Re: fatal: bad revision 'HEAD'

Jeff King [off-list ref] writes:

I was able to replicate your problem, but only if I set
branch.master.rebase to "true" in my user-wide git config (i.e.,
~/.gitconfig). It looks like "git pull" is not capable of handling a
rebase when you have no commits yet.

It does sound sick to store such a setting in $HOME/.gitconfig file, when
the variable is clearly per-repository.  As you wrote, you can easily
trigger it with an explicit --rebase, but again, it is insane to ask
"rebase" when you clearly do not have anything.

But just like we twisted the definition of merge to mean "merging
something into nothing yields that something", we could twist the
definition of rebase to mean "rebasing nothing on top of something result
in that something".  It sort of makes sense in a twisted way.

Two comments.

 * Is "rev-parse -q --verify" a safe test to guarantee that HEAD is
   unborn?  Shouldn't we be checking with "symbolic-ref" or something?

 * 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.  So how about
   doing something like the following instead?

	if on unborn branch
	then
		if test -f "$GIT_DIR/index"
                then
			die "refusing to update; you have a non-empty index"
		fi
	else
		... existing tests against HEAD ...
	fi

On Wed, Aug 12, 2009 at 12:37:44AM -0700, Junio C Hamano wrote:

But just like we twisted the definition of merge to mean "merging
something into nothing yields that something", we could twist the
definition of rebase to mean "rebasing nothing on top of something result
in that something".  It sort of makes sense in a twisted way.

I dunno. It doesn't seem all that twisted to me.

But like many of the "branch to be born" and "initial commit" edge cases
we have dealt with, it is not so much about somebody intentionally
triggering this as it is about doing something sane when some script
_does_ trigger it. And I think the sane thing is obvious and easy to do
here, so why not?

 * Is "rev-parse -q --verify" a safe test to guarantee that HEAD is
   unborn?  Shouldn't we be checking with "symbolic-ref" or something?

I'm not sure. The test in git-checkout, for example, seems to basically
just be looking up HEAD as a commit. If it doesn't work, then the branch
is to-be-born (see switch_branches in builtin-checkout.c).

Which is more or less what's happening here (except we don't check that
the type is a commit).

With symbolic-ref, I guess we could find out what the ref is, and check
to see if _that_ exists. But I can't think of a situation where that
would be meaningfully different than just resolving HEAD. Obviously
detached HEADs come to mind, but wouldn't you then by definition not be
a branch-to-be-born, which is what this rev-parse test would tell you?

 * 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.  So how about
   doing something like the following instead?

	if on unborn branch
	then
		if test -f "$GIT_DIR/index"
                then
			die "refusing to update; you have a non-empty index"
		fi
	else
		... existing tests against HEAD ...
	fi

Yeah, I think that is a better idea. Do you want to tweak the patch, or
should I re-submit?

-Peff