Re: Don't complain about changed file when it hasn't changed

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> Anyway, the proposed implementation misses a few subtleties, IMO:

Revised patch,


        Stefan


diff --git a/etc/NEWS b/etc/NEWS
index 1290fa4..cdabeda 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -213,6 +213,10 @@ In modes where form feed was treated as a whitespace character,
 It now deletes whitespace after the last form feed thus behaving the
 same as in modes where the character is not whitespace.
 
+** No more prompt about changed file when the file's content is unchanged.
+Instead of only checking the modification time, Emacs now also checks
+the file's actual content before prompting the user.
+
 \f
 * Changes in Specialized Modes and Packages in Emacs 25.2
 
diff --git a/lisp/userlock.el b/lisp/userlock.el
index a0c55fd..2295184 100644
--- a/lisp/userlock.el
+++ b/lisp/userlock.el
@@ -97,6 +97,28 @@ ask-user-about-lock-help
 
 (define-error 'file-supersession nil 'file-error)
 
+(defun userlock--check-content-unchanged (fn)
+  (with-demoted-errors "Unchanged content check: %S"
+    (save-restriction
+      (widen)
+      (let ((buf (current-buffer))
+            (cs buffer-file-coding-system)
+            (start (point-min))
+            (end (point-max)))
+        ;; FIXME: To avoid a slow `insert-file-contents' on large or
+        ;; remote files, it'd be good to include file size in the
+        ;; "visited-modtime" check.
+        (when (with-temp-buffer
+                (let ((coding-system-for-read cs))
+                  (insert-file-contents fn))
+                (when (= (buffer-size) (- end start)) ;Minor optimization.
+                  (= 0 (let ((case-fold-search nil))
+                         (compare-buffer-substrings
+                          buf start end
+                          (current-buffer) (point-min) (point-max))))))
+          (set-visited-file-modtime)
+          'unchanged)))))
+
 ;;;###autoload
 (defun ask-user-about-supersession-threat (fn)
   "Ask a user who is about to modify an obsolete buffer what to do.
@@ -106,30 +128,30 @@ ask-user-about-supersession-threat
 
 You can rewrite this to use any criterion you like to choose which one to do.
 The buffer in question is current when this function is called."
-  (discard-input)
-  (save-window-excursion
-    (let ((prompt
-	   (format "%s changed on disk; \
+  (unless (userlock--check-content-unchanged fn)
+    (discard-input)
+    (save-window-excursion
+      (let ((prompt
+             (format "%s changed on disk; \
 really edit the buffer? (y, n, r or C-h) "
-		   (file-name-nondirectory fn)))
-	  (choices '(?y ?n ?r ?? ?\C-h))
-	  answer)
-      (while (null answer)
-	(setq answer (read-char-choice prompt choices))
-	(cond ((memq answer '(?? ?\C-h))
-	       (ask-user-about-supersession-help)
-	       (setq answer nil))
-	      ((eq answer ?r)
-	       ;; Ask for confirmation if buffer modified
-	       (revert-buffer nil (not (buffer-modified-p)))
-	       (signal 'file-supersession
-		       (list "File reverted" fn)))
-	      ((eq answer ?n)
-	       (signal 'file-supersession
-		       (list "File changed on disk" fn)))))
-      (message
-       "File on disk now will become a backup file if you save these changes.")
-      (setq buffer-backed-up nil))))
+                     (file-name-nondirectory fn))))
+        (while
+            (let ((answer (read-char-choice prompt '(?y ?n ?r ?? ?\C-h))))
+              (cond ((memq answer '(?? ?\C-h))
+                     (ask-user-about-supersession-help)
+                     'repeat)
+                    ((eq answer ?r)
+                     ;; Ask for confirmation if buffer modified
+                     (revert-buffer nil (not (buffer-modified-p)))
+                     (signal 'file-supersession
+                             (list "File reverted" fn)))
+                    ((eq answer ?n)
+                     (signal 'file-supersession
+                             (list "File changed on disk" fn)))
+                    (t (null answer)))))
+        (message
+         "File on disk now will become a backup file if you save these changes.")
+        (setq buffer-backed-up nil)))))
 
 (defun ask-user-about-supersession-help ()
   (with-output-to-temp-buffer "*Help*"