Re: [PATCH v2 8/8] repack: disable writing bitmaps when doing a local geometric repack

[Taylor Blau <me@ttaylorr.com>]

On Wed, Apr 12, 2023 at 12:23:01PM +0200, Patrick Steinhardt wrote:
> Now there are two different ways to fix this. The first one would be to
> amend git-multi-pack-index(1) to disable writing bitmaps when we notice
> that we don't have full object coverage. But we ain't really got enough
> information there, and seeing that it is a low-level plumbing command it
> does not feel like the right place to fix this.

I might actually advocate that we either fix this in both places, or fix
it at the lower level only. I think that we would still be able to
trigger this problem by invoking `git multi-pack-index write
--bitmap --stdin-packs` directly.

> ---
>  builtin/repack.c            | 20 ++++++++++++++++++++
>  t/t7703-repack-geometric.sh | 27 +++++++++++++++++++++++++++
>  2 files changed, 47 insertions(+)
>
> diff --git a/builtin/repack.c b/builtin/repack.c
> index f57869f14a..07d92fdf87 100644
> --- a/builtin/repack.c
> +++ b/builtin/repack.c
> @@ -881,6 +881,26 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
>  	if (pack_kept_objects < 0)
>  		pack_kept_objects = write_bitmaps > 0 && !write_midx;
>
> +	if (write_midx && write_bitmaps && geometric_factor && po_args.local) {
> +		struct packed_git *p;
> +
> +		for (p = get_all_packs(the_repository); p; p = p->next) {
> +			if (p->pack_local)
> +				continue;
> +
> +			/*
> +			 * When asked to do a local repack, but we have
> +			 * packfiles that are inherited from an alternate, then
> +			 * we cannot guarantee that the multi-pack-index would
> +			 * have full coverage of all objects. We thus disable
> +			 * writing bitmaps in that case.
> +			 */
> +			warning(_("disabling bitmap writing, as some objects are not being packed"));
> +			write_bitmaps = 0;
> +			break;
> +		}
> +	}
> +

In terms of the higher-level fix here, though, I think that you could
reasonably assume that the alternate repository has at least one pack,
and that the combination of "write_midx && write_bitmaps &&
po.args_local" and "has any alternate(s)" is banned (or, at least, emits
the above warning and disables writing bitmaps).

But certainly ensuring that there are indeed packs in at least one of
the alternate(s) doesn't hurt either, so I don't mind this approach at
all.

One thing that I don't quite follow with this logic is why we need to
have geometric_factor set. You could (somewhat unreasonably) write a
MIDX containing a single pack (git repack -[A|a] --write-midx
--write-bitmap-index), or a MIDX containing just the new pack along with
all of the existing (local) packs, (git repack --write-midx
--write-bitmap-index).

So I think we'd want to drop the geometric_factor from the above
conditional. (And in the future, I think we typically refer to whether
or not the "geometry" pointer is NULL or not to indicate whether or not
we are doing a geometric repack, though the diff context doesn't give me
enough to know whether we have even attempted to set up that instance
yet, so this is fine, too).

Thanks,
Taylor