[PATCH v4 0/3] Make update refs more atomic

refs.c:ref_transaction_commit() intermingles doing updates and checks with actually applying changes to the refs in loops that abort on error. This is done one ref at a time and means that if an error is detected that will fail the operation partway through the list of refs to update we will end up with some changes applied to disk and others not.

Without having transaction support from the filesystem, it is hard to make an update that involves multiple refs to guarantee atomicity, but we can do a somewhat better than we currently do.

These patches change the update and delete functions to use a three call pattern of

1, lock 2, update, or flag for deletion 3, apply on disk (rename() or unlink())

When a transaction is commited we first do all the locking, preparations and most of the error checking before we actually start applying any changes to the filesystem store.

This means that more of the error cases that will fail the commit will trigger before we start doing any changes to the actual files.

This should make the changes of refs in refs_transaction_commit slightly more atomic.

Version 4:
* Fix a bug in fast-import.c:dump_tags and make sure no tests fail

Version 3:
* Rebased onto mhagger/ref-transactions.
* Removed the patch to do update/delete from a single loop.

Version 2:
Updates and fixes based on Junio's feedback.
* Fix the subject line for patches so they comply with the project standard.
* Redo the update/delete loops so that we maintain the correct order of
  operations. Perform all updates first, then perform the deletes.
* Add an additional patch that allows us to do the update/delete in the correct
  order from within a single loop by first sorting the refs so that deletes
  are after all non-deletes.

Ronnie Sahlberg (3):
  refs.c: split writing and commiting a ref into two separate functions
  refs.c: split delete_ref_loose() into a separate flag-for-deletion and
    commit phase
  refs.c: change ref_transaction_commit to run the commit loops once all
    work is finished

 branch.c               | 10 ++++--
 builtin/commit.c       |  5 +++
 builtin/fetch.c        |  7 +++-
 builtin/receive-pack.c |  4 +++
 builtin/replace.c      |  6 +++-
 builtin/tag.c          |  6 +++-
 fast-import.c          | 18 ++++++++--
 refs.c                 | 98 +++++++++++++++++++++++++++++++++-----------------
 refs.h                 |  6 ++++
 sequencer.c            |  4 +++
 walker.c               |  4 +++
 11 files changed, 129 insertions(+), 39 deletions(-)