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From: Jeff King <peff@peff.net>
To: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Cc: Stefan Beller <sbeller@google.com>,
	git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH 2/4] t5310: test delta reuse with bitmaps
Date: Sat, 1 Sep 2018 04:03:16 -0400	[thread overview]
Message-ID: <20180901080316.GA25852@sigill.intra.peff.net> (raw)
In-Reply-To: <20180901074813.GB25461@sigill.intra.peff.net>

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

  reply	other threads:[~2018-09-01  8:03 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-21 18:41 [PATCH] test-tool.h: include git-compat-util.h Jeff King
2018-08-21 19:03 ` Junio C Hamano
2018-08-21 19:06 ` [PATCH 1/6] t/perf: factor boilerplate out of test_perf Jeff King
2018-08-21 19:06 ` [PATCH 2/6] t/perf: factor out percent calculations Jeff King
2018-08-21 19:06 ` [PATCH 3/6] t/perf: add infrastructure for measuring sizes Jeff King
2018-08-22 13:40   ` Derrick Stolee
2018-08-22 15:31     ` Jeff King
2018-08-21 19:06 ` [PATCH 4/6] t/perf: add perf tests for fetches from a bitmapped server Jeff King
2018-08-21 19:07 ` [PATCH 5/6] pack-bitmap: save "have" bitmap from walk Jeff King
2018-08-21 19:47   ` Derrick Stolee
2018-08-21 19:54     ` Jeff King
2018-08-31 15:23   ` Ævar Arnfjörð Bjarmason
2018-08-31 22:55     ` Jeff King
2018-09-01  7:41       ` [PATCH 0/4] un-breaking pack-objects with bitmaps Jeff King
2018-09-01  7:44         ` [PATCH 1/4] bitmap_has_sha1_in_uninteresting(): drop BUG check Jeff King
2018-09-01  7:48         ` [PATCH 2/4] t5310: test delta reuse with bitmaps Jeff King
2018-09-01  8:03           ` Jeff King [this message]
2018-09-01 20:29             ` Ævar Arnfjörð Bjarmason
2018-09-01 22:46               ` Ben Peart
2018-09-02  5:51               ` Jeff King
2018-09-04 19:05             ` Stefan Beller
2018-09-04 19:45               ` Junio C Hamano
2018-09-04 20:02               ` Jeff King
2018-09-01  7:49         ` [PATCH 3/4] traverse_bitmap_commit_list(): don't free result Jeff King
2018-09-01  7:50         ` [PATCH 4/4] pack-bitmap: drop "loaded" flag Jeff King
2018-09-04 19:30         ` [PATCH 0/4] un-breaking pack-objects with bitmaps Stefan Beller
2018-09-04 20:03           ` Jeff King
2018-09-08  6:43         ` Ævar Arnfjörð Bjarmason
2018-09-10 16:53           ` Junio C Hamano
2018-09-10 18:48             ` Jeff King
2018-09-10 19:23               ` Junio C Hamano
2018-08-21 19:07 ` [PATCH 6/6] pack-objects: reuse on-disk deltas for thin "have" objects Jeff King
2018-08-21 19:43   ` Junio C Hamano
2018-08-21 19:50     ` Junio C Hamano
2018-08-21 20:07       ` Jeff King
2018-08-21 20:14         ` Jeff King
2018-08-21 20:52           ` Junio C Hamano
2018-08-21 21:30             ` Jeff King
2018-08-21 20:57         ` Junio C Hamano
2018-08-21 21:32           ` Jeff King
2018-08-23  0:43           ` [PATCH 0/9] trailer-parsing false positives Jeff King
2018-08-23  0:44             ` [PATCH 1/9] trailer: use size_t for string offsets Jeff King
2018-08-23  0:45             ` [PATCH 2/9] trailer: use size_t for iterating trailer list Jeff King
2018-08-23  0:46             ` [PATCH 3/9] trailer: pass process_trailer_opts to trailer_info_get() Jeff King
2018-08-23  0:48             ` [PATCH 4/9] interpret-trailers: tighten check for "---" patch boundary Jeff King
2018-08-23  0:49             ` [PATCH 5/9] interpret-trailers: allow suppressing "---" divider Jeff King
2018-08-23  0:50             ` [PATCH 6/9] pretty, ref-filter: format %(trailers) with no_divider option Jeff King
2018-08-23  0:50             ` [PATCH 7/9] sequencer: ignore "---" divider when parsing trailers Jeff King
2018-08-23  0:50             ` [PATCH 8/9] append_signoff: use size_t for string offsets Jeff King
2018-08-23  0:51             ` [PATCH 9/9] sequencer: handle ignore_footer when parsing trailers Jeff King
2018-08-23 18:30             ` [PATCH 0/9] trailer-parsing false positives Junio C Hamano
2018-08-24  7:26               ` Jeff King
2018-08-21 20:00     ` [PATCH 6/6] pack-objects: reuse on-disk deltas for thin "have" objects Jeff King

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