Reimplement auto-saving to visited files

This reacts to confusing behavior of 'auto-save-visited-file-name',
cf. Bug#25478.

* lisp/files.el (auto-save-visited-interval): New customization option.
(auto-save-visited-mode): New global minor mode.
(auto-save-visited-file-name): Make obsolete.
(auto-save--timer): New internal helper variable.

* doc/emacs/files.texi (Auto Save Files): Document
'auto-save-visited-mode' instead of obsolete
'auto-save-visited-file-name'.
(Auto Save Control): Document customization option
'auto-save-visited-interval'.

doc/emacs/files.texi

@@ -1040,14 +1040,23 @@ useful if it contains the deleted text.  To reenable auto-saving after
 this happens, save the buffer with @kbd{C-x C-s}, or use @kbd{C-u 1 M-x
 auto-save-mode}.
 
-@vindex auto-save-visited-file-name
+@vindex auto-save-visited-mode
   If you want auto-saving to be done in the visited file rather than
-in a separate auto-save file, set the variable
-@code{auto-save-visited-file-name} to a non-@code{nil} value.  In this
-mode, auto-saving is very similar to explicit saving.  However,
-differences still exist, in particular for modes which modify the
-buffer-saving process in non-trivial ways via various hooks
-(@pxref{Saving Buffers,,, elisp, The Emacs Lisp Reference Manual}).
+in a separate auto-save file, enable the global minor mode
+@code{auto-save-visited-mode}.  In this mode, auto-saving is identical
+to explicit saving.  Note that this mode is orthogonal to the
+@code{auto-save} mode described above; you can enable both at the same
+time.  However, if @code{auto-save} mode is active in some buffer and
+the obsolete @code{auto-save-visited-file-name} variable is set to a
+non-@code{nil} value, that buffer won't be affected by
+@code{auto-save-visited-mode}.
+
+  You can use the variable @code{auto-save-visited-interval} to
+customize the interval between auto-save operations in
+@code{auto-save-visited-mode}; by default it's five seconds.
+@code{auto-save-interval} and @code{auto-save-timeout} have no effect
+on @code{auto-save-visited-mode}.  @xref{Auto Save Control} for
+details on these variables.
 
 @vindex delete-auto-save-files
   A buffer's auto-save file is deleted when you save the buffer in its
@@ -1091,6 +1100,12 @@ makes sure all your work is saved if you go away from the terminal for
 a while; second, it may avoid some auto-saving while you are actually
 typing.
 
+@vindex auto-save-visited-interval
+  When @code{auto-save-visited-mode} is enabled, Emacs will auto-save
+file-visiting buffers after five seconds of idle time.  You can
+customize the variable @code{auto-save-visited-interval} to change the
+idle time interval.
+
   Emacs also does auto-saving whenever it gets a fatal error.  This
 includes killing the Emacs job with a shell command such as @samp{kill
 %emacs}, or disconnecting a phone line or network connection.

etc/NEWS

@@ -383,6 +383,12 @@ Strings such as ΌΣΟΣ are now correctly converted to Όσος when
 capitalized instead of incorrect Όσοσ (compare lowercase sigma at the
 end of the word).
 
+** Emacs can now auto-save buffers to visited files in a more robust
+manner via the new mode 'auto-save-visited-mode'.  Unlike
+'auto-save-visited-file-name', this mode uses the normal saving
+procedure and therefore obeys saving hooks.
+'auto-save-visited-file-name' is now obsolete.
+
 
 * Changes in Specialized Modes and Packages in Emacs 26.1
 

lisp/files.el

@@ -393,6 +393,46 @@ ignored."
   :initialize 'custom-initialize-delay
   :version "21.1")
 
+(defvar auto-save--timer nil "Timer for `auto-save-visited-mode'.")
+
+(defcustom auto-save-visited-interval 5
+  "Interval in seconds for `auto-save-visited-mode'.
+If `auto-save-visited-mode' is enabled, Emacs will save all
+buffers visiting a file to the visited file after it has been
+idle for `auto-save-visited-interval' seconds."
+  :group 'auto-save
+  :type 'number
+  :version "26.1"
+  :set (lambda (symbol value)
+         (set-default symbol value)
+         (when auto-save--timer
+           (timer-set-idle-time auto-save--timer value :repeat))))
+
+(define-minor-mode auto-save-visited-mode
+  "Toggle automatic saving to file-visiting buffers on or off.
+With a prefix argument ARG, enable regular saving of all buffers
+visiting a file if ARG is positive, and disable it otherwise.
+Unlike `auto-save-mode', this mode will auto-save buffer contents
+to the visited files directly and will also run all save-related
+hooks.  See Info node `Saving' for details of the save process.
+
+If called from Lisp, enable the mode if ARG is omitted or nil,
+and toggle it if ARG is `toggle'."
+  :group 'auto-save
+  :global t
+  (when auto-save--timer (kill-timer auto-save--timer))
+  (setq auto-save--timer
+        (when auto-save-visited-mode
+          (run-with-idle-timer
+           auto-save-visited-interval :repeat
+           #'save-some-buffers :no-prompt
+           (lambda ()
+             (not (and buffer-auto-save-file-name
+                       auto-save-visited-file-name)))))))
+
+(make-obsolete-variable 'auto-save-visited-file-name 'auto-save-visited-mode
+                        "Emacs 26.1")
+
 (defcustom save-abbrevs t
   "Non-nil means save word abbrevs too when files are saved.
 If `silently', don't ask the user before saving."