[PATCH RFC 0/5] Add --dry-run option to git-backfill(1)

[Cc'd Derrick Stolee for his work in the backfill(1) command]

This series adds a --dry-run option to git-backfill(1) that reports how many missing blobs would be fetched and, when the remote server supports the object-info capability, their total size:

        $ git backfill --dry-run
        After backfill, 48 blobs would be fetched (1.20 KiB).

If the server does not advertise object-info, only the count is shown.

I am not a git-backfill(1) user myself, but it seemed useful for users to know how much data a backfill would bring in before running it.

The number of missing blobs is the sum of the number of blobs to be fetched in each batch. The object-info capability lets us ask the server for the size of each blob without downloading it, so summing them gives an estimate of the total.

Note that this is an upper bound rather than the exact disk usage: object-info reports the uncompressed size of each object, while the objects end up stored compressed and possibly deltified in a packfile, so the space actually used on disk will usually be smaller.

Since I do not use backfill, feedback on whether this is useful, and on the output format, is very welcome.

Patches 1-3 are preparatory:

  [1/5] transport-internal: update fetch_object_info comment
        Fixes an outdated comment: object-info supports type as well as
        size.

  [2/5] fetch-object-info: add enum for fetch_object_info() statuses
  [3/5] fetch-object-info: return a status instead of dying
        Teach fetch_object_info() to return a status instead of dying
        when the server does not advertise object-info. The die() is
        kept in cat-file's remote-object-info path, so its behavior is
        unchanged.

Patches 4-5 add the option in two steps:

  [4/5] backfill: add --dry-run option
        Prints only the number of blobs that would be fetched.

  [5/5] backfill: report total size of missing blobs in --dry-run
        Also prints their total size when the server supports
        object-info, and falls back to the count alone otherwise.

Thanks.

Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
---
Pablo Sabater (5):
      transport-internal: update fetch_object_info comment
      fetch-object-info: add enum for fetch_object_info() statuses
      fetch-object-info: return a status instead of dying
      backfill: add --dry-run option
      backfill: report total size of missing blobs in --dry-run

 Documentation/git-backfill.adoc | 10 ++++-
 builtin/backfill.c              | 97 +++++++++++++++++++++++++++++++++++++++--
 builtin/cat-file.c              |  4 ++
 fetch-object-info.c             | 17 ++++----
 fetch-object-info.h             | 24 +++++++---
 t/t5620-backfill.sh             | 48 ++++++++++++++++++++
 transport-helper.c              |  6 +--
 transport-internal.h            | 12 ++---
 transport.c                     | 29 ++++++------
 transport.h                     |  7 +--
 10 files changed, 208 insertions(+), 46 deletions(-)

--- base-commit: 12cb6293d6288865c1a133cf22accbaf99d13eb6 change-id: 20260914-backfill-dryrun-fac997003322