Re: [PATCH 9/9] update-ref: implement interactive transaction handling

Patrick Steinhardt [off-list ref] writes:

The git-update-ref(1) command can only handle queueing transactions
right now via its "--stdin" parameter, but there is no way for users to
handle the transaction itself in a more explicit way. E.g. in a
replicated scenario, one may imagine a coordinator that spawns
git-update-ref(1) for multiple repositories and only if all agree that
an update is possible will the coordinator send a commit. Such a
transactional session could look like

    > start
    < start: ok
    > update refs/heads/master $OLD $NEW
    > prepare
    < prepare: ok
    # All nodes have returned "ok"
    > commit
    < commit: ok

or

    > start
    < start: ok
    > create refs/heads/master $OLD $NEW
    > prepare
    < fatal: cannot lock ref 'refs/heads/master': reference already exists
    # On all other nodes:
    > abort
    < abort: ok

In order to allow for such transactional sessions, this commit
introduces four new commands for git-update-ref(1), which matches those
we have internally already with the exception of "start":

    - start: start a new transaction

    - prepare: prepare the transaction, that is try to lock all
               references and verify their current value matches the
               expected one

    - commit: explicitly commit a session, that is update references to
              match their new expected state

    - abort: abort a session and roll back all changes

OK, this is a fun stuff, and explains why we wanted to do [7/9].