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https://issues.apache.org/jira/browse/HBASE-8340?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13632039#comment-13632039
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Jean-Daniel Cryans commented on HBASE-8340:
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The way I read it, emptyCompressionContext is set to false only when we were
able to next() all the way to the seek position.
> WAL 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(...):
> {code}
> if (compressionContext != null && emptyCompressionContext) {
> emptyCompressionContext = false;
> }
> return ...
> {code}
>
> In seek()
> {code}
> if (compressionContext != null && emptyCompressionContext) {
> while (next() != null) {
> if (getPosition() == pos) {
> emptyCompressionContext = false;
> break;
> }
> }
> ...
> reader.seek(pos);
> {code}
> 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 removed.
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