On Wed, Feb 11, 2026 at 01:44:59PM +0100, Patrick Steinhardt wrote:

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.

Nicely explained. I think it's reasonable to ask "well why are we doing
a revwalk, you just told me that --stdin-packs does not require a
(potentially expensive) traversal?". I think that the exposition you
provided here answers that question nicely.

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.

Great find!

We of course cannot passe "--exclude-promisor-objects" to fix this

s/passe/pass, though I think Junio noted this below.

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.

Yep, I think this is the right trade-off.

---
 builtin/pack-objects.c        | 10 ++++++++++
 t/t5331-pack-objects-stdin.sh | 18 ++++++++++++++++++
 2 files changed, 28 insertions(+)

The implementation and test look exactly as expected. Thanks for jumping
on this and fixing it!

Thanks,
Taylor