Subject: Re: [PATCH 3/5] describe: catch unborn branch in describe_blob()

From: Junio C Hamano <[email-masked]>
Date: Mon, 18 Aug 2025 14:19:42 -0700

Jeff King <[email-masked]> writes:

> --- a/builtin/describe.c
> +++ b/builtin/describe.c
> @@ -518,10 +518,16 @@ static void describe_blob(const struct object_id *oid, struct strbuf *dst)
>  {
>  	struct rev_info revs;
>  	struct strvec args = STRVEC_INIT;
> +	struct object_id head_oid;
>  	struct process_commit_data pcd = { *null_oid(the_hash_algo), oid, dst, &revs};

An unrelated tangent, but it seems that we are copying the object
name for the first member of this struct, even though 1/5 changed
the second one.

> +	if (repo_get_oid(the_repository, "HEAD", &head_oid))
> +		die(_("cannot search for blob '%s' on an unborn branch"),
> +		    oid_to_hex(oid));

Makes sense.  I briefly wondered if there is really a point in doing
the traversal only from HEAD, but this topic is not about enhancing
and making the "describe <blob>" more useful, but it is a strict
improvement.  When you first mentioned "resolve HEAD ourselves", I
somehow expected you to ask the ref subsystem to resolve HEAD, but
this should do fine, thanks to ref_rev_parse_rules[].

> diff --git a/t/t6120-describe.sh b/t/t6120-describe.sh
> index 470631d17d..feec57bcbc 100755
> --- a/t/t6120-describe.sh
> +++ b/t/t6120-describe.sh
> @@ -415,6 +415,14 @@ test_expect_success 'describe an unreachable blob' '
>  	test_grep "blob .$blob. not reachable from HEAD" actual
>  '
>  
> +test_expect_success 'describe blob on an unborn branch' '
> +	oldbranch=$(git symbolic-ref HEAD) &&
> +	test_when_finished "git symbolic-ref HEAD $oldbranch" &&
> +	git symbolic-ref HEAD refs/heads/does-not-exist &&
> +	test_must_fail git describe test-blob 2>actual &&
> +	test_grep "cannot search .* on an unborn branch" actual
> +'
> +
>  test_expect_success ULIMIT_STACK_SIZE 'name-rev works in a deep repo' '
>  	i=1 &&
>  	while test $i -lt 8000