From: Jeff King <peff@peff.net>
Date: Thu, 13 Aug 2009 02:31:37 GMT
Subject: Re: fatal: bad revision 'HEAD'
Message-ID: <20090813023137.GA17358@coredump.intra.peff.net>
In-Reply-To: <7vab24ve97.fsf@alter.siamese.dyndns.org>

On Wed, Aug 12, 2009 at 03:49:56PM -0700, Junio C Hamano wrote:

> When doing a "pull --rebase", we check to make sure that the index and
> working tree are clean. The index-clean check compares the index against
> HEAD. The test erroneously reports dirtiness if we don't have a HEAD yet.
> 
> In such an "unborn branch" case, by definition, a non-empty index won't
> be based on whatever we are pulling down from the remote, and will lose
> the local change.  Just check if $GIT_DIR/index exists and error out.
> 
> Signed-off-by: Jeff King <peff@peff.net>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Looks sane to me, but see below.

> +		# On an unborn branch
> +		if test -f "$GIT_DIR/index"
> +		then
> +			die "updating an unborn branch with changes added to the index"
> +		fi

I had a brief moment where I thought this might not actually be
sufficient. That is, can somebody create an unborn branch in an existing
repo (e.g., because they want an alternate project root) in such a way
that they have an index, but it is empty? In which case we should be
actually checking for index emptiness instead of existence.

I.e., I have done in the past (but not frequently):

  git symbolic-ref HEAD refs/heads/to-be-born

in an existing repository to create a new root. But I don't think we
really need to worry about that case:

  1. It is somewhat crazy and rare in the first place. Even more crazy
     and rare would be to "git pull --rebase" immediately afterwards.

  2. If you _did_ have an index, either:

     2a. You want to keep it, in which case your check is sane.

     2b. You want to start fresh, in which case your next step would be
         be "rm -f .git/index".

The only way this check would not be OK is is if you did something like:

  git symbolic-ref HEAD refs/heads/new-branch
  git ls-files | xargs git rm --cached
  git pull --rebase ...

which just seems crazy.

So I think I have convinced myself that what you have is fine, but I
just wanted a sanity check.

-Peff