On Sat, Sep 01, 2018 at 03:48:13AM -0400, Jeff King wrote: > Commit 6a1e32d532 (pack-objects: reuse on-disk deltas for > thin "have" objects, 2018-08-21) taught pack-objects a new > optimization trick. Since this wasn't meant to change > user-visible behavior, but only produce smaller packs more > quickly, testing focused on t/perf/p5311. > > However, since people don't run perf tests very often, we > should make sure that the feature is exercised in the > regular test suite. This patch does so.
This, by the way, is the crux of how such an obvious and severe bug made it to 'next'. The original series was tested quite extensively via t/perf and in production at GitHub. When I re-rolled v2, the only change was the addition of the assertion, so I didn't bother re-doing the perf tests, since they're slow and there wouldn't be a measurable impact. I did run the normal test suite (as I'm sure Junio did, too) as a double-check for correctness, but as we noticed, the code wasn't actually exercised there. Nor had I yet backported the revised series to the version we run at GitHub, so it hadn't been run there, either. And all of that coupled with the fact that it only triggers with bitmaps, so day-to-day use of the buggy Git (like Junio trying to push out the result ;) ) wouldn't show it. Anyway. Not that exciting, and kind of obviously dumb in retrospect. But I think it was worth analyzing to see what went wrong. If there's an immediate lesson, it is probably: add tests even for changes that aren't really user-visible to make sure the code is exercised. There may be a larger lesson about tracking code coverage, but I don't know that most general code coverage tools would have helped (any overall percentage number would be too large to move). A tool that looked at the diff and said "of the N lines you added/touched, this percent is exercised in the test suite" might have been useful. -Peff
