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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: [PATCH 0/4] un-breaking pack-objects with bitmaps
Date: Sat, 1 Sep 2018 03:41:45 -0400	[thread overview]
Message-ID: <20180901074145.GA24023@sigill.intra.peff.net> (raw)
In-Reply-To: <20180831225558.GA22917@sigill.intra.peff.net>

On Fri, Aug 31, 2018 at 06:55:58PM -0400, Jeff King wrote:

> On Fri, Aug 31, 2018 at 05:23:17PM +0200, Ævar Arnfjörð Bjarmason wrote:
> 
> > On Tue, Aug 21 2018, Jeff King wrote:
> > 
> > > +int bitmap_has_sha1_in_uninteresting(struct bitmap_index *bitmap_git,
> > > +				     const unsigned char *sha1)
> > > +{
> > > +	int pos;
> > > +
> > > +	if (!bitmap_git)
> > > +		return 0; /* no bitmap loaded */
> > > +	if (!bitmap_git->result)
> > > +		BUG("failed to perform bitmap walk before querying");
> > 
> > Some part of what calls this completely breaks pushing from the "next"
> > branch when you have local bitmaps (we *really* should have some tests
> > for this...).
> 
> Yikes, thanks for reporting. I agree we need better tests here.

OK, here is the fix. Since the problem is in 'next', this is done as a
patch on top of jk/pack-delta-reuse-with-bitmap. But since we're set to
rewind 'next' post-release anyway, we could squash it directly into
30cdc33fba from the original series. That would help later bisections
from running into it, which may be worth it as it's a pretty severe
breakage.  Or maybe not:

  1. The test suite doesn't actually fail, because it's toy repos are
     too small.

  2. It only triggers in the real-world if you have bitmaps turned on,
     which are not the default.

So it may not be that likely in practice to bother a hypothetical future
bisecting developer.

> [1] Actually, there is also prepare_bitmap_git(), but it is not really
>     for general use by callers. It should be made static, or better yet,
>     I suspect it can be folded into its callers.

This actually turned out not to work. There's a caller over in
pack-bitmap-write.c, and it makes things worse to try to expand the
logic there. So it technically _is_ possible to have a bitmap_index
without a "have" field, but it also doesn't make sense to ask about
"uninteresting" objects there. You haven't done (and cannot do) a
traversal on such an object.

Which I think goes back to Stefan's original question: is this just a
crappy API? And the answer is "yes, to some degree". There are really
two uses of bitmaps:

  - you want to do a traverse_commit_list() walk, but faster

  - you want to selectively query the on-disk bitmaps (e.g., you are
    walking for --contains and want to ask "do we have a bitmap for this
    object?"

Those currently use the same struct bitmap_index, but with two different
constructors (prepare_bitmap_git and prepare_bitmap_walk). It probably
ought to be two different ones (with the "walk" variant using the
"query" variant under the hood). I've punted on that full conversion for
now, but did clean up a few confusing bits.

  [1/4]: bitmap_has_sha1_in_uninteresting(): drop BUG check

    The actual fix. This should get merged to next ASAP (or the original
    topic just reverted).

  [2/4]: t5310: test delta reuse with bitmaps

    I did this separately to give us flexibility to squash or merge
    quickly. But it does find Ævar's bug on a git without patch 1.

  [3/4]: traverse_bitmap_commit_list(): don't free result

    The original assert should have simply been useless, but it was the
    surprising behavior of this function that turned it into a bug.

  [4/4]: pack-bitmap: drop "loaded" flag

    And this is just an annoyance I ran into, which is a fallout from
    our conversion to using an allocated bitmap_index struct.

 pack-bitmap.c           | 14 ++-----
 pack-bitmap.h           |  2 +-
 t/t5310-pack-bitmaps.sh | 93 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 97 insertions(+), 12 deletions(-)

-Peff

  reply	other threads:[~2018-09-01  7:41 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       ` Jeff King [this message]
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
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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