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From "Hong Tang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6834) TFile.append compares initial key against null lastKey
Date Tue, 22 Jun 2010 20:50:54 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6834?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12881352#action_12881352
] 

Hong Tang commented on HADOOP-6834:
-----------------------------------

In the first append, lastLen is 0, lastKey refers to a valid byte array whose bytes are all
zero. So the behavior is determined. However, I do agree that it is possible for a customary
comparator, an zero-length byte array may not be a legal key type. So it would be nice to
do a first-key check.

Ahad, would you be interested in providing a patch, or perhaps share with us a sample program
with customized comparator that fails TFile.Writer.append?

> TFile.append compares initial key against null lastKey  
> --------------------------------------------------------
>
>                 Key: HADOOP-6834
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6834
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: io
>    Affects Versions: 0.20.1, 0.20.2
>            Reporter: Ahad Rana
>
> The following code in TFile.KeyReigster.close: 
>             byte[] lastKey = lastKeyBufferOS.getBuffer();
>             int lastLen = lastKeyBufferOS.size();
>             if (tfileMeta.getComparator().compare(key, 0, len, lastKey, 0,
>                 lastLen) < 0) {
>               throw new IOException("Keys are not added in sorted order");
>             }
> compares the initial  key (passed in via  TFile.Writer.append) against a technically
NULL lastKey. lastKey is not initialized until after the first call to TFile.Writer.append.
The underlying RawComparator interface used for comparisons does not stipulate the proper
behavior when either length 1  or length 2 is zero. In the case of LongWritable, its WritableComparator
implementation does an unsafe read on the passed in byte arrays b1 and b2. Since TFile pre-allocates
the buffer used for storing lastKey, this passes a valid buffer with zero count to LongWritable's
comparator, which ignores length and thus produces incorrect results. 

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