Re: [PATCH] builtin/pack-objects: don't fetch objects when merging packs

[Junio C Hamano <gitster@pobox.com>]

Patrick Steinhardt <ps@pks.im> writes:

> The "--stdin-packs" option can be used to merge objects from multiple
> packfiles given via stdin into a new packfile. One big upside of this
> option is that we don't have to perform a complete rev walk to enumerate
> objects. Instead, we can simply enumerate all objects that are part of
> the specified packfiles, which can be significantly faster in very large
> repositories.
>
> There is one downside though: when we don't perform a rev walk we also
> don't have a good way to learn about the respective object's names. As a
> consequence, we cannot use the name hashes as a heuristic to get better
> delta selection.
>
> We try to offset this downside though by performing a localized rev
> walk: we queue all objects that we're about to repack as interesting,
> and all objects from excluded packfiles as uninteresting. We then
> perform a best-effort rev walk that allows us to fill in object names.
>
> There is one gotcha here though: when "--exclude-promisor-objects" has
> not been given we will perform backfill fetches for any promised objects
> that are missing. This used to not be an issue though as this option was
> mutually exclusive with "--stdin-packs". But that has changed recently,
> and starting with dcc9c7ef47 (builtin/repack: handle promisor packs with
> geometric repacking, 2026-01-05) we will now repack promisor packs
> during geometric compaction. The consequence is that a geometric repack
> may now perform a bunch of backfill fetches.
>
> We of course cannot passe "--exclude-promisor-objects" to fix this
> issue -- after all, the whole intent is to repack objects part of a
> promisor pack. But arguably we don't have to: the rev walk is intended
> as best effort, and we already configure it to ignore missing links to
> other objects. So we can adapt the walk to unconditionally disable
> fetching any missing objects.

"passe" -> "pass".

Other than that, very nicely described, and the implementation is
surprisingly simple (thanks to a single global variable, and
asumption that makes it safe to use such a single global variable,
i.e., there is just one packing operation running at a time).

Will queue.  Thanks.