[PATCH v2 2/2] worktree add: add --orphan flag

[Jacob Abel <jacobabel@nullpo.dev>]

Adds support for creating an orphan branch when adding a new worktree.
This functionality is equivalent to git checkout's --orphan flag.

The original reason this feature was implemented was to allow a user
to initialise a new repository using solely the worktree oriented
workflow. Example usage included below.

$ GIT_DIR=".git" git init --bare
$ git worktree add --orphan master master/

Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>
---
 Documentation/git-worktree.txt | 18 +++++++-
 builtin/worktree.c             | 81 ++++++++++++++++++++++++++++------
 t/t2400-worktree-add.sh        | 50 +++++++++++++++++++++
 3 files changed, 135 insertions(+), 14 deletions(-)

diff --git a/Documentation/git-worktree.txt b/Documentation/git-worktree.txt
index 4dd658012b..92bd75564f 100644
--- a/Documentation/git-worktree.txt
+++ b/Documentation/git-worktree.txt
@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git worktree add' [-f] [--detach] [--checkout] [--lock [--reason <string>]]
-		   [[-b | -B] <new-branch>] <path> [<commit-ish>]
+		   [[-b | -B | --orphan] <new-branch>] <path> [<commit-ish>]
 'git worktree list' [-v | --porcelain [-z]]
 'git worktree lock' [--reason <string>] <worktree>
 'git worktree move' <worktree> <new-path>
@@ -95,6 +95,17 @@ exist, a new branch based on `HEAD` is automatically created as if
 `-b <branch>` was given.  If `<branch>` does exist, it will be checked out
 in the new worktree, if it's not checked out anywhere else, otherwise the
 command will refuse to create the worktree (unless `--force` is used).
++
+------------
+$ git worktree add --orphan <branch> <path> [<commit-ish>]
+------------
++
+Create a worktree containing an orphan branch named `<branch>` based
+on `<commit-ish>`. If `<commit-ish>` is not specified, the new orphan branch
+will be created based on `HEAD`.
++
+Note that unlike with `-b` or `-B`, this operation will succeed even if
+`<commit-ish>` is a branch that is currently checked out somewhere else.

 list::

@@ -222,6 +233,11 @@ This can also be set up as the default behaviour by using the
 	With `prune`, do not remove anything; just report what it would
 	remove.

+--orphan <new-branch>::
+	With `add`, create a new orphan branch named `<new-branch>` in the new
+	worktree based on `<commit-ish>`. If `<commit-ish>` is omitted, it
+	defaults to `HEAD`.
+
 --porcelain::
 	With `list`, output in an easy-to-parse format for scripts.
 	This format will remain stable across Git versions and regardless of user
diff --git a/builtin/worktree.c b/builtin/worktree.c
index fccb17f070..303a8fb935 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -17,7 +17,7 @@

 #define BUILTIN_WORKTREE_ADD_USAGE \
 	N_("git worktree add [-f] [--detach] [--checkout] [--lock [--reason <string>]]\n" \
-	   "                 [[-b | -B] <new-branch>] <path> [<commit-ish>]")
+	   "                 [[-b | -B | --orphan] <new-branch>] <path> [<commit-ish>]")
 #define BUILTIN_WORKTREE_LIST_USAGE \
 	N_("git worktree list [-v | --porcelain [-z]]")
 #define BUILTIN_WORKTREE_LOCK_USAGE \
@@ -90,6 +90,8 @@ struct add_opts {
 	int detach;
 	int quiet;
 	int checkout;
+	int implicit;
+	const char *orphan_branch;
 	const char *keep_locked;
 };

@@ -364,6 +366,24 @@ static int checkout_worktree(const struct add_opts *opts,
 	return run_command(&cp);
 }

+static int make_worktree_orphan(const struct add_opts *opts,
+				struct strvec *child_env)
+{
+	int ret;
+	struct strbuf symref = STRBUF_INIT;
+	struct child_process cp = CHILD_PROCESS_INIT;
+	cp.git_cmd = 1;
+
+	validate_new_branchname(opts->orphan_branch, &symref, 0);
+	strvec_pushl(&cp.args, "symbolic-ref", "HEAD", symref.buf, NULL);
+	if (opts->quiet)
+		strvec_push(&cp.args, "--quiet");
+	strvec_pushv(&cp.env, child_env->v);
+	ret = run_command(&cp);
+	strbuf_release(&symref);
+	return ret;
+}
+
 static int add_worktree(const char *path, const char *refname,
 			const struct add_opts *opts)
 {
@@ -393,7 +413,7 @@ static int add_worktree(const char *path, const char *refname,
 			die_if_checked_out(symref.buf, 0);
 	}
 	commit = lookup_commit_reference_by_name(refname);
-	if (!commit)
+	if (!commit && !opts->implicit)
 		die(_("invalid reference: %s"), refname);

 	name = worktree_basename(path, &len);
@@ -482,10 +502,10 @@ static int add_worktree(const char *path, const char *refname,
 	strvec_pushf(&child_env, "%s=%s", GIT_WORK_TREE_ENVIRONMENT, path);
 	cp.git_cmd = 1;

-	if (!is_branch)
+	if (!is_branch && commit) {
 		strvec_pushl(&cp.args, "update-ref", "HEAD",
 			     oid_to_hex(&commit->object.oid), NULL);
-	else {
+	} else {
 		strvec_pushl(&cp.args, "symbolic-ref", "HEAD",
 			     symref.buf, NULL);
 		if (opts->quiet)
@@ -497,9 +517,13 @@ static int add_worktree(const char *path, const char *refname,
 	if (ret)
 		goto done;

-	if (opts->checkout &&
-	    (ret = checkout_worktree(opts, &child_env)))
-		goto done;
+	if (opts->checkout) {
+		ret = checkout_worktree(opts, &child_env);
+		if (opts->orphan_branch && !ret)
+			ret = make_worktree_orphan(opts, &child_env);
+		if (ret)
+			goto done;
+	}

 	is_junk = 0;
 	FREE_AND_NULL(junk_work_tree);
@@ -516,7 +540,7 @@ static int add_worktree(const char *path, const char *refname,
 	 * Hook failure does not warrant worktree deletion, so run hook after
 	 * is_junk is cleared, but do return appropriate code when hook fails.
 	 */
-	if (!ret && opts->checkout) {
+	if (!ret && opts->checkout && !opts->orphan_branch) {
 		struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;

 		strvec_pushl(&opt.env, "GIT_DIR", "GIT_WORK_TREE", NULL);
@@ -616,6 +640,8 @@ static int add(int ac, const char **av, const char *prefix)
 			   N_("create a new branch")),
 		OPT_STRING('B', NULL, &new_branch_force, N_("branch"),
 			   N_("create or reset a branch")),
+		OPT_STRING(0, "orphan", &opts.orphan_branch, N_("branch"),
+			   N_("new unparented branch")),
 		OPT_BOOL('d', "detach", &opts.detach, N_("detach HEAD at named commit")),
 		OPT_BOOL(0, "checkout", &opts.checkout, N_("populate the new working tree")),
 		OPT_BOOL(0, "lock", &keep_locked, N_("keep the new working tree locked")),
@@ -633,8 +659,16 @@ static int add(int ac, const char **av, const char *prefix)
 	memset(&opts, 0, sizeof(opts));
 	opts.checkout = 1;
 	ac = parse_options(ac, av, prefix, options, git_worktree_add_usage, 0);
-	if (!!opts.detach + !!new_branch + !!new_branch_force > 1)
-		die(_("options '%s', '%s', and '%s' cannot be used together"), "-b", "-B", "--detach");
+	opts.implicit = ac < 2;
+
+	if (!!opts.detach + !!new_branch + !!new_branch_force + !!opts.orphan_branch > 1)
+		die(_("options '%s', '%s', '%s', and '%s' cannot be used together"),
+		    "-b", "-B", "--orphan", "--detach");
+	if (opts.orphan_branch && opt_track)
+		die(_("'%s' and '%s' cannot be used together"), "--orphan", "--track");
+	if (opts.orphan_branch && !opts.checkout)
+		die(_("'%s' and '%s' cannot be used together"), "--orphan",
+		    "--no-checkout");
 	if (lock_reason && !keep_locked)
 		die(_("the option '%s' requires '%s'"), "--reason", "--lock");
 	if (lock_reason)
@@ -646,11 +680,18 @@ static int add(int ac, const char **av, const char *prefix)
 		usage_with_options(git_worktree_add_usage, options);

 	path = prefix_filename(prefix, av[0]);
-	branch = ac < 2 ? "HEAD" : av[1];
+	branch = opts.implicit ? "HEAD" : av[1];

 	if (!strcmp(branch, "-"))
 		branch = "@{-1}";

+	/*
+	 * When creating a new branch, new_branch now contains the branch to
+	 * create.
+	 *
+	 * Past this point, new_branch_force can be treated solely as a
+	 * boolean flag to indicate whether `-B` was selected.
+	 */
 	if (new_branch_force) {
 		struct strbuf symref = STRBUF_INIT;

@@ -663,7 +704,21 @@ static int add(int ac, const char **av, const char *prefix)
 		strbuf_release(&symref);
 	}

-	if (ac < 2 && !new_branch && !opts.detach) {
+	/*
+	 * As the orphan cannot be created until the contents of branch
+	 * are staged, opts.orphan_branch should be treated as both a boolean
+	 * indicating that `--orphan` was selected and as the name of the new
+	 * orphan branch from this point on.
+	 *
+	 * When creating a new orphan, force checkout regardless of whether
+	 * the existing branch is already checked out.
+	 */
+	if (opts.orphan_branch) {
+		new_branch = opts.orphan_branch;
+		opts.force = 1;
+	}
+
+	if (ac < 2 && !new_branch && !opts.detach && !opts.orphan_branch) {
 		const char *s = dwim_branch(path, &new_branch);
 		if (s)
 			branch = s;
@@ -686,7 +741,7 @@ static int add(int ac, const char **av, const char *prefix)
 	if (!opts.quiet)
 		print_preparing_worktree_line(opts.detach, branch, new_branch, !!new_branch_force);

-	if (new_branch) {
+	if (new_branch && !opts.orphan_branch) {
 		struct child_process cp = CHILD_PROCESS_INIT;
 		cp.git_cmd = 1;
 		strvec_push(&cp.args, "branch");
diff --git a/t/t2400-worktree-add.sh b/t/t2400-worktree-add.sh
index d587e0b20d..72930432f9 100755
--- a/t/t2400-worktree-add.sh
+++ b/t/t2400-worktree-add.sh
@@ -310,6 +310,26 @@ test_expect_success '"add" -B/--detach mutually exclusive' '
 	test_must_fail git worktree add -B poodle --detach bamboo main
 '

+test_expect_success '"add" --orphan/-b mutually exclusive' '
+	test_must_fail git worktree add --orphan poodle -b poodle bamboo main
+'
+
+test_expect_success '"add" --orphan/-B mutually exclusive' '
+	test_must_fail git worktree add --orphan poodle -B poodle bamboo main
+'
+
+test_expect_success '"add" --orphan/--detach mutually exclusive' '
+	test_must_fail git worktree add --orphan poodle --detach bamboo main
+'
+
+test_expect_success '"add" --orphan/--no-checkout mutually exclusive' '
+	test_must_fail git worktree add --orphan poodle --no-checkout bamboo main
+'
+
+test_expect_success '"add" -B/--detach mutually exclusive' '
+	test_must_fail git worktree add -B poodle --detach bamboo main
+'
+
 test_expect_success '"add -B" fails if the branch is checked out' '
 	git rev-parse newmain >before &&
 	test_must_fail git worktree add -B newmain bamboo main &&
@@ -330,6 +350,36 @@ test_expect_success 'add --quiet' '
 	test_must_be_empty actual
 '

+test_expect_success '"add --orphan"' '
+	test_when_finished "git worktree remove -f -f orphandir" &&
+	git worktree add --orphan neworphan orphandir main &&
+	echo refs/heads/neworphan >expected &&
+	git -C orphandir symbolic-ref HEAD >actual &&
+	test_cmp expected actual &&
+	git -C orphandir diff main
+'
+
+test_expect_success '"add --orphan" fails if the branch already exists' '
+	test_when_finished "git worktree remove -f -f orphandir" &&
+	git worktree add -b existingbranch orphandir main &&
+	test_must_fail git worktree add --orphan existingbranch orphandir2 main &&
+	test ! -d orphandir2
+'
+
+test_expect_success '"add --orphan" fails if the commit-ish doesnt exist' '
+	test_must_fail git worktree add --orphan badcommitish orphandir eee2222 &&
+	test ! -d orphandir
+'
+
+test_expect_success '"add --orphan" with empty repository' '
+	test_when_finished "rm -rf empty_repo" &&
+	echo refs/heads/newbranch >expected &&
+	GIT_DIR="empty_repo" git init --bare &&
+	git -C empty_repo  worktree add --orphan newbranch worktreedir &&
+	git -C empty_repo/worktreedir symbolic-ref HEAD >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'local clone from linked checkout' '
 	git clone --local here here-clone &&
 	( cd here-clone && git fsck )
--
2.37.4