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@xxxxxxxxxxxx>
---
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
--
2.36.1.94.gb0d54bedca
