[PATCH] textmodes/flyspell.el (flyspell-post-command-hook): Decided whether to check previous word while in correct buffer.

[Michal Nazarewicz <mpn@google.com>]

From: Michal Nazarewicz <mina86@mina86.com>

If command changed the buffer, the decision may be made based on the
current buffer even though it should based on the previous one.  This
may lead to false positives and more importantly to errors since
`flyspell-pre-point' is buffer local so it may have unsanitised value
(such as nil) in previous buffer.

To be honest, I'm not sure how this can happen since
`flyspell-pre-point' is set in previous buffer, but nonetheless, I've
been encountering the error for quite some time and finally decided to
fix it.  Interestingly, line making `flyspell-pre-point'
a buffer-local variable has a very revealing "Why?? --Stef" comment.

While at it, remove uneccessary (progn ...).
---
 lisp/ChangeLog             | 14 ++++++++++++++
 lisp/textmodes/flyspell.el | 27 +++++++++++++--------------
 2 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 5b24164..ccd4c13 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,17 @@
+2013-03-26  Michal Nazarewicz  <mina86@mina86.com>
+
+	* textmodes/flyspell.el (flyspell-post-command-hook): Decided whether
+	to check previous word while in correct buffer.  If command changed
+	the buffer, the decision may be made based on the current buffer even
+	though it should based on the previous one.  This may lead to false
+	positives and more importantly to errors since `flyspell-pre-point' is
+	buffer local so it may have unsanitised value (such as nil) in
+	previous buffer.  To be honest, I'm not sure how this can happen since
+	`flyspell-pre-point' is set in previous buffer, but nonetheless, I've
+	been encountering the error for quite some time and finally decided to
+	fix it.  Interestingly, line making `flyspell-pre-point'
+	a buffer-local variable has a very revealing "Why?? --Stef" comment.
+
 2013-03-26  Stefan Monnier  <monnier@iro.umontreal.ca>
 
 	* desktop.el (desktop--v2s): Rename from desktop-internal-v2s.
diff --git a/lisp/textmodes/flyspell.el b/lisp/textmodes/flyspell.el
index 6ab3e3d..698dac0 100644
--- a/lisp/textmodes/flyspell.el
+++ b/lisp/textmodes/flyspell.el
@@ -955,12 +955,12 @@ Mostly we check word delimiters."
       (let ((command this-command)
             ;; Prevent anything we do from affecting the mark.
             deactivate-mark)
-        (if (flyspell-check-pre-word-p)
-            (with-current-buffer flyspell-pre-buffer
-              '(flyspell-debug-signal-pre-word-checked)
+        (when (buffer-live-p flyspell-pre-buffer)
+          (with-current-buffer flyspell-pre-buffer
+            (when (flyspell-check-pre-word-p)
               (save-excursion
                 (goto-char flyspell-pre-point)
-                (flyspell-word))))
+                (flyspell-word)))))
         (if (flyspell-check-word-p)
             (progn
               '(flyspell-debug-signal-word-checked)
@@ -974,16 +974,15 @@ Mostly we check word delimiters."
               ;; FLYSPELL-CHECK-PRE-WORD-P
               (setq flyspell-pre-pre-buffer (current-buffer))
               (setq flyspell-pre-pre-point  (point)))
-          (progn
-            (setq flyspell-pre-pre-buffer nil)
-            (setq flyspell-pre-pre-point  nil)
-            ;; when a word is not checked because of a delayed command
-            ;; we do not disable the ispell cache.
-            (if (and (symbolp this-command)
-                     (get this-command 'flyspell-delayed))
-                (progn
-                  (setq flyspell-word-cache-end -1)
-                  (setq flyspell-word-cache-result '_)))))
+          (setq flyspell-pre-pre-buffer nil)
+          (setq flyspell-pre-pre-point  nil)
+          ;; when a word is not checked because of a delayed command
+          ;; we do not disable the ispell cache.
+          (if (and (symbolp this-command)
+                   (get this-command 'flyspell-delayed))
+              (progn
+                (setq flyspell-word-cache-end -1)
+                (setq flyspell-word-cache-result '_))))
         (while (and (not (input-pending-p)) (consp flyspell-changes))
           (let ((start (car (car flyspell-changes)))
                 (stop  (cdr (car flyspell-changes))))
-- 
1.8.1.3.718.g2a547ef