ArrayIndexOutOfBoundsException when adding histograms and iterating about hdrhistogram HOT 6 CLOSED

I should have said it seems to happen inside AbstractHistogram#add, as I can't make it happen during debug, but I was able to "restrict" the area to some application code that loops over a concurrentskiplistmap tail view and does the AbstractHistogram#add.

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After reviewing the AbstractHistogram#add code it was clear that the AIOOBE could have not come from there, and the source is indeed AbstractHistogramIterator#next, called a few lines after when computing the mean of the aggregated histogram.

So, I was able to pin down the bug back into AbstractHistogram#add, which computes an higher total count in case of concurrently modified "from" histograms: pull request #25 should fix it.

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This is integrated into the recently pushed v2.0.1

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@giltene thanks! I'm going to upgrade to v2.0.2 to see if that helps

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Going back to the original way you use this stuff though, I think you may want to start using IntervalHistogramRecorder instead of doing potentially lossy adds on live atomic histograms. IntervalHistogramRecorder guarantees a stable view of the interval histogram, and avoids exposure to concurrent modifications.

With this feature now available, I'm considering whether throwing a concurrent modification exception is not the better way to go when such a copy creates an inconsistent totalCount. We can use the current (your) logic that establishes the totalCount during the loop to cross-check agains the other totalCount, and throw an exception when they don't match.

What do you think?

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Thanks for the heads up @giltene. I will look into IntervalHistogramRecorder, but I think you should keep the current exception-free behaviour: after all, the atomic histograms are the ones concurrently modified (and they are meant to be so), while the add to target histogram is single threaded, so a ConcurrentModificationException would be imho unexpected.

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