t7703: demonstrate object corruption with pack.packSizeLimit

When doing a `--geometric=<d>` repack, `git repack` determines a
splitting point among packs ordered by their object count such that:

  - each pack above the split has at least `<d>` times as many objects
    as the next-largest pack by object count, and
  - the first pack above the split has at least `<d>` times as many
    object as the sum of all packs below the split line combined

`git repack` then creates a pack containing all of the objects contained
in packs below the split line by running `git pack-objects
--stdin-packs` underneath. Once packs are moved into place, then any
packs below the split line are removed, since their objects were just
combined into a new pack.

But `git repack` tries to be careful to avoid removing a pack that it
just wrote, by checking:

    struct packed_git *p = geometry->pack[i];
    if (string_list_has_string(&names, hash_to_hex(p->hash)))
      continue;

in the `delete_redundant` and `geometric` conditional towards the end of
`cmd_repack`.

But it's possible to trick `git repack` into not recognizing a pack that
it just wrote when `names` is out-of-order (which violates
`string_list_has_string()`'s assumption that the list is sorted and thus
binary search-able).

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):

  - 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 <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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