Subject: Re: [PATCH 1/2] midx: fix `BUG()` when getting preferred pack without a reverse index

From: Taylor Blau <[email-masked]>
Date: Tue, 9 Dec 2025 19:22:11 -0500

On Mon, Dec 08, 2025 at 07:27:14PM +0100, Patrick Steinhardt wrote:
> The function `midx_preferred_pack()` returns the preferred pack for a
> given multi-pack index. To compute the preferred pack we:
>
>   1. Look up the position of the first object indexed by the multi-pack
>      index.
>
>   2. Convert this position from pseudo-pack order into MIDX order.
>
>   3. We then look up pack that corresponds to this MIDX index.

I think the implementation of midx_preferred_pack() works a little bit
differently than is described here. I often get confused when working in
this area juggling between the various object/pack orderings in my head.

midx_preferred_pack() cares about converting from the first position in
pseudo-pack order back into MIDX object order. To do that, we convert
the pseudo-pack position into a MIDX one, and then lookup the pack that
represents that object.

Effectively we're doing something like:

    uint32_t pseudo_pack_pos = m->num_objects_in_base;
    uint32_t midx_pos = pack_pos_to_midx(m, pseudo_pack_pos);
    uint32_t pack_int_id = nth_midxed_pack_int_id(m, midx_pos);

> This reliably returns the preferred pack given that all of its contained
> objects will be up front in pseudo-pack order.
>
> The second step that turns the pseudo-pack order into MIDX order
> requires the reverse index though, which may not exist for example when
> the MIDX does not have a bitmap. And in that case one may easily hit a
> bug:
>
>     BUG: ../pack-revindex.c:491: pack_pos_to_midx: reverse index not yet loaded

That makes sense, we can't convert a pseudo-pack position into a
MIDX-relative position without a reverse index to tell us where that bit
goes.

> In theory, `midx_preferred_pack()` already knows to handle the case
> where no reverse index exists, as it calls `load_midx_revindex()` before
> calling into `midx_preferred_pack()`. [...]

Right, it handles this case by returning -1 to indicate that it didn't
have enough information to determine the preferred pack.

> [...] But we only check for negative
> return values there, even though the function returns a positive error
> code in case the reverse index does not exist.

Ah. It looks like that was changed in 5a6072f631d (fsck: validate .rev
file header, 2023-04-17), but it looks like the caller here did not
learn about that change. It may be worth mentioning that commit in your
patch message.

While reviewing, I wanted to make sure that there weren't any other
callers of load_midx_revindex() that were also missing this check. The
return value of that function is propagated through the two expected
functions:

    $ git grep -p load_revindex_from_disk
    pack-revindex.c=struct revindex_header {
    pack-revindex.c:static int load_revindex_from_disk(const struct git_hash_algo *algo,
    pack-revindex.c=int load_pack_revindex_from_disk(struct packed_git *p)
    pack-revindex.c:        ret = load_revindex_from_disk(p->repo->hash_algo,
    pack-revindex.c=int load_midx_revindex(struct multi_pack_index *m)
    pack-revindex.c:        ret = load_revindex_from_disk(m->source->odb->repo->hash_algo,

, and checking through the callers of those two functions, all are
prepared to handle a >0 return value.

> diff --git a/midx.c b/midx.c
> index 24e1e72175..b681b18fc1 100644
> --- a/midx.c
> +++ b/midx.c
> @@ -686,7 +686,7 @@ int midx_preferred_pack(struct multi_pack_index *m, uint32_t *pack_int_id)
>  {
>  	if (m->preferred_pack_idx == -1) {
>  		uint32_t midx_pos;
> -		if (load_midx_revindex(m) < 0) {
> +		if (load_midx_revindex(m)) {
>  			m->preferred_pack_idx = -2;
>  			return -1;
>  		}
> diff --git a/pack-revindex.h b/pack-revindex.h
> index 422c2487ae..0042892091 100644
> --- a/pack-revindex.h
> +++ b/pack-revindex.h
> @@ -72,7 +72,8 @@ int verify_pack_revindex(struct packed_git *p);
>   * multi-pack index by mmap-ing it and assigning pointers in the
>   * multi_pack_index to point at it.
>   *
> - * A negative number is returned on error.
> + * A negative number is returned on error. A positive number is returned in
> + * case the multi-pack-index does not have a reverse index.

Makes sense.

> diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
> index 93f319a4b2..9492a9737b 100755
> --- a/t/t5319-multi-pack-index.sh
> +++ b/t/t5319-multi-pack-index.sh
> @@ -350,7 +350,20 @@ test_expect_success 'preferred pack from existing MIDX without bitmaps' '
>  		# the new MIDX
>  		git multi-pack-index write --preferred-pack=pack-$pack.pack
>  	)
> +'
>
> +test_expect_success 'preferred pack cannot be determined without bitmap' '
> +	test_when_finished "rm -fr preferred-can-be-queried" &&
> +	git init preferred-can-be-queried &&
> +	(
> +		cd preferred-can-be-queried &&
> +		test_commit initial &&
> +		git repack -Adl --write-midx --no-write-bitmap-index &&
> +		test_must_fail test-tool read-midx --preferred-pack .git/objects 2>err &&
> +		test_grep "could not determine MIDX preferred pack" err &&

Looks good. I think that it's fine to end the test here, since we have
extensive coverage that we can determine the preferred pack when there
is a MIDX and matching bitmap, but I definitely don't feel strongly
about it.

Thanks,
Taylor