checkout: forbid "-B <branch>" from touching a branch used elsewhere

"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 <willem.verstraeten@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -63,7 +63,9 @@ $ git checkout <branch>
 ------------
 +
 that is to say, the branch is not reset/created unless "git checkout" is
-successful.
+successful (e.g., when the branch is in use in another worktree, not
+just the current branch stays the same, but the branch is not reset to
+the start-point, either).
 
 'git checkout' --detach [<branch>]::
 'git checkout' [--detach] <commit>::

Documentation/git-switch.txt

@@ -59,13 +59,18 @@ out at most one of `A` and `B`, in which case it defaults to `HEAD`.
 -c <new-branch>::
 --create <new-branch>::
 	Create a new branch named `<new-branch>` starting at
-	`<start-point>` before switching to the branch. This is a
+	`<start-point>` before switching to the branch. This is the
-	convenient shortcut for:
+	transactional equivalent of
 +
 ------------
 $ git branch <new-branch>
 $ git switch <new-branch>
 ------------
++
+that is to say, the branch is not reset/created unless "git switch" is
+successful (e.g., when the branch is in use in another worktree, not
+just the current branch stays the same, but the branch is not reset to
+the start-point, either).
 
 -C <new-branch>::
 --force-create <new-branch>::

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;

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
 '
 

t/t2400-worktree-add.sh

@@ -126,6 +126,28 @@ test_expect_success 'die the same branch is already checked out' '
 	)
 '
 
+test_expect_success 'refuse to reset a branch in use elsewhere' '
+	(
+		cd here &&
+
+		# we know we are on detached HEAD but just in case ...
+		git checkout --detach HEAD &&
+		git rev-parse --verify HEAD >old.head &&
+
+		git rev-parse --verify refs/heads/newmain >old.branch &&
+		test_must_fail git checkout -B newmain 2>error &&
+		git rev-parse --verify refs/heads/newmain >new.branch &&
+		git rev-parse --verify HEAD >new.head &&
+
+		grep "already used by worktree at" error &&
+		test_cmp old.branch new.branch &&
+		test_cmp old.head new.head &&
+
+		# and we must be still on the same detached HEAD state
+		test_must_fail git symbolic-ref HEAD
+	)
+'
+
 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) &&