Subject: [PATCH 2/2] checkout: forbid "-B <branch>" from touching a branch used elsewhere

From: Junio C Hamano <[email-masked]>
Date: Thu, 23 Nov 2023 15:00:31 +0900

"git checkout -B <branch> [<start-point>]", being a "forced" version
of "-b", switches to the <branch>, after optionally resetting its
tip to the <start-point>, even if the <branch> is in use in another
worktree, which is somewhat unexpected.

Protect the <branch> using the same logic that forbids "git checkout
<branch>" from touching a branch that is in use elsewhere.

This is a breaking change that may deserve backward compatibliity
warning in the Release Notes.  The "--ignore-other-worktrees" option
can be used as an escape hatch if the finger memory of existing
users depend on the current behaviour of "-B".

Reported-by: Willem Verstraeten <[email-masked]>
Signed-off-by: Junio C Hamano <[email-masked]>
---

 * The documentation might also need updates, but I didn't look at.

 builtin/checkout.c      | 7 +++++++
 t/t2060-switch.sh       | 2 ++
 t/t2400-worktree-add.sh | 8 ++++++++
 3 files changed, 17 insertions(+)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index b4ab972c5a..8a8ad23e98 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -1600,6 +1600,13 @@ static int checkout_branch(struct checkout_opts *opts,
 	if (new_branch_info->path && !opts->force_detach && !opts->new_branch)
 		die_if_switching_to_a_branch_in_use(opts, new_branch_info->path);
 
+	/* "git checkout -B <branch>" */
+	if (opts->new_branch_force) {
+		char *full_ref = xstrfmt("refs/heads/%s", opts->new_branch);
+		die_if_switching_to_a_branch_in_use(opts, full_ref);
+		free(full_ref);
+	}
+
 	if (!new_branch_info->commit && opts->new_branch) {
 		struct object_id rev;
 		int flag;
diff --git a/t/t2060-switch.sh b/t/t2060-switch.sh
index e247a4735b..c91c4db936 100755
--- a/t/t2060-switch.sh
+++ b/t/t2060-switch.sh
@@ -170,8 +170,10 @@ test_expect_success 'switch back when temporarily detached and checked out elsew
 	# we test in both worktrees to ensure that works
 	# as expected with "first" and "next" worktrees
 	test_must_fail git -C wt1 switch shared &&
+	test_must_fail git -C wt1 switch -C shared &&
 	git -C wt1 switch --ignore-other-worktrees shared &&
 	test_must_fail git -C wt2 switch shared &&
+	test_must_fail git -C wt2 switch -C shared &&
 	git -C wt2 switch --ignore-other-worktrees shared
 '
 
diff --git a/t/t2400-worktree-add.sh b/t/t2400-worktree-add.sh
index df4aff7825..bbcb2d3419 100755
--- a/t/t2400-worktree-add.sh
+++ b/t/t2400-worktree-add.sh
@@ -126,6 +126,14 @@ test_expect_success 'die the same branch is already checked out' '
 	)
 '
 
+test_expect_success 'refuse to reset a branch in use elsewhere' '
+	(
+		cd here &&
+		test_must_fail git checkout -B newmain 2>actual &&
+		grep "already used by worktree at" actual
+	)
+'
+
 test_expect_success SYMLINKS 'die the same branch is already checked out (symlink)' '
 	head=$(git -C there rev-parse --git-path HEAD) &&
 	ref=$(git -C there symbolic-ref HEAD) &&
-- 
2.43.0