Re: MSan failures in pack-bitmap

> On Fri, Jun 07, 2024 at 07:43:56PM -0700, Kyle Lippincott wrote:
>
> > I believe what's happening is that pack-bitmap.c:2091 grows the packs
> > list and sets up some of the fields, but doesn't set pack_int_id. We
> > then use it at pack-bitmap.c:1888.
> >
> > I investigated, but couldn't prove to myself what value should be
> > placed there while growing it, or if it's incorrect to read from it in
> > this case (so we shouldn't be in pack-bitmap.c:1888 with this pack).
>
> Hmm, I'm not sure.
>
> In reuse_partial_packfile_from_bitmap(), the code path that creates the
> struct only kicks in when the "multi_pack_reuse" flag isn't set. Which
> generally would correspond to whether we have a midx. And then the code
> in try_partial_reuse() that uses the struct similarly checks
> bitmap_is_midx() before looking at the pack_int_id field.
>
> But that changed in 795006fff4 (pack-bitmap: gracefully handle missing
> BTMP chunks, 2024-04-15), where we also disable multi_pack_reuse if we
> have a midx but it has no BTMP chunk. So we end up in the non-multi code
> path to create the struct, but then try_partial_reuse() still realizes
> we have a midx and uses that code path.
>
> I guess this gets into the "we have a midx, but are only doing reuse out
> of a single pack" case. Which I think is supported, but I'm not familiar
> enough with the code to know where the assumption is going wrong.