/**
    * 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.io;
  
  import java.io.IOException;
  
  import org.apache.hadoop.fs.FSDataInputStream;
  import org.apache.hadoop.fs.FileSystem;
  import org.apache.hadoop.fs.Path;
  import org.apache.hadoop.hbase.fs.HFileSystem;
  
  import com.google.common.annotations.VisibleForTesting;
  
  /**
   * Wrapper for input stream(s) that takes care of the interaction of FS and HBase checksums,
   * as well as closing streams. Initialization is not thread-safe, but normal operation is;
   * see method comments.
   */
  public class FSDataInputStreamWrapper {
    private final HFileSystem hfs;
    private final Path path;
    private final FileLink link;
    private final boolean doCloseStreams;
  
    /** Two stream handles, one with and one without FS-level checksum.
     * HDFS checksum setting is on FS level, not single read level, so you have to keep two
     * FS objects and two handles open to interleave different reads freely, which is very sad.
     * This is what we do:
     * 1) First, we need to read the trailer of HFile to determine checksum parameters.
     *  We always use FS checksum to do that, so ctor opens {@link #stream}.
     * 2.1) After that, if HBase checksum is not used, we'd just always use {@link #stream};
     * 2.2) If HBase checksum can be used, we'll open {@link #streamNoFsChecksum},
     *  and close {@link #stream}. User MUST call prepareForBlockReader for that to happen;
     *  if they don't, (2.1) will be the default.
     * 3) The users can call {@link #shouldUseHBaseChecksum()}, and pass its result to
     *  {@link #getStream(boolean)} to get stream (if Java had out/pointer params we could
     *  return both in one call). This stream is guaranteed to be set.
     * 4) The first time HBase checksum fails, one would call {@link #fallbackToFsChecksum(int)}.
     * That will take lock, and open {@link #stream}. While this is going on, others will
     * continue to use the old stream; if they also want to fall back, they'll also call
     * {@link #fallbackToFsChecksum(int)}, and block until {@link #stream} is set.
     * 5) After some number of checksumOk() calls, we will go back to using HBase checksum.
     * We will have 2 handles; however we presume checksums fail so rarely that we don't care.
     */
    private volatile FSDataInputStream stream = null;
    private volatile FSDataInputStream streamNoFsChecksum = null;
    private Object streamNoFsChecksumFirstCreateLock = new Object();
  
    // The configuration states that we should validate hbase checksums
    private boolean useHBaseChecksumConfigured;
  
    // Record the current state of this reader with respect to
    // validating checkums in HBase. This is originally set the same
    // value as useHBaseChecksumConfigured, but can change state as and when
    // we encounter checksum verification failures.
    private volatile boolean useHBaseChecksum;
  
    // In the case of a checksum failure, do these many succeeding
    // reads without hbase checksum verification.
    private volatile int hbaseChecksumOffCount = -1;
  
    public FSDataInputStreamWrapper(FileSystem fs, Path path) throws IOException {
      this(fs, null, path, false);
    }
  
    public FSDataInputStreamWrapper(FileSystem fs, Path path, boolean dropBehind) throws IOException {
      this(fs, null, path, dropBehind);
    }
  
    public FSDataInputStreamWrapper(FileSystem fs, FileLink link) throws IOException {
      this(fs, link, null, false);
    }
    public FSDataInputStreamWrapper(FileSystem fs, FileLink link,
                                    boolean dropBehind) throws IOException {
      this(fs, link, null, dropBehind);
    }
  
    private FSDataInputStreamWrapper(FileSystem fs, FileLink link,
                                     Path path, boolean dropBehind) throws IOException {
      assert (path == null) != (link == null);
      this.path = path;
      this.link = link;
      this.doCloseStreams = true;
      // If the fs is not an instance of HFileSystem, then create an instance of HFileSystem
     // that wraps over the specified fs. In this case, we will not be able to avoid
     // checksumming inside the filesystem.
     this.hfs = (fs instanceof HFileSystem) ? (HFileSystem)fs : new HFileSystem(fs);
 
     // Initially we are going to read the tail block. Open the reader w/FS checksum.
     this.useHBaseChecksumConfigured = this.useHBaseChecksum = false;
     this.stream = (link != null) ? link.open(hfs) : hfs.open(path);
     try {
       this.stream.setDropBehind(dropBehind);
     } catch (Exception e) {
       // Skipped.
     }
   }
 
 
   /**
    * Prepares the streams for block reader. NOT THREAD SAFE. Must be called once, after any
    * reads finish and before any other reads start (what happens in reality is we read the
    * tail, then call this based on what's in the tail, then read blocks).
    * @param forceNoHBaseChecksum Force not using HBase checksum.
    */
   public void prepareForBlockReader(boolean forceNoHBaseChecksum) throws IOException {
     if (hfs == null) return;
     assert this.stream != null && !this.useHBaseChecksumConfigured;
     boolean useHBaseChecksum =
         !forceNoHBaseChecksum && hfs.useHBaseChecksum() && (hfs.getNoChecksumFs() != hfs);
 
     if (useHBaseChecksum) {
       FileSystem fsNc = hfs.getNoChecksumFs();
       this.streamNoFsChecksum = (link != null) ? link.open(fsNc) : fsNc.open(path);
       this.useHBaseChecksumConfigured = this.useHBaseChecksum = useHBaseChecksum;
       // Close the checksum stream; we will reopen it if we get an HBase checksum failure.
       this.stream.close();
       this.stream = null;
     }
   }
 
   /** For use in tests. */
   @VisibleForTesting
   public FSDataInputStreamWrapper(FSDataInputStream fsdis) {
     this(fsdis, fsdis);
   }
 
   /** For use in tests. */
   @VisibleForTesting
   public FSDataInputStreamWrapper(FSDataInputStream fsdis, FSDataInputStream noChecksum) {
     doCloseStreams = false;
     stream = fsdis;
     streamNoFsChecksum = noChecksum;
     path = null;
     link = null;
     hfs = null;
     useHBaseChecksumConfigured = useHBaseChecksum = false;
   }
 
   /**
    * @return Whether we are presently using HBase checksum.
    */
   public boolean shouldUseHBaseChecksum() {
     return this.useHBaseChecksum;
   }
 
   /**
    * Get the stream to use. Thread-safe.
    * @param useHBaseChecksum must be the value that shouldUseHBaseChecksum has returned
    *  at some point in the past, otherwise the result is undefined.
    */
   public FSDataInputStream getStream(boolean useHBaseChecksum) {
     return useHBaseChecksum ? this.streamNoFsChecksum : this.stream;
   }
 
   /**
    * Read from non-checksum stream failed, fall back to FS checksum. Thread-safe.
    * @param offCount For how many checksumOk calls to turn off the HBase checksum.
    */
   public FSDataInputStream fallbackToFsChecksum(int offCount) throws IOException {
     // checksumOffCount is speculative, but let's try to reset it less.
     boolean partOfConvoy = false;
     if (this.stream == null) {
       synchronized (streamNoFsChecksumFirstCreateLock) {
         partOfConvoy = (this.stream != null);
         if (!partOfConvoy) {
           this.stream = (link != null) ? link.open(hfs) : hfs.open(path);
         }
       }
     }
     if (!partOfConvoy) {
       this.useHBaseChecksum = false;
       this.hbaseChecksumOffCount = offCount;
     }
     return this.stream;
   }
 
   /** Report that checksum was ok, so we may ponder going back to HBase checksum. */
   public void checksumOk() {
     if (this.useHBaseChecksumConfigured && !this.useHBaseChecksum
         && (this.hbaseChecksumOffCount-- < 0)) {
       // The stream we need is already open (because we were using HBase checksum in the past).
       assert this.streamNoFsChecksum != null;
       this.useHBaseChecksum = true;
     }
   }
 
   /** Close stream(s) if necessary. */
   public void close() throws IOException {
     if (!doCloseStreams) return;
     try {
       if (stream != streamNoFsChecksum && streamNoFsChecksum != null) {
         streamNoFsChecksum.close();
         streamNoFsChecksum = null;
       }
     } finally {
       if (stream != null) {
         stream.close();
         stream = null;
       }
     }
   }
 
   public HFileSystem getHfs() {
     return this.hfs;
   }
 }