Subject: [PATCH v6 8/8] branch: protect branches checked out in all worktrees

From: Anders Kaseorg <[email-masked]>
Date: Fri, 12 Nov 2021 19:33:58 -0800

Refuse to force-move a branch over the currently checked out branch of
any working tree, not just the current one.

Signed-off-by: Anders Kaseorg <[email-masked]>
---
 branch.c          | 13 +++++++++----
 t/t3200-branch.sh |  7 +++++++
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/branch.c b/branch.c
index c7b9ba0e10..3a7d205fa4 100644
--- a/branch.c
+++ b/branch.c
@@ -199,7 +199,8 @@ int validate_branchname(const char *name, struct strbuf *ref)
  */
 int validate_new_branchname(const char *name, struct strbuf *ref, int force)
 {
-	const char *head;
+	struct worktree **worktrees;
+	const struct worktree *wt;
 
 	if (!validate_branchname(name, ref))
 		return 0;
@@ -208,9 +209,13 @@ int validate_new_branchname(const char *name, struct strbuf *ref, int force)
 		die(_("a branch named '%s' already exists"),
 		    ref->buf + strlen("refs/heads/"));
 
-	head = resolve_ref_unsafe("HEAD", 0, NULL, NULL);
-	if (!is_bare_repository() && head && !strcmp(head, ref->buf))
-		die(_("cannot force update the current branch"));
+	worktrees = get_worktrees();
+	wt = find_shared_symref(worktrees, "HEAD", ref->buf);
+	if (wt && !wt->is_bare)
+		die(_("cannot force update the branch '%s'"
+		      "checked out at '%s'"),
+		    ref->buf + strlen("refs/heads/"), wt->path);
+	free_worktrees(worktrees);
 
 	return 1;
 }
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index 6496e5dd38..797a2c752d 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -168,6 +168,13 @@ test_expect_success 'git branch -M foo bar should fail when bar is checked out'
 	test_must_fail git branch -M bar foo
 '
 
+test_expect_success 'git branch -M foo bar should fail when bar is checked out in worktree' '
+	git branch -f bar &&
+	test_when_finished "git worktree remove wt && git branch -D wt" &&
+	git worktree add wt &&
+	test_must_fail git branch -M bar wt
+'
+
 test_expect_success 'git branch -M baz bam should succeed when baz is checked out' '
 	git checkout -b baz &&
 	git branch bam &&
-- 
2.33.1