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https://issues.apache.org/jira/browse/HBASE-8340?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13631991#comment-13631991
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Sergey Shelukhin commented on HBASE-8340:
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bq. I'm not sure what you mean here, calling next() just once we only populate
the dict with whatever entries were needed to write the first WAL entry, and
then you miss all the other entries.
The current code sets the boolean flag after one next, though, that's what I
meant
> 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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