feat: try to recover from missing tempdir #22573

Problem:
If vim_tempdir mysteriously goes missing (typically by "antivirus" on
Windows), any plugins using tempname() will be broken for the rest of
the session. #1432 #9833 https://groups.google.com/g/vim_use/c/ef55jNm5czI
Steps:
    mkdir foo
    TMPDIR=./foo nvim
    :echo tempname()
    !rm -r foo
    :echo tempname()
    tempname() still uses the foo path even though it was deleted.

Solution:
- Don't assume that vim_tempdir exists.
- If it goes missing once, retry vim_mktempdir and log (silently) an error.
- If it goes missing again, retry vim_mktempdir and show an error.

Rejected in Vim for performance reasons:
  https://groups.google.com/g/vim_use/c/qgRob9SWDv8/m/FAOFVVcDTv0J
  https://groups.google.com/g/vim_dev/c/cogp-Vye4oo/m/d_SVFXBbnnoJ
But, logging shows that `vim_gettempdir` is not called frequently.

Fixes #1432
Fixes #9833
Fixes #11250
Related: stdpath("run") f50135a32e

src/nvim/fileio.c

@@ -5369,10 +5369,21 @@ void vim_deltempdir(void)
 /// Creates the directory on the first call.
 char *vim_gettempdir(void)
 {
-  if (vim_tempdir == NULL) {
+  static int notfound = 0;
+  bool exists = false;
+  if (vim_tempdir == NULL || !(exists = os_isdir(vim_tempdir))) {
+    if (vim_tempdir != NULL && !exists) {
+      notfound++;
+      if (notfound == 1) {
+        ELOG("tempdir disappeared (antivirus or broken cleanup job?): %s", vim_tempdir);
+      }
+      if (notfound > 1) {
+        msg_schedule_semsg("E5431: tempdir disappeared (%d times)", notfound);
+      }
+      XFREE_CLEAR(vim_tempdir);
+    }
     vim_mktempdir();
   }
-
   return vim_tempdir;
 }