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From "Yang Yang (JIRA)" <j...@apache.org>
Subject [jira] Created: (MAPREDUCE-1487) io.DataInputBuffer.getLength() semantic wrong/confused
Date Fri, 12 Feb 2010 10:12:28 GMT
io.DataInputBuffer.getLength() semantic wrong/confused

                 Key: MAPREDUCE-1487
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1487
             Project: Hadoop Map/Reduce
          Issue Type: Bug
    Affects Versions: 0.20.1
         Environment: linux
            Reporter: Yang Yang

I was trying Google Protocol Buffer as a value type on hadoop,
then when I used it in reducer, the parser always failed.
while it worked fine on a plain inputstream reader or mapper.

the reason is that the reducer interface in Task.java gave a buffer larger than an actual
encoded record to the parser, and the parser does not stop until it reaches
the buffer end, so it parsed some  junk bytes.

the root cause is due to hadoop.io.DataInputBuffer.java :

in 0.20.1  DataInputBuffer.java  line 47:

    public void reset(byte[] input, int start, int length) {
      this.buf = input;
      this.count = start+length;
      this.mark = start;
      this.pos = start;

    public byte[] getData() { return buf; }
    public int getPosition() { return pos; }
    public int getLength() { return count; }

we see that the above logic seems to assume that "getLength()" returns the total ** capacity
***, not the actual content length, of the buffer, yet latter code
seems to assume the semantic that "length" is actual content length, i.e. end - start :

 /** Resets the data that the buffer reads. */
  public void reset(byte[] input, int start, int length) {
    buffer.reset(input, start, length);

i.e. if u call reset( getPosition(), getLength() ) on the same buffer again and again, the
"length" would be infinitely increased.

this confusion in semantic is reflected in  many places, at leat in IFile.java, and Task.java,
where it caused the original issue.
around line 980 of Task.java, we see
   valueIn.reset(nextValueBytes.getData(), nextValueBytes.getPosition(), nextValueBytes.getLength())
if the position above is not empty, the above actually sets a buffer too long, causing the
reported issue.

changing the Task.java as a hack , to 
      valueIn.reset(nextValueBytes.getData(), nextValueBytes.getPosition(), nextValueBytes.getLength()
- nextValueBytes.getPosition());

fixed the issue, but the semantic of DataInputBuffer should be fixed and streamlined

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