Fixes: debbugs:20832

* lisp/calendar/todo-mode.el (todo-show): Signal an error
if buffer for adding new todo file is empty but modified.

diff --git a/lisp/calendar/todo-mode.el b/lisp/calendar/todo-mode.el
index dcc960fb9bfa81a6aa1737218cb12a6886198476..6e975249a3fa3169e6b607b73df1007f561e5d4a 100644 (file)
--- a/lisp/calendar/todo-mode.el
+++ b/lisp/calendar/todo-mode.el
@@ -743,6 +743,11 @@ corresponding todo file, displaying the corresponding category."
            (setq todo-category-number (todo-category-number cat)))
          ;; If this is a new todo file, add its first category.
          (when (zerop (buffer-size))
+            ;; Don't confuse an erased buffer with a fresh buffer for
+            ;; adding a new todo file -- it might have been erased by
+            ;; mistake or due to a bug (e.g. Bug#20832).
+            (when (buffer-modified-p)
+              (error "Buffer is empty but modified, please report a bug"))
            (let (cat-added)
              (unwind-protect
                  (setq todo-category-number