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From "Sergey Shelukhin (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-8340) WAL log compression handling of seeks seems to be either inefficient or incorrect
Date Fri, 12 Apr 2013 22:56:17 GMT
Sergey Shelukhin created HBASE-8340:

             Summary: WAL log compression handling of seeks seems to be either inefficient
or incorrect
                 Key: HBASE-8340
                 URL: https://issues.apache.org/jira/browse/HBASE-8340
             Project: HBase
          Issue Type: Bug
            Reporter: Sergey Shelukhin

In next(...):
    if (compressionContext != null && emptyCompressionContext) {
      emptyCompressionContext = false;
    return ...
In seek()
    if (compressionContext != null && emptyCompressionContext) {
      while (next() != null) {
        if (getPosition() == pos) {
          emptyCompressionContext = false;

So, seek will seek the file directly if either any next, or any seek, has been called before.

I am not sure what this code is for, but my best guess is that it is to populate the dictionary
for compression.
If it is so, it would seem that one next() call (or even one seek() call) would not be enough,
and seek must always use next(), otherwise it is incorrect.

If we assume that one next() is enough to be able to use reader.seek, as the current code
would seem to imply, then there's no need for the first seek to call next() in a loop - it
can call next once and then do reader.seek.

Note: even in case if all of this works fine because external usage creates the object and
does one seek before any next-s, and no seeks after (the only bug-free pattern currently possible
with both methods used if I'm not mistaken), then the code needs to be tightened and bug potential

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