worktree: fix worktree add race

Git runs a stat loop to find a worktree name that's available and
then does mkdir on the found name. Turn it to mkdir loop to avoid
another invocation of worktree add finding the same free name and
creating the directory first.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -268,10 +268,10 @@ static int add_worktree(const char *path, const char *refname,
 	struct strbuf sb_git = STRBUF_INIT, sb_repo = STRBUF_INIT;
 	struct strbuf sb = STRBUF_INIT;
 	const char *name;
-	struct stat st;
 	struct child_process cp = CHILD_PROCESS_INIT;
 	struct argv_array child_env = ARGV_ARRAY_INIT;
-	int counter = 0, len, ret;
+	unsigned int counter = 0;
+	int len, ret;
 	struct strbuf symref = STRBUF_INIT;
 	struct commit *commit = NULL;
 	int is_branch = 0;
@@ -295,8 +295,12 @@ static int add_worktree(const char *path, const char *refname,
 	if (safe_create_leading_directories_const(sb_repo.buf))
 		die_errno(_("could not create leading directories of '%s'"),
 			  sb_repo.buf);
-	while (!stat(sb_repo.buf, &st)) {
+
+	while (mkdir(sb_repo.buf, 0777)) {
 		counter++;
+		if ((errno != EEXIST) || !counter /* overflow */)
+			die_errno(_("could not create directory of '%s'"),
+				  sb_repo.buf);
 		strbuf_setlen(&sb_repo, len);
 		strbuf_addf(&sb_repo, "%d", counter);
 	}
@@ -306,8 +310,6 @@ static int add_worktree(const char *path, const char *refname,
 	atexit(remove_junk);
 	sigchain_push_common(remove_junk_on_signal);
 
-	if (mkdir(sb_repo.buf, 0777))
-		die_errno(_("could not create directory of '%s'"), sb_repo.buf);
 	junk_git_dir = xstrdup(sb_repo.buf);
 	is_junk = 1;