zhihai xu created MAPREDUCE-6481:
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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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