Re: [PATCH 4/4] repack: retain cruft packs in MIDXs containing kept packs

[Taylor Blau <ttaylorr@openai.com>]

On Wed, Sep 30, 2026 at 04:53:11PM -0400, Jeff King wrote:
> OK. This makes sense to me, but two questions:
>
>   1. Is this going to kick in racily because of the .keep that we
>      temporarily install during pushes? That could cause unexpected
>      performance changes in a big repo when the midx sometimes has to
>      randomly include cruft packs.

Excellent point. If we happen to race between repacking and receiving an
incoming push, it is certainly not the case that we should have to then
thrust the cruft pack onto the MIDX, and defeat the whole point of this
series ;-).

>   2. I'd have thought that the solution would be to treat .keep packs
>      like other included follow-packs: traverse them in the usual way.
>      But maybe there are good reasons we didn't do that in the first
>      place.

Yeah, I think that's right. The following round passes excluded kept
packs into the geometric repack's traversal using '!' for packs outside
the existing MIDX and '^' for those already covered by it. That lets us
copy their ancestors out of cruft without copying the kept objects
themselves. Non-geometric repacks still conservatively retain cruft,
since kept objects may remain untraversed.

There is, however, a pre-existing corner case with --pack-kept-objects
and a cruft pack that is also kept. Such a pack can enter the MIDX
without being traversed. This change doesn't address that case. Handling
it cleanly looks more involved, so I'd leave it for a separate
follow-up.

Thanks,
Taylor