Support deleting symlinks in Lwt_io.with_temp_dir on Windows

If a Windows symlink points to a directory, then rmdir is the correct
function to call. If it points to a file, then unlink is the correct
function. When pointing to a file, the symlink can also have the
read-only attribute, which makes it subject of the little chmod dance.

CHANGES

@@ -12,6 +12,8 @@
  the behavior of Lwt_io.with_temp_dir following symlinks to
  directories on Win32. (#883, Antonin Décimo)
 
+ * Support deleting symlinks on Windows during cleanup of Lwt_io.with_temp_dir (#886, Antonin Décimo)
+
 ====== Additions ======
 
  * Lwt_bytes.blit_from_string: string complement of Lwt_bytes.blit (#882, Hugo Heuzard).

src/unix/lwt_io.ml

@@ -1509,11 +1509,16 @@ let rec delete_recursively directory =
  Lwt.return ()
  else
  let path = Filename.concat directory entry in
- Lwt_unix.lstat path >>= fun stat ->
- if stat.Lwt_unix.st_kind = Lwt_unix.S_DIR then
- delete_recursively path
- else
- unlink path
+ Lwt_unix.lstat path >>= fun {Lwt_unix.st_kind; _} ->
+ match st_kind with
+ | S_DIR -> delete_recursively path
+ | S_LNK when (Sys.win32 || Sys.cygwin) ->
+ Lwt_unix.stat path >>= fun {Lwt_unix.st_kind; _} ->
+ begin match st_kind with
+ | S_DIR -> Lwt_unix.rmdir path
+ | _ -> unlink path
+ end
+ | _ -> unlink path
  end >>= fun () ->
  Lwt_unix.rmdir directory