[PATCH] checkout: fix BUG() case in 9081a421a6

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Fix a regression in my 9081a421a6d (checkout: fix "branch info" memory
leaks, 2021-11-16) where I'd assumed that the old_branch_info.path
would have to start with refs/heads/*, but as has been reported[1]
that's not the case.

As a test case[2] to reproduce this shows the second "git checkout"
here runs into the BUG() in the pre-image. The test being added is
amended from[2] and will pass both with this change, and before
9081a421a6. I.e. our behavior now is again the same as before that
commit.

1. https://bugzilla.redhat.com/show_bug.cgi?id=2042920
2. https://lore.kernel.org/git/YemTGQZ97vAPUPY0@pobox.com/

Reported-by: Petr Šplíchal <psplicha@redhat.com>
Reported-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---

On Thu, Jan 20 2022, Todd Zullinger wrote:

> Hi,
>
> A bug was filed in the Fedora/Red Hat bugzilla today for the
> git 2.35.0 rc (rc1, but it's the same in rc2).  Petr (Cc'd),
> ran the following
>
>     git clone https://github.com/psss/fmf /tmp/fmf
>     cd /tmp/fmf
>     cp .git/refs/remotes/origin/HEAD .git/refs/heads/__DEFAULT__
>     git checkout -f __DEFAULT__
>     git checkout -f __DEFAULT__
>
> The second git checkout call runs into the BUG() call added
> in 9081a421a6 (checkout: fix "branch info" memory leaks,
> 2021-11-16):
>
>     BUG: builtin/checkout.c:1098: should be able to skip
>     past 'refs/heads/' in 'refs/remotes/origin/master'!
>     Aborted (core dumped)
>
> This worked in 2.34.1, so it's new to 2.35.0.  Should this
> work or does the manual copy to setup a branch fall into a
> new category of "don't do that"?
>
> (It's novel to get a bug report from rc testing of a distro
> build -- that doesn't happen often.)

Thanks to both you and Petr for the report and easy to reproduce case,
and sorry about causing it.

In retrospec it's a rather obvious thinko. Here's a minimal fix for
it, along with a derived test case that I made more exhaustive to
check the state of the repo before, after, and in-between the two "git
checkout" commands. As noted it'll also pass with 9081a421a6d
reverted, showing that our behavior is the same as before that commit.

 builtin/checkout.c         | 11 ++++-------
 t/t2018-checkout-branch.sh | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 6a5dd2a2a22..52a47ef40e1 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -1090,13 +1090,10 @@ static int switch_branches(const struct checkout_opts *opts,
 		FREE_AND_NULL(old_branch_info.path);
 
 	if (old_branch_info.path) {
-		const char *const prefix = "refs/heads/";
-		const char *p;
-		if (skip_prefix(old_branch_info.path, prefix, &p))
-			old_branch_info.name = xstrdup(p);
-		else
-			BUG("should be able to skip past '%s' in '%s'!",
-			    prefix, old_branch_info.path);
+		const char *p = old_branch_info.path;
+
+		skip_prefix(old_branch_info.path, "refs/heads/", &p);
+		old_branch_info.name = xstrdup(p);
 	}
 
 	if (opts->new_orphan_branch && opts->orphan_from_empty_tree) {
diff --git a/t/t2018-checkout-branch.sh b/t/t2018-checkout-branch.sh
index 3e93506c045..82df9b8bf64 100755
--- a/t/t2018-checkout-branch.sh
+++ b/t/t2018-checkout-branch.sh
@@ -85,6 +85,39 @@ test_expect_success 'setup' '
 	git branch -m branch1
 '
 
+test_expect_success REFFILES 'checkout a branch without refs/heads/* prefix' '
+	git clone --no-tags . repo-odd-prefix &&
+	(
+		cd repo-odd-prefix &&
+
+		cp .git/refs/remotes/origin/HEAD .git/refs/heads/a-branch &&
+
+		echo branch1 >expect.ref &&
+		git rev-parse --abbrev-ref HEAD >actual.ref &&
+		test_cmp expect.ref actual.ref &&
+
+		git checkout -f a-branch &&
+
+		echo origin/branch1 >expect.ref &&
+		git rev-parse --abbrev-ref HEAD >actual.ref &&
+		test_cmp expect.ref actual.ref &&
+
+		git checkout -f a-branch &&
+
+		cat >expect <<-EOF &&
+		$(git rev-parse HEAD) commit	refs/heads/a-branch
+		$(git rev-parse HEAD) commit	refs/heads/branch1
+		$(git rev-parse HEAD) commit	refs/remotes/origin/HEAD
+		$(git rev-parse HEAD) commit	refs/remotes/origin/branch1
+		EOF
+		git for-each-ref >actual &&
+		test_cmp expect actual &&
+
+		git rev-parse --abbrev-ref HEAD >actual &&
+		test_cmp expect.ref actual.ref
+	)
+'
+
 test_expect_success 'checkout -b to a new branch, set to HEAD' '
 	test_when_finished "
 		git checkout branch1 &&
-- 
2.35.0.rc1.864.g57621b115b6