[PATCH 0/6] refs: introduce support for partial reference transactions

Git's reference updates are traditionally atomic - when updating multiple references in a transaction, either all updates succeed or none do. While this behavior is generally desirable, it can be limiting in certain scenarios, particularly with the reftable backend where batching multiple reference updates is more efficient than performing them sequentially.

This series introduces support for partial reference transactions, allowing individual reference updates to fail while letting others proceed. This capability is exposed through git-update-ref's `--allow-partial` flag, which can be used in `--stdin` mode to batch updates and handle failures gracefully.

The changes are structured to carefully build up this functionality:

First, we clean up and consolidate the reference update checking logic. This includes removing duplicate checks in the files backend and moving refname tracking to the generic layer, which simplifies the codebase and prepares it for the new feature.

We then restructure the reftable backend's transaction preparation code, extracting the update validation logic into a dedicated function. This not only improves code organization but sets the stage for implementing partial transaction support.

With this groundwork in place, we implement the core partial transaction support in the refs subsystem. This adds the necessary infrastructure to track and report rejected updates while allowing transactions to proceed. All reference backends are modified to support this behavior when enabled.

Finally, we expose this functionality to users through git-update-ref(1)'s `--allow-partial` flag, complete with test coverage and documentation. The flag is specifically limited to `--stdin` mode where batching multiple updates is most relevant.

This enhancement improves Git's flexibility in handling reference updates while maintaining the safety of atomic transactions by default. It's particularly valuable for tools and workflows that need to handle reference update failures gracefully without abandoning the entire batch of updates.

This series is based on top of bc204b7427 (The seventh batch, 2025-02-03). There were no conflicts noticed with topics in 'seen' or 'next'.

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Karthik Nayak (6):
      refs/files: remove duplicate check in `split_symref_update()`
      refs: move duplicate refname update check to generic layer
      refs/files: remove duplicate duplicates check
      refs/reftable: extract code from the transaction preparation
      refs: implement partial reference transaction support
      update-ref: add --allow-partial flag for stdin mode

 Documentation/git-update-ref.txt |  12 +-
 builtin/update-ref.c             |  53 ++++-
 refs.c                           |  58 ++++-
 refs.h                           |  22 ++
 refs/files-backend.c             |  97 ++------
 refs/packed-backend.c            |  49 ++--
 refs/refs-internal.h             |  19 +-
 refs/reftable-backend.c          | 494 +++++++++++++++++++--------------------
 t/t1400-update-ref.sh            | 191 +++++++++++++++
 9 files changed, 633 insertions(+), 362 deletions(-)
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base-commit: bc204b742735ae06f65bb20291c95985c9633b7f change-id: 20241206-245-partially-atomic-ref-updates-9fe8b080345c

Thanks
- Karthik