Re: [PATCH v3 2/2] builtin/pack-objects.c: introduce `pack.recentObjectsHook`

> On Fri, May 12, 2023 at 05:24:56PM -0400, Jeff King wrote:
>
> > > This patch introduces a new configuration, `pack.recentObjectsHook`
> > > which allows the caller to specify a program (or set of programs) whose
> > > output is treated as a set of objects to treat as recent, regardless of
> > > their true age.
> >
> > I was going to complain about putting this in the "pack" section,
> > because I thought by touching reachable.c, we'd also affect git-prune.
> > But I don't think we do, because it does its own direct mtime check on
> > the loose objects.
> >
> > But I'm not sure that's the right behavior.
> >
> > It feels like even before your patch, this is a huge gap in our
> > object-retention strategy.  During repacking, we try to avoid dropping
> > objects which are reachable from recent-but-unreachable things we're
> > keeping (since otherwise it effectively corrupts those recent objects,
> > making them less valuable to keep). But git-prune will happily drop them
> > anyway!
> >
> > And I think the same thing would apply to your hook. If the hook says
> > "object XYZ is precious even if unreachable, keep it", then git-prune
> > ignoring that seems like it would be a source of errors.
> >
> > I suspect both could be fixed by having git-prune trigger the same
> > add_unseen_recent_objects_to_traversal() call either as part of
> > the perform_reachability_traversal() walk, or maybe in its own walk (I
> > think maybe it has to be its own because the second walk should avoid
> > complaining about missing objects).
>
> <phew> I am happy to say that I was wrong here, and git-prune behaves as
> it should, courtesy of d3038d22f9 (prune: keep objects reachable from
> recent objects, 2014-10-15). The magic happens in mark_reachable_objects(),
> which handles walking the recent objects by calling...you guessed it,
> add_unseen_recent_objects_to_traversal().

> So it does the right thing now, and your patch should kick in
> automatically for git-prune, too. But I think we'd want two things:
>
>   1. Should the config variable name be made more generic to match?
>      Maybe "core" is too broad (though certainly I'd expect it to apply
>      anywhere in Git where we check recent-ness of objects), but perhaps
>      "gc" would make sense (even though it is not strictly part of the
>      gc command, it is within that realm of concepts).