Subject: Re: [PATCH 2/3] t7703: demonstrate object corruption with pack.packSizeLimit

From: Junio C Hamano <[email-masked]>
Date: Fri, 20 May 2022 13:54:36 -0700

Taylor Blau <[email-masked]> writes:

> When this happens in just the right circumstances, it is possible to
> remove a pack that we just wrote, leading to object corruption.
>
> Luckily, this is quite difficult to provoke in practice (for a couple of
> reasons):

I'd call it unlucky that it is hard to trigger, actually ;-) With a
system like Git with more than a few handful of users, even an issue
that is hard-to-trigger is bound to hit somebody every day, but it
it is hard to trigger without the right condition, it is hard to
debug.

Thanks for finding and fixing (presumably we need a call to keep the
list sorted at the right places?)

>   - we ordinarily write just one pack, so `names` usually contains just
>     one entry, and is thus sorted
>   - when we do write more than one pack (e.g., due to `--max-pack-size`)
>     we have to: (a) write a pack identical to one that already
>     exists, (b) have that pack be below the split line, and (c) have
>     the set of packs written by `pack-objects` occur in an order which
>     tricks `string_list_has_string()`.
>
> Demonstrate the above scenario in a failing test, which causes `git
> repack --geometric` to write a pack which occurs below the split line,
> _and_ fail to recognize that it wrote that pack.
>
> The following patch will fix this bug.
>
> Signed-off-by: Taylor Blau <[email-masked]>
> ---
>  t/t7703-repack-geometric.sh | 47 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 47 insertions(+)
>
> diff --git a/t/t7703-repack-geometric.sh b/t/t7703-repack-geometric.sh
> index 91bb2b37a8..2cd1de7295 100755
> --- a/t/t7703-repack-geometric.sh
> +++ b/t/t7703-repack-geometric.sh
> @@ -7,6 +7,7 @@ test_description='git repack --geometric works correctly'
>  GIT_TEST_MULTI_PACK_INDEX=0
>  
>  objdir=.git/objects
> +packdir=$objdir/pack
>  midx=$objdir/pack/multi-pack-index
>  
>  test_expect_success '--geometric with no packs' '
> @@ -230,4 +231,50 @@ test_expect_success '--geometric chooses largest MIDX preferred pack' '
>  	)
>  '
>  
> +test_expect_failure '--geometric with pack.packSizeLimit' '
> +	git init pack-rewrite &&
> +	test_when_finished "rm -fr pack-rewrite" &&
> +	(
> +		cd pack-rewrite &&
> +
> +		test-tool genrandom foo 1048576 >foo &&
> +		test-tool genrandom bar 1048576 >bar &&
> +
> +		git add foo bar &&
> +		test_tick &&
> +		git commit -m base &&
> +
> +		git rev-parse HEAD:foo HEAD:bar >p1.objects &&
> +		git rev-parse HEAD HEAD^{tree} >p2.objects &&
> +
> +		# These two packs each contain two objects, so the following
> +		# `--geometric` repack will try to combine them.
> +		p1="$(git pack-objects $packdir/pack <p1.objects)" &&
> +		p2="$(git pack-objects $packdir/pack <p2.objects)" &&
> +
> +		# Remove any loose objects in packs, since we do not want extra
> +		# copies around (which would mask over potential object
> +		# corruption issues).
> +		git prune-packed &&
> +
> +		# Both p1 and p2 will be rolled up, but pack-objects will write
> +		# three packs:
> +		#
> +		#   - one containing object "foo",
> +		#   - another containing object "bar",
> +		#   - a final pack containing the commit and tree objects
> +		#     (identical to p2 above)
> +		git repack --geometric 2 -d --max-pack-size=1048576 &&
> +
> +		# Ensure `repack` can detect that the third pack it wrote
> +		# (containing just the tree and commit objects) was identical to
> +		# one that was below the geometric split, so that we can save it
> +		# from deletion.
> +		#
> +		# If `repack` fails to do that, we will incorrectly delete p2,
> +		# causing object corruption.
> +		git fsck
> +	)
> +'
> +
>  test_done