Subject: [PATCH v4 18/26] switch: only allow explicit detached HEAD

From: Nguyễn Thái Ngọc Duy <[email-masked]>
Date: Sun, 17 Mar 2019 19:49:18 +0700

"git checkout <commit>" will checkout the commit in question and
detach HEAD from the current branch. It is naturally a right thing to
do once you get git references. But detached HEAD is a scary concept
to new users because we show a lot of warnings and stuff, and it could
be hard to get out of (until you know better).

To keep switch a bit more friendly to new users, we only allow
entering detached HEAD mode when --detach is given. "git
switch" must take a branch (unless you create a new branch,
then of course switch can take any commit-ish)

Signed-off-by: Nguyễn Thái Ngọc Duy <[email-masked]>
---
 builtin/checkout.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 73b7373414..d72e28aabb 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -45,6 +45,7 @@ struct checkout_opts {
 	int merge;
 	int force;
 	int force_detach;
+	int implicit_detach;
 	int writeout_stage;
 	int overwrite_ignore;
 	int ignore_skipworktree;
@@ -1298,6 +1299,29 @@ static int switch_unborn_to_new_branch(const struct checkout_opts *opts)
 	return status;
 }
 
+static void die_expecting_a_branch(const struct branch_info *branch_info)
+{
+	struct object_id oid;
+	char *to_free;
+
+	if (dwim_ref(branch_info->name, strlen(branch_info->name), &oid, &to_free) == 1) {
+		const char *ref = to_free;
+
+		if (skip_prefix(ref, "refs/tags/", &ref))
+			die(_("a branch is expected, got tag '%s'"), ref);
+		if (skip_prefix(ref, "refs/remotes/", &ref))
+			die(_("a branch is expected, got remote branch '%s'"), ref);
+		die(_("a branch is expected, got '%s'"), ref);
+	}
+	if (branch_info->commit)
+		die(_("a branch is expected, got commit '%s'"), branch_info->name);
+	/*
+	 * This case should never happen because we already die() on
+	 * non-commit, but just in case.
+	 */
+	die(_("a branch is expected, got '%s'"), branch_info->name);
+}
+
 static int checkout_branch(struct checkout_opts *opts,
 			   struct branch_info *new_branch_info)
 {
@@ -1345,6 +1369,14 @@ static int checkout_branch(struct checkout_opts *opts,
 	    !opts->force_detach)
 		die(_("missing branch or commit argument"));
 
+	if (!opts->implicit_detach &&
+	    !opts->force_detach &&
+	    !opts->new_branch &&
+	    !opts->new_branch_force &&
+	    new_branch_info->name &&
+	    !new_branch_info->path)
+		die_expecting_a_branch(new_branch_info);
+
 	if (new_branch_info->path && !opts->force_detach && !opts->new_branch &&
 	    !opts->ignore_other_worktrees) {
 		int flag;
@@ -1602,6 +1634,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	opts.no_dwim_new_local_branch = 0;
 	opts.switch_branch_doing_nothing_is_ok = 1;
 	opts.accept_pathspec = 1;
+	opts.implicit_detach = 1;
 
 	options = parse_options_dup(checkout_options);
 	options = add_common_options(&opts, options);
@@ -1633,6 +1666,7 @@ int cmd_switch(int argc, const char **argv, const char *prefix)
 	opts.no_dwim_new_local_branch = 0;
 	opts.accept_pathspec = 0;
 	opts.switch_branch_doing_nothing_is_ok = 0;
+	opts.implicit_detach = 0;
 
 	options = parse_options_dup(switch_options);
 	options = add_common_options(&opts, options);
-- 
2.21.0.548.gd3c7d92dc2