On Sat, Mar 02, 2013 at 12:18:59AM +0100, David Madore wrote:

> I'd like to suggest a very simple, but IMHO quite useful, additional
> option to git-update-ref: an option --ff-only which would cause the
> command to refuse unless the current ref is an ancestor of the new
> one.
> The reason I think it would be useful: I occasionally wish to perform
> a trivial (i.e., fast-forward) merge of some branch into another
> (e.g., after a git-fetch) without checking it out.  Now git-update-ref
> is perfect for that, but there is always the possibility of getting
> something wrong (which one would not have with git merge --ff-only,
> but the latter requires checking out the branch), and this option
> would avoid tedious verifications.

The update-ref command is plumbing, which is supposed to do one small,
well-defined job. But you can compose many plumbing commands to do what
you want:

  # input

  # where are we now?
  old=`git rev-parse --verify $ref` || exit 1

  # is it a fast-forward?
  if ! git merge-base --is-ancestor $old $new; then
    echo >&2 "Not a fast-forward"
    exit 1

  # update it; we do not have to worry about race conditions because
  # update-ref will abort if somebody touched the ref in the meantime
  git update-ref $ref $new $old

Yes, it's three commands instead of one, but it's much more flexible
(you get to write your own message, you can use the same "merge-base" to
handle the "already up to date" case, etc).

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