worktree add: add --lock option

As explained in the document. This option has an advantage over the
command sequence "git worktree add && git worktree lock": there will be
no gap that somebody can accidentally "prune" the new worktree (or soon,
explicitly "worktree remove" it).

"worktree add" does keep a lock on while it's preparing the worktree.
If --lock is specified, this lock remains after the worktree is created.

Suggested-by: David Taylor <David.Taylor@dell.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

diff --git a/Documentation/git-worktree.txt b/Documentation/git-worktree.txt
index 553cf84..b472acc 100644
--- a/Documentation/git-worktree.txt
+++ b/Documentation/git-worktree.txt
@@ -9,7 +9,7 @@
 SYNOPSIS
 --------
 [verse]
-'git worktree add' [-f] [--detach] [--checkout] [-b <new-branch>] <path> [<branch>]
+'git worktree add' [-f] [--detach] [--checkout] [--lock] [-b <new-branch>] <path> [<branch>]
 'git worktree list' [--porcelain]
 'git worktree lock' [--reason <string>] <worktree>
 'git worktree prune' [-n] [-v] [--expire <expire>]
@@ -107,6 +107,11 @@
 	such as configuring sparse-checkout. See "Sparse checkout"
 	in linkgit:git-read-tree[1].
 
+--lock::
+	Keep the working tree locked after creation. This is the
+	equivalent of `git worktree lock` after `git worktree add`,
+	but without race condition.
+
 -n::
 --dry-run::
 	With `prune`, do not remove anything; just report what it would

builtin/worktree.c

diff --git a/builtin/worktree.c b/builtin/worktree.c
index 9993ded..05ade54 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -24,6 +24,7 @@ struct add_opts {
 	int force;
 	int detach;
 	int checkout;
+	int keep_locked;
 	const char *new_branch;
 	int force_new_branch;
 };
@@ -242,7 +243,10 @@ static int add_worktree(const char *path, const char *refname,
 	 * after the preparation is over.
 	 */
 	strbuf_addf(&sb, "%s/locked", sb_repo.buf);
-	write_file(sb.buf, "initializing");
+	if (!opts->keep_locked)
+		write_file(sb.buf, "initializing");
+	else
+		write_file(sb.buf, "added with --lock");
 
 	strbuf_addf(&sb_git, "%s/.git", path);
 	if (safe_create_leading_directories_const(sb_git.buf))
@@ -303,9 +307,11 @@ static int add_worktree(const char *path, const char *refname,
 	junk_git_dir = NULL;
 
 done:
-	strbuf_reset(&sb);
-	strbuf_addf(&sb, "%s/locked", sb_repo.buf);
-	unlink_or_warn(sb.buf);
+	if (ret || !opts->keep_locked) {
+		strbuf_reset(&sb);
+		strbuf_addf(&sb, "%s/locked", sb_repo.buf);
+		unlink_or_warn(sb.buf);
+	}
 	argv_array_clear(&child_env);
 	strbuf_release(&sb);
 	strbuf_release(&symref);
@@ -328,6 +334,7 @@ static int add(int ac, const char **av, const char *prefix)
 			   N_("create or reset a branch")),
 		OPT_BOOL(0, "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", &opts.keep_locked, N_("keep the new working tree locked")),
 		OPT_END()
 	};
 

t/t2025-worktree-add.sh

diff --git a/t/t2025-worktree-add.sh b/t/t2025-worktree-add.sh
index b618d6b..b5c47ac 100755
--- a/t/t2025-worktree-add.sh
+++ b/t/t2025-worktree-add.sh
@@ -63,6 +63,12 @@
 	)
 '
 
+test_expect_success '"add" worktree with lock' '
+	git rev-parse HEAD >expect &&
+	git worktree add --detach --lock here-with-lock master &&
+	test -f .git/worktrees/here-with-lock/locked
+'
+
 test_expect_success '"add" worktree from a subdir' '
 	(
 		mkdir sub &&