bug#44864: 27.1; advice.el: ad-with-originals deprecated, but no advice on replacement

[Stefan Monnier <monnier@iro.umontreal.ca>]

> That sounds eminently sensible.  Scouring the rest of Dave's file, I
> found the following comment at the beginning of the function you've
> just quoted from:

Hmm... seems relevant indeed.

> (defun multi-install-mode (mode &optional chunk-fn base)
>   "Add MODE to the multiple major modes supported by the current buffer.
> CHUNK-FN, if non-nil, is a function to select the mode of a chunk,
> added to the list `multi-chunk-fns'.  BASE non-nil means that this
> is the base mode."
>   (unless (memq mode multi-indirect-buffers-alist) ; be idempotent
>     ;; This is part of a grim hack for lossage in AUCTeX, which
>     ;; bogusly advises `hack-one-local-variable'.  This loses, due to
>     ;; the way advice works, when we run `multi-hack-local-variables'
>     ;; below -- there ought to be a way round this, probably with CL's
>     ;; flet.  Any subsequent use of it then fails because advice has
>     ;; captured the now-unbound variable `late-hack'...  Thus ensure
>     ;; we've loaded the mode in advance to get any autoloads sorted
>     ;; out.  Do it generally in case other modes have similar
>     ;; problems.  [The AUCTeX stuff is in support of an undocumented
>     ;; feature which is unnecessary and, anyway, wouldn't need advice
>     ;; to implement.  Unfortunately the maintainer seems not to
>     ;; understand the local variables mechanism and wouldn't remove
>     ;; this.  To invoke minor modes, you should just use `mode:' in
>     ;; `local variables'.]

I'm not sure I understand the details of the problem described, but
I think I see the general idea.  Indeed when multi-mode.el does:

    (let ((late-hack (symbol-function 'hack-one-local-variable)))
      (fset 'hack-one-local-variable
            (lambda (var val)
              (unless (eq var 'mode)
                (funcall late-hack var val))))
      (unwind-protect
          (hack-local-variables)
        (fset 'hack-one-local-variable late-hack))))

if the code run within `hack-local-variables` ends up looking at the
definition of `hack-one-local-variable` and saving it somewhere for
later use (e.g. if `defadvice` is used at this moment, maybe because
AUCTeX was autoloaded during the course of running
`hack-local-variables`), then we could have a problem.  To some extent
the final `fset` above should hide the problem, but it probably just
ends up putting advice in an inconsistent state or worse (depending on
the version of `advice`).

I think with the new version of `advice.el` (introduced when
`nadvice.el` was introduced, to make them interact tolerably) the
problem should be minor: the final `fset` should not put `advice.el` in
an inconsistent state but should simply undo the `defadvice` that
happened during `hack-local-variables` (which is still undesirable,
arguably, but should be mostly harmless as long as you don't rely on
the feature implemented by that advice).

> So I'm guessing that's what he's referring to.

Indeed.

You might like to try the patch below (guaranteed 100% untested).
Do you think it would be worthwhile to add it to GNU ELPA?


        Stefan


diff --git a/multi-mode.el b/multi-mode.el
index cece376..06291cf 100644
--- a/multi-mode.el
+++ b/multi-mode.el
@@ -1,6 +1,6 @@
-;;; multi-mode.el --- support for multiple major modes
+;;; multi-mode.el --- support for multiple major modes  -*- lexical-binding: t; -*-
 
-;; Copyright (C) 2003, 2004, 2007, 2009  Free Software Foundation, Inc.
+;; Copyright (C) 2003-2020  Free Software Foundation, Inc.
 
 ;; Author: Dave Love <fx@gnu.org>
 ;; Keywords: languages, extensions, files
@@ -164,27 +164,26 @@ Buffer local.")
   "Original value of `imenu-create-index-function' for the buffer's mode.")
 (make-variable-buffer-local 'multi-late-index-function)
 
+(define-obsolete-variable-alias 'multi-alist 'multi-mode-alist nil)
 (defvar multi-mode-alist nil
   "Alist of elements (MODE . FUNCTION) specifying a buffer's multiple modes.
 MODE is a major mode and FUNCTION is a function used as an element of
 `multi-chunk-fns' or nil.  Use nil if MODE is detected by another element
 of the alist.")
 
-(if (fboundp 'define-obsolete-variable-alias)
-    (define-obsolete-variable-alias 'multi-alist 'multi-mode-alist)
-  (make-obsolete-variable 'multi-alist 'multi-mode-alist))
-
 ;; See the commentary below.
 (defun multi-hack-local-variables ()
   "Like `hack-local-variables', but ignore `mode' items."
-  (let ((late-hack (symbol-function 'hack-one-local-variable)))
-    (fset 'hack-one-local-variable
-	  (lambda (var val)
-	    (unless (eq var 'mode)
-	      (funcall late-hack var val))))
+  (let ((f (lambda (orig-fun var val)
+	     (unless (eq var 'mode)
+	       (funcall orig-fun var val)))))
     (unwind-protect
-	(hack-local-variables)
-      (fset 'hack-one-local-variable late-hack))))
+	(progn
+	  (advice-add 'hack-one-local-variable :around f)
+	  (hack-local-variables))
+      (advice-remove 'hack-one-local-variable f))))
+
+(defvar multi-mode)
 
 (defun multi-install-mode (mode &optional chunk-fn base)
   "Add MODE to the multiple major modes supported by the current buffer.
@@ -235,21 +234,7 @@ is the base mode."
 	      (funcall mode))
 	    ;; Now we can make it local:
 	    (set (make-local-variable 'multi-mode) t)
-	    ;; Use file's local variables section to set variables in
-	    ;; this buffer.  (Don't just copy local variables from the
-	    ;; base buffer because it may have set things locally that
-	    ;; we don't want in the other modes.)  We need to prevent
-	    ;; `mode' being processed and re-setting the major mode.
-	    ;; It all goes badly wrong if `hack-one-local-variable' is
-	    ;; advised.  The appropriate mechanism to get round this
-	    ;; appears to be `ad-with-originals', but we don't want to
-	    ;; pull in the advice package unnecessarily.  `flet'-like
-	    ;; mechanisms lose with advice because `fset' acts on the
-	    ;; advice anyway.
-	    (if (featurep 'advice)
-		(ad-with-originals (hack-one-local-variable)
-		  (multi-hack-local-variables))
-	      (multi-hack-local-variables))
+	    (multi-hack-local-variables)
 	    ;; Indentation should first narrow to the chunk.  Modes
 	    ;; should normally just bind `indent-line-function' to
 	    ;; handle indentation.
@@ -290,9 +275,9 @@ is the base mode."
 	    ;; Kill the base buffer along with the indirect one; careful not
 	    ;; to infloop.
 	    (add-hook 'kill-buffer-hook
-		      '(lambda ()
-			 (setq kill-buffer-hook nil)
-			 (kill-buffer (buffer-base-buffer)))
+		      (lambda ()
+			(setq kill-buffer-hook nil)
+			(kill-buffer (buffer-base-buffer)))
 		      t t)
 	    ;; This should probably be at the front of the hook list, so
 	    ;; that other hook functions get run in the (perhaps)
@@ -306,7 +291,7 @@ is the base mode."
 	    (setq buffer-file-coding-system coding)
 	    ;; For benefit of things like VC
 	    (setq buffer-file-name file)
-	    (vc-find-file-hook))
+	    (vc-refresh-state))
 	  ;; Propagate updated values of the relevant buffer-local
 	  ;; variables to the indirect buffers.
 	  (dolist (x alist)
@@ -373,8 +358,7 @@ Fontifies chunk-by-chunk within the region from START for up to
 Works piece-wise in all the chunks with the same major mode.
 Assigned to `imenu-create-index-function'."
   (let ((selected-mode major-mode)
-	imenu-alist			; accumulator
-	last mode)
+	imenu-alist)			; accumulator
     (multi-map-over-chunks
      (point-min) (point-max)
      (lambda ()
@@ -454,8 +438,7 @@ Destructively modifies `multi-mode-list' to avoid consing in
 Return a list (MODE START END), the value returned by the function in the
 list for which START is closest to POS (and before it); i.e. the innermost
 mode is selected.  POS defaults to point."
-  (let ((fns multi-chunk-fns)
-	(start (point-min))
+  (let ((start (point-min))
 	(mode (with-current-buffer (multi-base-buffer)
 		major-mode))
 	(end (point-max))
@@ -500,28 +483,6 @@ mode is selected.  POS defaults to point."
     (fundamental-mode)
     (error "`multi-mode-alist' not defined for multi-mode")))
 
-;; In 21.3, Flyspell breaks things, apparently by getting an error in
-;; post-command-hook and thus clobbering it.  In development code it
-;; doesn't do that, but does check indirect buffers it shouldn't.  I'm
-;; not sure exactly how this happens, but checking flyspell-mode in
-;; the hook functions cures this.  For the moment, we'll hack this up.
-;; (Let's not bring advice into it...)
-(eval-after-load "flyspell"
-  '(progn
-     (defalias 'flyspell-post-command-hook
-       `(lambda ()
-	  ,(concat (documentation 'flyspell-post-command-hook)
-		   "\n\n[Wrapped by multi-mode.]")
-	  (if flyspell-mode
-	   (funcall ,(symbol-function 'flyspell-post-command-hook)))))
-
-     (defalias 'flyspell-pre-command-hook
-       `(lambda ()
-	  (concat (documentation 'flyspell-pre-command-hook)
-		  "\n\n[Wrapped by multi-mode.]")
-	  (if 'flyspell-mode
-	      (funcall ,(symbol-function 'flyspell-pre-command-hook)))))))
-
 ;; This is useful in composite modes to determine whether a putative
 ;; major mode is safe to invoke.
 (defun multi-mode-major-mode-p (value)