Pablo Sabater [off-list ref] writes:

This patch series is a continuation of Eric Ju's
(eric.peijian@gmail.com) and Calvin Wan's (calvinwan@google.com) patch
series [1] and [2] respectively.

Sometimes it is beneficial to retrieve information about an object
without having to download it completely. The server logic for
retrieving size has already been implemented and merged in a2ba162cda
(object-info: support for retrieving object info, 2021-04-20) [3].
This patch series implement the client option for it.

Eric's series adds the remote-object-info command to cat-file
--batch-command. This command allows the client to make an object-info
command request to a server that supports protocol v2.

If the server uses protocol v2 but does not support the object-info
capability, cat-file --batch-command will die.

If a user attempts to use remote-object-info with protocol v1, cat-file
--batch-command will die.

Currently, only the size (%(objectsize)) is supported end to end in this
implementation. The type (%(objecttype)) is known by the client's
allow-list and request path but is not supported on the server side
nor the response parsing. A follow up series will add full end-to-end
support for %(objecttype).

The default format for remote-object-info is set to "%(objectname)
%(objectsize)". Once %(objecttype) is supported, the default format will
be unified accordingly.

If the batch command format includes unsupported fields such as
%(objecttype), %(objectsize:disk), or %(deltabase), the command will
return empty strings for each unsupported field.

This series completes Eric's work mainly with the refactor of the
validation of the placeholder with an allow-list that filters what the
client asks with what the server is capable of provide following Jeff
King's idea [4].

GitHub CI: https://github.com/pabloosabaterr/git/actions/runs/29404390713

[1]: https://lore.kernel.org/git/20250221190451.12536-1-eric.peijian@gmail.com/ (local)
[2]: https://lore.kernel.org/git/20220728230210.2952731-1-calvinwan@google.com/#t (local)
[3]: https://git.kernel.org/pub/scm/git/git.git/commit/?id=a2ba162cda2acc171c3e36acbbc854792b093cb7
[4]: https://lore.kernel.org/git/20250313060250.GH94015@coredump.intra.peff.net/ (local)

Changes since v17:

At 10th commit: transport: add client support for object-info
- style
- enforced the server response handling, not allowing bare "<oid>" or
  responses with a different number of attributes different from the
  number asked.

Some small comments/nits from my side, mostly looks good otherwise :)