bug#25104: 25.1; cursor-sensor--detect can go beyond buffer bounds

[npostavs]

tags: patch

>From emacs -Q, insert the following in *scratch*, and evaluate it
twice.

(with-current-buffer (get-buffer-create "foo")
  (erase-buffer)
  (insert (make-string 200 ?\n))
  (cursor-sensor-mode +1)
  (display-buffer (current-buffer)))

The second time an error is triggered, here is a backtrace:

Debugger entered--Lisp error: (args-out-of-range 0)
  get-char-property(0 cursor-sensor-functions)
  cursor-sensor--detect(#<window 9 on *Backtrace*>)
  run-hook-with-args(cursor-sensor--detect #<window 9 on *Backtrace*>)
  redisplay--pre-redisplay-functions((#<window 9 on *Backtrace*>))
  apply(redisplay--pre-redisplay-functions (#<window 9 on *Backtrace*>))
  #[128 "\300\301\"\210\300\302\"\207" [apply 
redisplay--pre-redisplay-functions ignore nil] 4 nil nil]((#<window 9 on 
*Backtrace*>))
  redisplay_internal\ \(C\ function\)()

cursor-sensor-mode is a new feature in Emacs 25, so I propose the
following patch for emacs-25:

>From 67817cd0727a1b257952afb93fc9a8238965b3c4 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Sun, 4 Dec 2016 01:06:52 -0500
Subject: [PATCH v1] Don't assume window-point and point are the same

The function `cursor-sensor--detect' calls `bobp' to decide whether to
check properties at (1- (window-point)).  However, (window-point) may be
at beginning of buffer, even if (point) is not.  In this case an
`args-out-of-range' error will be signaled.

* lisp/emacs-lisp/cursor-sensor.el (cursor-sensor--detect): Check the
value of (window-point) against (point-min), rather than (bobp) to
decide if (1- (window-point)) is accessible.
---
 lisp/emacs-lisp/cursor-sensor.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/emacs-lisp/cursor-sensor.el b/lisp/emacs-lisp/cursor-sensor.el
index 2231179..28b6188 100644
--- a/lisp/emacs-lisp/cursor-sensor.el
+++ b/lisp/emacs-lisp/cursor-sensor.el
@@ -114,7 +114,7 @@ cursor-sensor--detect
            ;; non-sticky on both ends, but that means get-pos-property might
            ;; never see it.
            (new (or (get-char-property point 'cursor-sensor-functions)
-                    (unless (bobp)
+                    (unless (<= (point-min) point)
                       (get-char-property (1- point) 
'cursor-sensor-functions))))
            (old (window-parameter window 'cursor-sensor--last-state))
            (oldposmark (car old))
-- 
2.9.3