Merge branch 'ms/worktree-add-atomic-mkdir'

"git worktree add" used to do a "find an available name with stat
and then mkdir", which is race-prone.  This has been fixed by using
mkdir and reacting to EEXIST in a loop.

* ms/worktree-add-atomic-mkdir:
  worktree: fix worktree add race

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;