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

From: Taylor Blau <me@ttaylorr.com>
Date: Apr 12, 2023, 22:01 UTC
Message-ID: <ZDcqIjSUMW4sKNXE@nand.local>
In-Reply-To: <b74d0a037b07706d07fad7b438fa0cb211726575.1681294715.git.ps@pks.im>

On Wed, Apr 12, 2023 at 12:23:01PM +0200, Patrick Steinhardt wrote:

Show 5 quoted lines

> 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.

Show 33 quoted lines

> ---
>  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