WORK ON MAKING THIS APPEND FASTER. DOING WAY TOO MUCH WORK WITH CPs, PBing, etc.
       atHeadOfRingBufferEventHandlerAppend();
 
       long start = EnvironmentEdgeManager.currentTime();
       byte [] encodedRegionName = entry.getKey().getEncodedRegionName();
       long regionSequenceId = WALKey.NO_SEQUENCE_ID;
       try {
         // We are about to append this edit; update the region-scoped sequence number.  Do it
         // here inside this single appending/writing thread.  Events are ordered on the ringbuffer
         // so region sequenceids will also be in order.
         regionSequenceId = entry.stampRegionSequenceId();
         // Edits are empty, there is nothing to append.  Maybe empty when we are looking for a
         // region sequence id only, a region edit/sequence id that is not associated with an actual
         // edit. It has to go through all the rigmarole to be sure we have the right ordering.
         if (entry.getEdit().isEmpty()) {
           return;
         }
 
         // Coprocessor hook.
         if (!coprocessorHost.preWALWrite(entry.getHRegionInfo(), entry.getKey(),
             entry.getEdit())) {
           if (entry.getEdit().isReplay()) {
             // Set replication scope null so that this won't be replicated
             entry.getKey().setScopes(null);
           }
         }
         if (!listeners.isEmpty()) {
           for (WALActionsListener i: listeners) {
             // TODO: Why does listener take a table description and CPs take a regioninfo?  Fix.
             i.visitLogEntryBeforeWrite(entry.getHTableDescriptor(), entry.getKey(),
               entry.getEdit());
           }
         }
 
         writer.append(entry);
         assert highestUnsyncedSequence < entry.getSequence();
         highestUnsyncedSequence = entry.getSequence();
         sequenceIdAccounting.update(encodedRegionName, entry.getFamilyNames(), regionSequenceId,
           entry.isInMemstore());
         coprocessorHost.postWALWrite(entry.getHRegionInfo(), entry.getKey(), entry.getEdit());
         // Update metrics.
         postAppend(entry, EnvironmentEdgeManager.currentTime() - start);
       } catch (Exception e) {
         String msg = "Append sequenceId=" + regionSequenceId + ", requesting roll of WAL";
         LOG.warn(msg, e);
         requestLogRoll();
         throw new DamagedWALException(msg, e);
       }
       numEntries.incrementAndGet();
     }
 
     @Override
     public void onStart() {
       for (SyncRunner syncRunner: this.syncRunners) syncRunner.start();
     }
 
     @Override
     public void onShutdown() {
       for (SyncRunner syncRunner: this.syncRunners) syncRunner.interrupt();
     }
   }
 
   /**
    * Exposed for testing only.  Use to tricks like halt the ring buffer appending.
    */
   @VisibleForTesting
   void atHeadOfRingBufferEventHandlerAppend() {
     // Noop
   }
 
   private static IOException ensureIOException(final Throwable t) {
     return (t instanceof IOException)? (IOException)t: new IOException(t);
   }
 
   private static void usage() {
     System.err.println("Usage: FSHLog <ARGS>");
     System.err.println("Arguments:");
     System.err.println(" --dump  Dump textual representation of passed one or more files");
     System.err.println("         For example: " +
       "FSHLog --dump hdfs://example.com:9000/hbase/.logs/MACHINE/LOGFILE");
     System.err.println(" --split Split the passed directory of WAL logs");
     System.err.println("         For example: " +
       "FSHLog --split hdfs://example.com:9000/hbase/.logs/DIR");
   }
 
   /**
    * Pass one or more log file names and it will either dump out a text version
    * on <code>stdout</code> or split the specified log files.
    *
    * @param args
    * @throws IOException
    */
   public static void main(String[] args) throws IOException {
     if (args.length < 2) {
       usage();
       System.exit(-1);
     }
     // either dump using the WALPrettyPrinter or split, depending on args
     if (args[0].compareTo("--dump") == 0) {
       WALPrettyPrinter.run(Arrays.copyOfRange(args, 1, args.length));
     } else if (args[0].compareTo("--perf") == 0) {
       LOG.fatal("Please use the WALPerformanceEvaluation tool instead. i.e.:");
       LOG.fatal("\thbase org.apache.hadoop.hbase.wal.WALPerformanceEvaluation --iterations " +
           args[1]);
       System.exit(-1);
     } else if (args[0].compareTo("--split") == 0) {
       Configuration conf = HBaseConfiguration.create();
       for (int i = 1; i < args.length; i++) {
         try {
           Path logPath = new Path(args[i]);
           FSUtils.setFsDefault(conf, logPath);
           split(conf, logPath);
         } catch (IOException t) {
           t.printStackTrace(System.err);
           System.exit(-1);
         }
       }
     } else {
       usage();
       System.exit(-1);
     }
   }
 
   /**
    * This method gets the pipeline for the current WAL.
    */
   @VisibleForTesting
   DatanodeInfo[] getPipeLine() {
     if (this.hdfs_out != null) {
       if (this.hdfs_out.getWrappedStream() instanceof DFSOutputStream) {
         return ((DFSOutputStream) this.hdfs_out.getWrappedStream()).getPipeline();
       }
     }
     return new DatanodeInfo[0];
   }
 
   /**
    *
    * @return last time on checking low replication
    */
   public long getLastTimeCheckLowReplication() {
     return this.lastTimeCheckLowReplication;
   }
 }