Re: Bug in `git branch --delete main` when on other orphan branch

[Taylor Blau <me@ttaylorr.com>]

On Tue, Nov 01, 2022 at 06:15:33AM -0400, Jeff King wrote:
> On Fri, Oct 28, 2022 at 10:46:37PM -0700, Martin von Zweigbergk wrote:
>
> > I did this:
> > git init test
> > cd test
> > echo a > file
> > git add file
> > git commit -m a
> > git checkout --orphan other
> > git branch --delete main
> >
> > The last command fails with:
> > fatal: Couldn't look up commit object for HEAD
> >
> > That's a bug, right? I can of course work around it with `rm
> > .git/refs/heads/main`.
>
> Sort of. This is part of the "is the thing we are deleting merged into
> HEAD" check. It tries to look up the HEAD and calls die() when it can't.
> The more correct thing, I think, would be for it to just return "nope,
> there is no HEAD so nothing is merged into it".

Yeah, I think that it's fair to call being unable to find HEAD in 'git
branch -d' when we are detached a bug. Indeed, if we can't find a HEAD,
then that's fine (there is just nothing merged into it, as you note).

> And in that case I think the HEAD check calling die() is actively doing
> the wrong thing, and would prevent an otherwise successful deletion.
>
> The fix might be as simple as:
>
> diff --git a/builtin/branch.c b/builtin/branch.c
> index 15be0c03ef..f6ff9084c8 100644
> --- a/builtin/branch.c
> +++ b/builtin/branch.c
> @@ -235,11 +235,8 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
>  	}
>  	branch_name_pos = strcspn(fmt, "%");
>
> -	if (!force) {
> +	if (!force)
>  		head_rev = lookup_commit_reference(the_repository, &head_oid);
> -		if (!head_rev)
> -			die(_("Couldn't look up commit object for HEAD"));
> -	}
>
>  	for (i = 0; i < argc; i++, strbuf_reset(&bname)) {
>  		char *target = NULL;
>
> as the later code seems to do the right thing with the NULL head_rev. It
> would definitely need more careful investigation (and tests!) to confirm
> that, though.

Yeah, that looks reasonable to me. Presumably we want a small test, as
well, but I doubt that is any more complicated than:

--- 8< ---
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index 7f605f865b..6ace22f7ce 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -279,6 +279,14 @@ test_expect_success 'git branch -M and -C fail on detached HEAD' '
 	test_cmp expect err
 '

+test_expect_success 'git branch -d on detached HEAD' '
+	test_when_finished "git checkout main && git branch -D other" &&
+	git branch other &&
+	git checkout --orphan orphan &&
+	test_must_fail git branch -d other 2>err &&
+	grep "not fully merged" err
+'
+
 test_expect_success 'git branch -v -d t should work' '
 	git branch t &&
 	git rev-parse --verify refs/heads/t &&
--- >8 ---

I'm happy to wrap all of that up into a patch, and equally happy for you
to do so (feel free to forge my S-o-b here if you do).

Thanks,
Taylor