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

diff --combined builtin/worktree.c
index 6cc094a453806308442d054a7d1c073ecf18b87b,85a604cfe98c03d9f1c9fb88347cb8a9ece45b9f..d2a7e2f3f18ba411d065a52ac868db612de5beaa
--- 1/builtin/worktree.c
--- 2/builtin/worktree.c
+++ b/builtin/worktree.c
@@@ -268,10 -268,10 +268,10 @@@ static int add_worktree(const char *pat
        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 +295,12 @@@
        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 +310,6 @@@
        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;
  
@@@ -402,7 -404,6 +404,7 @@@ done
                        cp.dir = path;
                        cp.env = env;
                        cp.argv = NULL;
 +                      cp.trace2_hook_name = "post-checkout";
                        argv_array_pushl(&cp.args, absolute_path(hook),
                                         oid_to_hex(&null_oid),
                                         oid_to_hex(&commit->object.oid),