worktree add: add --orphan flag

Add support for creating an orphan branch when adding a new worktree.
The functionality of this flag is equivalent to git switch's --orphan
option.

Current Behavior:
% git -C foo.git --no-pager branch -l
+ main
% git -C foo.git worktree add main/
Preparing worktree (new branch 'main')
HEAD is now at 6c93a75 a commit
%

% git init bar.git
Initialized empty Git repository in /path/to/bar.git/
% git -C bar.git --no-pager branch -l

% git -C bar.git worktree add main/
Preparing worktree (new branch 'main')
fatal: not a valid object name: 'HEAD'
%

New Behavior:

% git -C foo.git --no-pager branch -l
+ main
% git -C foo.git worktree add main/
Preparing worktree (new branch 'main')
HEAD is now at 6c93a75 a commit
%

% git init --bare bar.git
Initialized empty Git repository in /path/to/bar.git/
% git -C bar.git --no-pager branch -l

% git -C bar.git worktree add main/
Preparing worktree (new branch 'main')
fatal: invalid reference: HEAD
% git -C bar.git worktree add --orphan -b main/
Preparing worktree (new branch 'main')
% git -C bar.git worktree add --orphan -b newbranch worktreedir/
Preparing worktree (new branch 'newbranch')
%

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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>]
+		   [--orphan] [(-b | -B) <new-branch>] <path> [<commit-ish>]
 'git worktree list' [-v | --porcelain [-z]]
 'git worktree lock' [--reason <string>] <worktree>
 'git worktree move' <worktree> <new-path>
@@ -222,6 +222,10 @@ 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::
+	With `add`, make the new worktree and index empty, associating
+	the worktree with a new orphan/unborn branch named `<new-branch>`.
+
 --porcelain::
 	With `list`, output in an easy-to-parse format for scripts.
 	This format will remain stable across Git versions and regardless of user

builtin/worktree.c

@@ -22,7 +22,8 @@
 
 #define BUILTIN_WORKTREE_ADD_USAGE \
 	N_("git worktree add [-f] [--detach] [--checkout] [--lock [--reason <string>]]\n" \
-	   "                 [(-b | -B) <new-branch>] <path> [<commit-ish>]")
+	   "                 [--orphan] [(-b | -B) <new-branch>] <path> [<commit-ish>]")
+
 #define BUILTIN_WORKTREE_LIST_USAGE \
 	N_("git worktree list [-v | --porcelain [-z]]")
 #define BUILTIN_WORKTREE_LOCK_USAGE \
@@ -95,6 +96,7 @@ struct add_opts {
 	int detach;
 	int quiet;
 	int checkout;
+	int orphan;
 	const char *keep_locked;
 };
 
@@ -368,6 +370,22 @@ static int checkout_worktree(const struct add_opts *opts,
 	return run_command(&cp);
 }
 
+static int make_worktree_orphan(const char * ref, const struct add_opts *opts,
+				struct strvec *child_env)
+{
+	struct strbuf symref = STRBUF_INIT;
+	struct child_process cp = CHILD_PROCESS_INIT;
+
+	validate_new_branchname(ref, &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);
+	strbuf_release(&symref);
+	cp.git_cmd = 1;
+	return run_command(&cp);
+}
+
 static int add_worktree(const char *path, const char *refname,
 			const struct add_opts *opts)
 {
@@ -397,7 +415,7 @@ 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->orphan)
 		die(_("invalid reference: %s"), refname);
 
 	name = worktree_basename(path, &len);
@@ -486,10 +504,10 @@ 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)
@@ -501,6 +519,10 @@ static int add_worktree(const char *path, const char *refname,
 	if (ret)
 		goto done;
 
+	if (opts->orphan &&
+	    (ret = make_worktree_orphan(refname, opts, &child_env)))
+		goto done;
+
 	if (opts->checkout &&
 	    (ret = checkout_worktree(opts, &child_env)))
 		goto done;
@@ -520,7 +542,7 @@ done:
 	 * 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) {
 		struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
 
 		strvec_pushl(&opt.env, "GIT_DIR", "GIT_WORK_TREE", NULL);
@@ -568,7 +590,7 @@ static void print_preparing_worktree_line(int detach,
 		else {
 			struct commit *commit = lookup_commit_reference_by_name(branch);
 			if (!commit)
-				die(_("invalid reference: %s"), branch);
+				BUG(_("unreachable: invalid reference: %s"), branch);
 			fprintf_ln(stderr, _("Preparing worktree (detached HEAD %s)"),
 				  repo_find_unique_abbrev(the_repository, &commit->object.oid, DEFAULT_ABBREV));
 		}
@@ -620,6 +642,7 @@ 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_BOOL(0, "orphan", &opts.orphan, N_("create unborn/orphaned 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")),
@@ -640,6 +663,17 @@ static int add(int ac, const char **av, const char *prefix)
 	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");
+	if (opts.detach && opts.orphan)
+		die(_("options '%s', and '%s' cannot be used together"),
+		    "--orphan", "--detach");
+	if (opts.orphan && opt_track)
+		die(_("'%s' and '%s' cannot be used together"), "--orphan", "--track");
+	if (opts.orphan && !opts.checkout)
+		die(_("'%s' and '%s' cannot be used together"), "--orphan",
+		    "--no-checkout");
+	if (opts.orphan && ac == 2)
+		die(_("'%s' and '%s' cannot be used together"), "--orphan",
+		    _("<commit-ish>"));
 	if (lock_reason && !keep_locked)
 		die(_("the option '%s' requires '%s'"), "--reason", "--lock");
 	if (lock_reason)
@@ -668,13 +702,17 @@ static int add(int ac, const char **av, const char *prefix)
 		strbuf_release(&symref);
 	}
 
-	if (ac < 2 && !new_branch && !opts.detach) {
+	if (opts.orphan && !new_branch) {
+		int n;
+		const char *s = worktree_basename(path, &n);
+		new_branch = xstrndup(s, n);
+	} else if (new_branch || opts.detach || opts.orphan) {
+		// No-op
+	} else if (ac < 2) {
 		const char *s = dwim_branch(path, &new_branch);
 		if (s)
 			branch = s;
-	}
+	} else if (ac == 2) {
-
-	if (ac == 2 && !new_branch && !opts.detach) {
 		struct object_id oid;
 		struct commit *commit;
 		const char *remote;
@@ -688,10 +726,17 @@ static int add(int ac, const char **av, const char *prefix)
 			}
 		}
 	}
+
+	if (!opts.orphan && !lookup_commit_reference_by_name(branch)) {
+		die(_("invalid reference: %s"), branch);
+	}
+
 	if (!opts.quiet)
 		print_preparing_worktree_line(opts.detach, branch, new_branch, !!new_branch_force);
 
-	if (new_branch) {
+	if (opts.orphan) {
+		branch = new_branch;
+	} else if (new_branch) {
 		struct child_process cp = CHILD_PROCESS_INIT;
 		cp.git_cmd = 1;
 		strvec_push(&cp.args, "branch");

t/t2400-worktree-add.sh

@@ -312,6 +312,10 @@ test_wt_add_excl () {
 test_wt_add_excl -b poodle -B poodle bamboo main
 test_wt_add_excl -b poodle --detach bamboo main
 test_wt_add_excl -B poodle --detach bamboo main
+test_wt_add_excl --orphan --detach bamboo
+test_wt_add_excl --orphan --no-checkout bamboo
+test_wt_add_excl --orphan bamboo main
+test_wt_add_excl --orphan -b bamboo wtdir/ main
 
 test_expect_success '"add -B" fails if the branch is checked out' '
 	git rev-parse newmain >before &&
@@ -341,6 +345,62 @@ test_expect_success 'add --quiet -b' '
 	test_must_be_empty actual
 '
 
+test_expect_success '"add --orphan"' '
+	test_when_finished "git worktree remove -f -f orphandir" &&
+	git worktree add --orphan -b neworphan orphandir &&
+	echo refs/heads/neworphan >expected &&
+	git -C orphandir symbolic-ref HEAD >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success '"add --orphan (no -b)"' '
+	test_when_finished "git worktree remove -f -f neworphan" &&
+	git worktree add --orphan neworphan &&
+	echo refs/heads/neworphan >expected &&
+	git -C neworphan symbolic-ref HEAD >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success '"add --orphan --quiet"' '
+	test_when_finished "git worktree remove -f -f orphandir" &&
+	git worktree add --quiet --orphan -b neworphan orphandir 2>log.actual &&
+	test_must_be_empty log.actual &&
+	echo refs/heads/neworphan >expected &&
+	git -C orphandir symbolic-ref HEAD >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success '"add --orphan" fails if the branch already exists' '
+	test_when_finished "git branch -D existingbranch" &&
+	git worktree add -b existingbranch orphandir main &&
+	git worktree remove orphandir &&
+	test_must_fail git worktree add --orphan -b existingbranch 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 -b newbranch worktreedir &&
+	git -C empty_repo/worktreedir symbolic-ref HEAD >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success '"add" worktree with orphan branch and lock' '
+	git worktree add --lock --orphan -b orphanbr orphan-with-lock &&
+	test_when_finished "git worktree unlock orphan-with-lock || :" &&
+	test -f .git/worktrees/orphan-with-lock/locked
+'
+
+test_expect_success '"add" worktree with orphan branch, lock, and reason' '
+	lock_reason="why not" &&
+	git worktree add --detach --lock --reason "$lock_reason" orphan-with-lock-reason main &&
+	test_when_finished "git worktree unlock orphan-with-lock-reason || :" &&
+	test -f .git/worktrees/orphan-with-lock-reason/locked &&
+	echo "$lock_reason" >expect &&
+	test_cmp expect .git/worktrees/orphan-with-lock-reason/locked
+'
+
 test_expect_success 'local clone from linked checkout' '
 	git clone --local here here-clone &&
 	( cd here-clone && git fsck )
@@ -457,6 +517,14 @@ setup_remote_repo () {
 	)
 }
 
+test_expect_success '"add" <path> <remote/branch> w/ no HEAD' '
+	test_when_finished rm -rf repo_upstream repo_local foo &&
+	setup_remote_repo repo_upstream repo_local &&
+	git -C repo_local config --bool core.bare true &&
+	git -C repo_local branch -D main &&
+	git -C repo_local worktree add ./foo repo_upstream/foo
+'
+
 test_expect_success '--no-track avoids setting up tracking' '
 	test_when_finished rm -rf repo_upstream repo_local foo &&
 	setup_remote_repo repo_upstream repo_local &&