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Date: Tue, 10 Mar 2009 22:42:50 -0700 (PDT)
From: "Chris Douglas (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Created: (HADOOP-5459) CRC errors not detected reading
 intermediate output into memory with problematic length
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CRC errors not detected reading intermediate output into memory with problematic length
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                 Key: HADOOP-5459
                 URL: https://issues.apache.org/jira/browse/HADOOP-5459
             Project: Hadoop Core
          Issue Type: Bug
    Affects Versions: 0.20.0
            Reporter: Chris Douglas
            Priority: Blocker


It's possible that the expected, uncompressed length of the segment is less than the available/decompressed data. This can happen in some worst-cases for compression, but it is exceedingly rare. It is also possible (though also fantastically unlikely) for the data to deflate to a size greater than that reported by the map. CRC errors will remain undetected because IFileInputStream does not validate the checksum until the end of the stream, and close() does not advance the stream to the end of the segment. The (abbreviated) read loop fetching data in shuffleInMemory:

{code}
int n = input.read(shuffleData, 0, shuffleData.length);
while (n > 0) { 
  bytesRead += n;
  n = input.read(shuffleData, bytesRead, 
                 (shuffleData.length-bytesRead));
} 
{code}

Will read only up to the expected length. Without reading the whole segment, the checksum is not validated. Even if IFileInputStream instances are closed, they should always validate checksums.

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