Jonathan Nieder wrote:

This series teaches the tag, replace, commit, cherry-pick,
fast-import, checkout -b, branch, receive-pack, and http-fetch
commands and all update_ref and delete_ref callers to use the ref
transaction API instead of lock_ref_sha1.

The main user-visible change should be some cosmetic changes to error
messages.  The series also combines multiple ref updates into a single
transaction in 'git http-fetch -w' and when writing tags in
fast-import but otherwise doesn't change the granularity of
transactions.

Reviewed at https://code-review.googlesource.com/#/q/topic:ref-transaction-1

----------------------------------------------------------------
Ronnie Sahlberg (20):
      refs.c: change ref_transaction_create to do error checking and return status
      refs.c: update ref_transaction_delete to check for error and return status
      refs.c: make ref_transaction_begin take an err argument
      refs.c: add transaction.status and track OPEN/CLOSED
      tag.c: use ref transactions when doing updates
      replace.c: use the ref transaction functions for updates
      commit.c: use ref transactions for updates
      sequencer.c: use ref transactions for all ref updates
      fast-import.c: change update_branch to use ref transactions
      branch.c: use ref transaction for all ref updates
      refs.c: change update_ref to use a transaction
      receive-pack.c: use a reference transaction for updating the refs
      fast-import.c: use a ref transaction when dumping tags
      walker.c: use ref transaction for ref updates
      refs.c: make lock_ref_sha1 static
      refs.c: remove the update_ref_lock function
      refs.c: remove the update_ref_write function
      refs.c: remove lock_ref_sha1
      refs.c: make prune_ref use a transaction to delete the ref
      refs.c: make delete_ref use a transaction

And here are the patches.