bug#20832: 25.0.50; todo-show accidentally deleted my todo file.

[Nicolas Richard]

I managed to lose my todo-mode.el todo file. I don't really know how
this happened, I guess it was a combination of showing the todo file
while in the minibuffer and hitting C-g, but I noticed it too late, and
then could not reproduce.

In any case, I suspect that todo-mode.el itself deleted it, so I suggest
the following patch as a safety measure.

>From 9088fc5037f587eab00c083d703b0a8cbf3eaf31 Mon Sep 17 00:00:00 2001
From: Nicolas Richard <youngfrog@members.fsf.org>
Date: Thu, 28 May 2015 08:57:45 +0200
Subject: [PATCH] (todo-show): Throw an error if buffer is empty but was
 modified.

* lisp/calendar/todo-mode.el (todo-show): Throw an error if buffer
is empty but was modified.
---
 lisp/calendar/todo-mode.el | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lisp/calendar/todo-mode.el b/lisp/calendar/todo-mode.el
index 7369ec2..85d4a80 100644
--- a/lisp/calendar/todo-mode.el
+++ b/lisp/calendar/todo-mode.el
@@ -743,6 +743,8 @@ 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))
+            (when (buffer-modified-p)
+              (error "Buffer is empty but modified. Aborting."))
            (let (cat-added)
              (unwind-protect
                  (setq todo-category-number
-- 
1.9.1