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War story: the hardest bug I ever debugged

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462 points jakevoytko | 2 comments | 24 Mar 25 14:37 UTC | HN request time: 0s | source
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latexr ◴[27 Mar 25 09:53 UTC] No.43491859[source]▶
> I do it a few more times. It’s not always the 20th iteration, but it usually happens sometime between the 10th and 40th iteration. Sometimes it never happend. Okay, the bug is nondeterministic.

That’s an incorrect assumption. Just because your test case isn’t triggering the bug reliably, it does not mean the bug is nondeterministic.

That is like saying the “OpenOffice can’t print on Tuesdays” is non deterministic because you can’t reproduce it everyday. It is deterministic, you just need to find the right set of circumstances.

https://beza1e1.tuxen.de/lore/print_on_tuesday.html

From the writing it appears the author found one way to reproduce the bug sometimes and then relied on it for every test. Another approach would have been to tweak their test case until they found a situation which reproduced the bug more or less often, trying to find the threshold that causes it and continuing to deduce from there.

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artimaeis ◴[27 Mar 25 12:56 UTC] No.43493111[source]▶
If I understood correctly - the Math.Abs() value would be positive roughly half the time, regardless of the steps taken to get there. That seems definitively nondeterministic.
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1. latexr ◴[27 Mar 25 13:50 UTC] No.43493654[source]▶
You don’t call Math.abs() on its own, you need to give it a number. Regardless if it is positive or negative, it should always return a positive (that’s what an absolute value is). The issue here is that it was returning a negative number when given a negative value, which is wrong:

> We rerun the repro. We look at the logged value. Math.abs() is returning negative values for negative inputs. We reload and run it again. Math.abs() is returning negative values for negative inputs. We reload and run it again. Math.abs() is returning negative values for negative inputs.

Regardless, that is beside the point. I was not arguing either way if this was a deterministic bug or not, I was pointing out that the author’s conclusion does not follow from the premise. Even if the bug had turned out to be nondeterministic, they had not done the necessary steps to confidently make that assertion. There is a chasm of difference between “this bug is nondeterministic” and “I haven’t yet determined the conditions that reproduce this bug”.

replies(1): >>43493958 #
2. amne ◴[27 Mar 25 14:20 UTC] No.43493958[source]▶
With the appropiate butterfly wing flap everything is deterministic.

https://xkcd.com/378/