[PATCH 0/7] refs: add reflog support to `git refs migrate`

The `git refs migrate` command was introduced in 25a0023f28 (builtin/refs: new command to migrate ref storage formats, 2024-06-06) to support migrating from one reference backend to another.

One limitation of the feature was that it didn't support migrating repositories which contained reflogs. This isn't a requirement on the server side as repositories are stored as bare repositories (which do not contain any reflogs). Clients however generally use reflogs and until now couldn't use the `git refs migrate` command to migrate their repositories to the new reftable format.

One of the issues for adding reflog support is that the ref transactions
don't support reflogs additions:
  1. While there is REF_LOG_ONLY flag, there is no function to utilize
  the flag and add reflogs.
  2. reference backends generally sort the updates by the refname. This
  wouldn't work for reflogs which need to ensure that they maintain the
  order of creation.
  3. In the files backend, reflog entries are added by obtaining locks
  on the refs themselves. This means each update in the transaction, will
  obtain a ref_lock. This paradigm fails to accompany the fact that there
  could be multiple reflog updates for a refname in a single transaction.
  4. The backends check for duplicate entries, which doesn't make sense
  in the context of adding multiple reflogs for a given refname.

We overcome these issue we make the following changes:
  - Update the ref_update structure to also include the committer
  information. Using this, we can add a new function which only adds
  reflog updates to the transaction.
  - Add an index field to the ref_update structure, this will help order
  updates in pre-defined order, this fixes #2.
  - While the ideal fix for #3 would be to actually introduce reflog
  locks, this wouldn't be possible without breaking backward
  compatibility. So we add a count field to the existing ref_lock. With
  this, multiple reflog updates can share a single ref_lock.

Overall, this series is a bit more involved, and I would appreciate it if it receives a bit more scrutiny.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
Karthik Nayak (7):
      refs: include committer info in `ref_update` struct
      refs: add `index` field to `struct ref_udpate`
      refs/files: add count field to ref_lock
      refs: extract out refname verification in transactions
      refs: introduce the `ref_transaction_update_reflog` function
      refs: allow multiple reflog entries for the same refname
      refs: add support for migrating reflogs

 Documentation/git-refs.txt |   2 -
 refs.c                     | 204 ++++++++++++++++++++++++++++++++-------------
 refs.h                     |  12 +++
 refs/files-backend.c       | 144 ++++++++++++++++++++------------
 refs/refs-internal.h       |  24 ++++--
 refs/reftable-backend.c    |  47 +++++++++--
 t/t1460-refs-migrate.sh    |  73 +++++++++++-----
 7 files changed, 360 insertions(+), 146 deletions(-)
---

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base-commit: e66fd72e972df760a53c3d6da023c17adfc426d6 change-id: 20241111-320-git-refs-migrate-reflogs-a53e3a6cffc9

Thanks
- Karthik