[PATCH 2/2] worktree add -B: do the checkout test before update branch

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

If --force is not given but -B is, we should not proceed if the given
branch is already checked out elsewhere. add_worktree() has this test,
but it kicks in too late when "git branch --force" is already
executed. As a result, even though we correctly refuse to create a new
worktree, we have already updated the branch and mess up the other
checkout.

Repeat the die_if_checked_out() test again for this specific case before
"git branch" runs.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Something extra while I was studying this code. I'm not so sure if
 this is the right way.
 
 Another option is do it in "git branch" which rejects with "Cannot
 force update the current branch", but only for current worktree.

 builtin/worktree.c      | 11 ++++++++++-
 t/t2025-worktree-add.sh |  7 +++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/builtin/worktree.c b/builtin/worktree.c
index 6b9c946..20cf67a 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -334,9 +334,18 @@ static int add(int ac, const char **av, const char *prefix)
 	branch = ac < 2 ? "HEAD" : av[1];
 
 	opts.force_new_branch = !!new_branch_force;
-	if (opts.force_new_branch)
+	if (opts.force_new_branch) {
+		struct strbuf symref = STRBUF_INIT;
+
 		opts.new_branch = new_branch_force;
 
+		if (!opts.force &&
+		    !strbuf_check_branch_ref(&symref, opts.new_branch) &&
+		    ref_exists(symref.buf))
+			die_if_checked_out(symref.buf);
+		strbuf_release(&symref);
+	}
+
 	if (ac < 2 && !opts.new_branch && !opts.detach) {
 		int n;
 		const char *s = worktree_basename(path, &n);
diff --git a/t/t2025-worktree-add.sh b/t/t2025-worktree-add.sh
index a4d36c0..cbfa41e 100755
--- a/t/t2025-worktree-add.sh
+++ b/t/t2025-worktree-add.sh
@@ -193,6 +193,13 @@ test_expect_success '"add" -B/--detach mutually exclusive' '
 	test_must_fail git worktree add -B poodle --detach bamboo master
 '
 
+test_expect_success '"add -B" fails if the branch is checked out' '
+	git rev-parse newmaster >before &&
+	test_must_fail git worktree add -B newmaster bamboo master &&
+	git rev-parse newmaster >after &&
+	test_cmp before after
+'
+
 test_expect_success 'add -B' '
 	git worktree add -B poodle bamboo2 master^ &&
 	git -C bamboo2 symbolic-ref HEAD >actual &&
-- 
2.7.0.377.g4cd97dd