Re: [PATCH] builtin/repack.c: invalidate MIDX only when necessary

[Taylor Blau <me@ttaylorr.com>]

On Tue, Aug 25, 2020 at 01:29:01PM -0400, Jeff King wrote:
> On Tue, Aug 25, 2020 at 01:10:13PM -0400, Derrick Stolee wrote:
>
> > > If we do care, though, that implies some cooperation between the
> > > deletion process and the midx code. Perhaps it even argues that repack
> > > should refuse to delete such a single pack at all, since it _isn't_
> > > redundant. It's part of a midx, and the caller should rewrite the midx
> > > first itself, and _then_ look for redundant packs.
> >
> > It is worth noting that we _do_ have a way to integrate the delete and
> > write code using 'git multi-pack-index expire'. One way to resolve this
> > atomicity would be to do the following inside the repack command:
> >
> >  1. Create and index the new pack.
> >  2. git multi-pack-index write
> >  3. git multi-pack-index expire
>
> Given that discussion elsewhere points to git-repack only really
> deleting packs in all-in-one mode (and not ever a single pack), it seems
> like we can really be much simpler here. If we're not deleting packs via
> all-in-one, there's no need to touch the midx at all. If we are, then
> it's reasonable to delete the midx immediately (after having written our
> new pack but before deleting) since our new single pack idx is as good
> or better.
>
> I.e., drop step 2 above, and make step 3 just clear_midx_file(). Which
> is roughly what the code does now, isn't it? Or is there some reason
> that "expire" is more interesting than just clearing?

It's not clear to me whether you're talking about my patch, or what a
more full integration with 'git repack' looks like.

If you are talking about my patch, I disagree: checking that the MIDX
doesn't know about a pack we're dropping *is* useful even without
all-in-one, because of '.keep' packs (as demonstrated by the new test in
my patch).

To me, this patch seems like an incremental step in the direction that
we ultimately want to be going, but it's hard to untangle whether the
ensuing discussion is targeted at my patch, or the ultimate goal.

> And if anybody does want to drop single packs, etc, they can do so by
> generating a sensible midx separately from the repack operation (and
> probably doing so before dropping the packs for atomicity).
>
> -Peff

Thanks,
Taylor