Subject: Re: [PATCH v2 1/8] packfile: introduce 'find_kept_pack_entry()'

From: Taylor Blau <[email-masked]>
Date: Tue, 16 Feb 2021 16:48:17 -0500

On Tue, Feb 16, 2021 at 04:42:38PM -0500, Jeff King wrote:
> On Wed, Feb 03, 2021 at 10:58:50PM -0500, Taylor Blau wrote:
>
> > Future callers will want a function to fill a 'struct pack_entry' for a
> > given object id but _only_ from its position in any kept pack(s).
> >
> > In particular, an new 'git repack' mode which ensures the resulting
>
> Nit (not worth re-rolling): s/an new/a new/

Oops. Good eyes.

> > There is a gotcha when looking up objects that are duplicated in kept
> > and non-kept packs, particularly when the MIDX stores the non-kept
> > version and the caller asked for kept objects only. This could be
> > resolved by teaching the MIDX to resolve duplicates by always favoring
> > the kept pack (if one exists), but this breaks an assumption in existing
> > MIDXs, and so it would require a format change.
>
> I don't think this would be possible without a major rethink of how
> midxs work. The "keep" property of a pack is not set in stone when the
> midx is created. You could add a ".keep" file to one of its packs later,
> or even mark one as an in-core keep on the fly. But the duplicate
> resolution happens at creation.
>
> So maybe your "breaks an assumption" is the notion that we do not store
> duplicate information at all in the midx. If so, then I agree. :) But
> I'd also call fixing that more than just a format change.

That's part of it, indeed. The part that I was referring to is that
existing MIDX readers expect duplicates to be resolved in a certain way
(effectively in favor of the pack with the lowest mtime). So the easy
part is indicating a format change which tells new readers how to expect
ties to be broken.

But (as you note) that's only part of the problem: even if we say "ties
are resolved in favor of the lowest mtime pack, or a .keep one, if it
exists", then which ones are kept and which aren't? Even *if* we wrote
that down (which I'm not suggesting we do), kept-ness isn't an immutable
property of the pack, and so I think relying on it is a tricky direction
to take.

> (None of which changes your point, which isn't that it isn't worth
> pursuing that direction).

Yeah; my hope in writing some of this down in the above paragraph is
that it would make clear to future readers that such a MIDX change would
resolve some complexity here, but the complexity it adds in the MIDX
code isn't worth the tradeoff.

> -Peff

Thanks,
Taylor