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https://issues.apache.org/jira/browse/LUCENE-4639?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13537742#comment-13537742
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Dawid Weiss commented on LUCENE-4639:
-------------------------------------
I looked at that code in Harmony and I sustain my opinion that this code
fragment is irrelevant. They pick a per-vm seed and then a counter starting
from that. If you create enough files the counter will roll over to next digits
anyway so they add the 10000 for aesthetic reasons more than anything. More: if
the seeds for two or more vm's are picked close to each other they'll soon
enter a tight race to allocate new files. This isn't something you want to
reproduce, it's something you want to avoid.
The process of generating temporary file names should avoid conflicts with
other processes and with itself. A congruential random generator with a long
cycle should be more than fine to avoid such conflicts (the probability of
picking an identical seed in the master generator is unavoidable but very
tiny), I don't think there is any need to have a "vm-identification" string as
they do in Harmony.
> 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,
> 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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