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Hadoop is the distributed computing framework of Apache; hadoop-core contains the filesystem, job tracker and map/reduce modules
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/**
 * 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.fs;

import java.io.IOException;
import java.io.InputStream;
import java.util.zip.Checksum;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.util.StringUtils;

/**
 * This is a generic input stream for verifying checksums for
 * data before it is read by a user.
 */

abstract public class FSInputChecker extends FSInputStream {
  public static final Log LOG 
  = LogFactory.getLog(FSInputChecker.class);
  
  /** The file name from which data is read from */
  protected Path file;
  private Checksum sum;
  private boolean verifyChecksum = true;
  private byte[] buf;
  private byte[] checksum;
  private int pos;
  private int count;
  
  private int numOfRetries;
  
  // cached file position
  private long chunkPos = 0;
  
  /** Constructor
   * 
   * @param file The name of the file to be read
   * @param numOfRetries Number of read retries when ChecksumError occurs
   */
  protected FSInputChecker( Path file, int numOfRetries) {
    this.file = file;
    this.numOfRetries = numOfRetries;
  }
  
  /** Constructor
   * 
   * @param file The name of the file to be read
   * @param numOfRetries Number of read retries when ChecksumError occurs
   * @param sum the type of Checksum engine
   * @param chunkSize maximun chunk size
   * @param checksumSize the number byte of each checksum
   */
  protected FSInputChecker( Path file, int numOfRetries, 
      boolean verifyChecksum, Checksum sum, int chunkSize, int checksumSize ) {
    this(file, numOfRetries);
    set(verifyChecksum, sum, chunkSize, checksumSize);
  }
  
  /** Reads in next checksum chunk data into buf at offset
   * and checksum into checksum.
   * The method is used for implementing read, therefore, it should be optimized
   * for sequential reading
   * @param pos chunkPos
   * @param buf desitination buffer
   * @param offset offset in buf at which to store data
   * @param len maximun number of bytes to read
   * @return number of bytes read
   */
  abstract protected int readChunk(long pos, byte[] buf, int offset, int len,
      byte[] checksum) throws IOException;

  /** Return position of beginning of chunk containing pos. 
   *
   * @param pos a postion in the file
   * @return the starting position of the chunk which contains the byte
   */
  abstract protected long getChunkPosition(long pos);

  /** Return true if there is a need for checksum verification */
  protected synchronized boolean needChecksum() {
    return verifyChecksum && sum != null;
  }
  
  /**
   * Read one checksum-verified byte
   * 
   * @return     the next byte of data, or -1 if the end of the
   *             stream is reached.
   * @exception  IOException  if an I/O error occurs.
   */

  public synchronized int read() throws IOException {
    if (pos >= count) {
      fill();
      if (pos >= count) {
        return -1;
      }
    }
    return buf[pos++] & 0xff;
  }
  
  /**
   * Read checksum verified bytes from this byte-input stream into 
   * the specified byte array, starting at the given offset.
   *
   *  This method implements the general contract of the corresponding
   * {@link InputStream#read(byte[], int, int) read} method of
   * the {@link InputStream} class.  As an additional
   * convenience, it attempts to read as many bytes as possible by repeatedly
   * invoking the read method of the underlying stream.  This
   * iterated read continues until one of the following
   * conditions becomes true: 

   *
   *    The specified number of bytes have been read,
   *
   *   
 The read method of the underlying stream returns
   *   -1, indicating end-of-file.
   *
   * 
 If the first read on the underlying stream returns
   * -1 to indicate end-of-file then this method returns
   * -1.  Otherwise this method returns the number of bytes
   * actually read.
   *
   * @param      b     destination buffer.
   * @param      off   offset at which to start storing bytes.
   * @param      len   maximum number of bytes to read.
   * @return     the number of bytes read, or -1 if the end of
   *             the stream has been reached.
   * @exception  IOException  if an I/O error occurs.
   *             ChecksumException if any checksum error occurs
   */
  public synchronized int read(byte[] b, int off, int len) throws IOException {
    // parameter check
    if ((off | len | (off + len) | (b.length - (off + len))) < 0) {
      throw new IndexOutOfBoundsException();
    } else if (len == 0) {
      return 0;
    }

    int n = 0;
    for (;;) {
      int nread = read1(b, off + n, len - n);
      if (nread <= 0) 
        return (n == 0) ? nread : n;
      n += nread;
      if (n >= len)
        return n;
    }
  }
  
  /**
   * Fills the buffer with a chunk data. 
   * No mark is supported.
   * This method assumes that all data in the buffer has already been read in,
   * hence pos > count.
   */
  private void fill(  ) throws IOException {
    assert(pos>=count);
    // fill internal buffer
    count = readChecksumChunk(buf, 0, buf.length);
  }
  
  /*
   * Read characters into a portion of an array, reading from the underlying
   * stream at most once if necessary.
   */
  private int read1(byte b[], int off, int len)
  throws IOException {
    int avail = count-pos;
    if( avail <= 0 ) {
      if(len>=buf.length) {
        // read a chunk to user buffer directly; avoid one copy
        int nread = readChecksumChunk(b, off, len);
        return nread;
      } else {
        // read a chunk into the local buffer
        fill();
        if( count <= 0 ) {
          return -1;
        } else {
          avail = count;
        }
      }
    }
    
    // copy content of the local buffer to the user buffer
    int cnt = (avail < len) ? avail : len;
    System.arraycopy(buf, pos, b, off, cnt);
    pos += cnt;
    return cnt;    
  }
  
  /* Read up one checksum chunk to array b at pos off
   * It requires a checksum chunk boundary
   * in between  
   * and it stops reading at the boundary or at the end of the stream;
   * Otherwise an IllegalArgumentException is thrown.
   * This makes sure that all data read are checksum verified.
   * 
   * @param b   the buffer into which the data is read.
   * @param off the start offset in array b
   *            at which the data is written.
   * @param len the maximum number of bytes to read.
   * @return    the total number of bytes read into the buffer, or
   *            -1 if there is no more data because the end of
   *            the stream has been reached.
   * @throws IOException if an I/O error occurs.
   */ 
  private int readChecksumChunk(byte b[], int off, int len)
  throws IOException {
    // invalidate buffer
    count = pos = 0;
          
    int read = 0;
    boolean retry = true;
    int retriesLeft = numOfRetries; 
    do {
      retriesLeft--;

      try {
        read = readChunk(chunkPos, b, off, len, checksum);
        if( read > 0 ) {
          if( needChecksum() ) {
            sum.update(b, off, read);
            verifySum(chunkPos);
          }
          chunkPos += read;
        } 
        retry = false;
      } catch (ChecksumException ce) {
          LOG.info("Found checksum error: b[" + off + ", " + (off+read) + "]="
              + StringUtils.byteToHexString(b, off, off + read), ce);
          if (retriesLeft == 0) {
            throw ce;
          }
          
          // try a new replica
          if (seekToNewSource(chunkPos)) {
            // Since at least one of the sources is different, 
            // the read might succeed, so we'll retry.
            seek(chunkPos);
          } else {
            // Neither the data stream nor the checksum stream are being read
            // from different sources, meaning we'll still get a checksum error 
            // if we try to do the read again.  We throw an exception instead.
            throw ce;
          }
        }
    } while (retry);
    return read;
  }
  
  /* verify checksum for the chunk.
   * @throws ChecksumException if there is a mismatch
   */
  private void verifySum(long errPos) throws ChecksumException {
    long crc = getChecksum();
    long sumValue = sum.getValue();
    sum.reset();
    if (crc != sumValue) {
      throw new ChecksumException(
          "Checksum error: "+file+" at "+errPos, errPos);
    }
  }
  
  /* calculate checksum value */
  private long getChecksum() {
    return checksum2long(checksum);
  }

  /** Convert a checksum byte array to a long */
  static public long checksum2long(byte[] checksum) {
    long crc = 0L;
    for(int i=0; in bytes of data from the
   * input stream.
   *
   * This method may skip more bytes than are remaining in the backing
   * file. This produces no exception and the number of bytes skipped
   * may include some number of bytes that were beyond the EOF of the
   * backing file. Attempting to read from the stream after skipping past
   * the end will result in -1 indicating the end of the file.
   *
   *
If n is negative, no bytes are skipped.
   *
   * @param      n   the number of bytes to be skipped.
   * @return     the actual number of bytes skipped.
   * @exception  IOException  if an I/O error occurs.
   *             ChecksumException if the chunk to skip to is corrupted
   */
  public synchronized long skip(long n) throws IOException {
    if (n <= 0) {
      return 0;
    }

    seek(getPos()+n);
    return n;
  }

  /**
   * Seek to the given position in the stream.
   * The next read() will be from that position.
   * 
   * 
This method may seek past the end of the file.
   * This produces no exception and an attempt to read from
   * the stream will result in -1 indicating the end of the file.
   *
   * @param      pos   the postion to seek to.
   * @exception  IOException  if an I/O error occurs.
   *             ChecksumException if the chunk to seek to is corrupted
   */

  public synchronized void seek(long pos) throws IOException {
    if( pos<0 ) {
      return;
    }
    // optimize: check if the pos is in the buffer
    long start = chunkPos - this.count;
    if( pos>=start && pos 0) {
      readFully(this, new byte[delta], 0, delta);
    }
  }

  /**
   * A utility function that tries to read up to len bytes from
   * stm
   * 
   * @param stm    an input stream
   * @param buf    destiniation buffer
   * @param offset offset at which to store data
   * @param len    number of bytes to read
   * @return actual number of bytes read
   * @throws IOException if there is any IO error
   */
  protected static int readFully(InputStream stm, 
      byte[] buf, int offset, int len) throws IOException {
    int n = 0;
    for (;;) {
      int nread = stm.read(buf, offset + n, len - n);
      if (nread <= 0) 
        return (n == 0) ? nread : n;
      n += nread;
      if (n >= len)
        return n;
    }
  }
  
  /**
   * Set the checksum related parameters
   * @param verifyChecksum whether to verify checksum
   * @param sum which type of checksum to use
   * @param maxChunkSize maximun chunk size
   * @param checksumSize checksum size
   */
  final protected synchronized void set(boolean verifyChecksum,
      Checksum sum, int maxChunkSize, int checksumSize ) {
    this.verifyChecksum = verifyChecksum;
    this.sum = sum;
    this.buf = new byte[maxChunkSize];
    this.checksum = new byte[checksumSize];
    this.count = 0;
    this.pos = 0;
  }

  final public boolean markSupported() {
    return false;
  }
  
  final public void mark(int readlimit) {
  }
  
  final public void reset() throws IOException {
    throw new IOException("mark/reset not supported");
  }
  

  /* reset this FSInputChecker's state */
  private void resetState() {
    // invalidate buffer
    count = 0;
    pos = 0;
    // reset Checksum
    if (sum != null) {
      sum.reset();
    }
  }
}