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https://issues.apache.org/jira/browse/LUCENE-4639?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13537383#comment-13537383
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Shai Erera commented on LUCENE-4639:
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Dawid, I've decided not to change the "magic formula". For two reasons:
# That's not the reason I opened this issue - all I want is to be able to run
the same test w/ same seed on Windows and not get Access is denied. And in
light of all the recent comments, I think it's better if the rest of the logic
stays as-is.
# I think that the magic formula tries to reduce the chance for collision by
using the higher bits of the integer (div by 64K is like >>16). Not sure how
different it is for an int, as it's only 4 bytes, so after the division you're
still left w/ the same range of values as if you did & 0xFFFF. Maybe the code
originally pulled a long? Hmm ... but I think that won't matter either.
Anyway, if we want to change any of this randomness / magic formulas, maybe we
should just set counter statically to System.currentTimeMillis() or
System.nanoTime() & 0xFFFF. But I'm too tired to think about this now. For now,
I just want to allow rerunning tests on Windows.
I'll attach a patch shortly.
> Improving _TestUtil.getTempDir
> ------------------------------
>
> Key: LUCENE-4639
> URL: https://issues.apache.org/jira/browse/LUCENE-4639
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Shai Erera
> Assignee: Shai Erera
> Priority: Minor
> Attachments: LUCENE-4639.patch, LUCENE-4639.patch, LUCENE-4639.patch
>
>
> Spinoff from here:
> http://lucene.472066.n3.nabble.com/TestUtil-getTempFile-may-fail-on-quot-Access-Denied-quot-td4028048.html.
> _TestUtil.getTempDir uses createTempFile and then deletes the file. While
> this usually works, if someone runs tests by multiple JVMs and does not
> ensure each JVM gets an isolated temp.dir to work in, that my result in two
> JVMs sharing the same directory.
> Also, on Windows, if you call getTempDir on an existing directory, you get an
> "Access is denied" exception.
> Dawid proposed a simple solution to just call mkdirs() continuously until
> success. I'd like to try that.
> Also, I think that genTempFile could use some house cleaning, e.g.:
> * tempFileLocker can be just an Object instance? Why do we need a class?
> * If we initialize counter and counterBase in a static clause, we can avoid
> checking if counter==0 as well as passing Random to genTempFile (that will
> remove any suspicion that it does anything randomly)
> ** Also, instead of synchronizing on tempFileLocker, can we just use
> AtomicInteger for the counter?
> I'll modify getTempDir first. It documents "does not create the directory", I
> want to make sure no test fails due that.
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