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From "zhihai xu (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MAPREDUCE-6481) LineRecordReader may give incomplete record and wrong position/key information for uncompressed input sometimes.
Date Thu, 17 Sep 2015 05:52:45 GMT
zhihai xu created MAPREDUCE-6481:
------------------------------------

             Summary: LineRecordReader may give incomplete record and wrong position/key information
for uncompressed input sometimes.
                 Key: MAPREDUCE-6481
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6481
             Project: Hadoop Map/Reduce
          Issue Type: Bug
          Components: mrv2
    Affects Versions: 2.7.0
            Reporter: zhihai xu
            Assignee: zhihai xu
            Priority: Critical


LineRecordReader may give incomplete record and wrong position/key information for uncompressed
input sometimes.
There are two issues:
# LineRecordReader may give incomplete record: some characters cut off at the end of record.
# LineRecordReader may give wrong position/key information.
The first issue only happens for Custom Delimiter, which is caused by the following code at
{{LineReader#readCustomLine}}:
{code}
    if (appendLength > 0) {
        if (ambiguousByteCount > 0) {
          str.append(recordDelimiterBytes, 0, ambiguousByteCount);
          //appending the ambiguous characters (refer case 2.2)
          bytesConsumed += ambiguousByteCount;
          ambiguousByteCount=0;
        }
        str.append(buffer, startPosn, appendLength);
        txtLength += appendLength;
      }
{code}
If {{appendLength}} is 0 and {{ambiguousByteCount}} is not 0, this bug will be triggered.
For example, input is "123456789aab", Custom Delimiter is "ab", bufferSize is 10 and splitLength
is 12, the correct record should be "123456789a" with length 10, but we get incomplete record
"123456789" with length 9 from current code.

The second issue can happen for both Custom Delimiter and Default Delimiter, which is caused
by the code in {{UncompressedSplitLineReader#readLine}}.
{{UncompressedSplitLineReader#readLine}} may report wrong size information at some corner
cases. The reason is {{unusedBytes}} in the following code:
{code}
bytesRead += unusedBytes;
unusedBytes = bufferSize - getBufferPosn();
bytesRead -= unusedBytes;
{code}
If the last bytes read (bufferLength) is less than bufferSize, the previous {{unusedBytes}}
will be wrong, which should be {{bufferLength}} - {{bufferPosn}} instead of bufferSize - {{bufferPosn}}.
It will return larger value.
For example, input is "1234567890ab12ab345", Custom Delimiter is "ab", bufferSize is 10 and
two splits:first splitLength is 15 and second splitLength 4:
the current code will give the following result:
First record: Key:0 Value:"1234567890"
Second record: Key:12 Value:"12"
Third Record: Key:21 Value:"345"
You can see the Key for the third record is wrong, it should be 16 instead of 21. It is due
to wrong {{unusedBytes}}. {{fillBuffer}} read 10 bytes for the first time, for the second
times, it only read 5 bytes, which is 5 bytes less than the bufferSize. That is why the key
we get is 5 bytes larger than the correct one.



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