Subject: 27.0.50; desktop-locals-to-save won't consider buffer-undo-list as local

From: Terje Larsen <[email-masked]>
Date: Sat, 24 Nov 2018 21:54:12 +0100

Add buffer-undo-list to be saved by desktop-save:
(cl-pushnew 'buffer-undo-list desktop-locals-to-save :test #'equal)

Open a file:
C-x C-f ~/sample.txt

Input text such as:
Lorem ipsum dolor sit amet

Remove the two last words:
M-2 M-DEL

Save the desktop:
M-x desktop-save

... Restart Emacs ...

Load the desktop:
M-x desktop-read

Execute undo operation:
C-/

At this point I expect the undo operations from the previous session
to be successful.

It seems like this has been mentioned at StackExchange amongst other
places before:
https://emacs.stackexchange.com/questions/3725/why-is-the-buffer-undo-list-excluded-from-buffer-local-variables

I can verify that the issue stems from desktop-save relying on the C-function
buffer-local-variables and this function does not include the variable
buffer-undo-list.
Due to this it will be ignored from being saved by the desktop-save command.

Proposed in the StackExchange post is to append the buffer-undo-list
in the desktop-save
function. But perhaps it should be included in the C-function? Or does
that perhaps cause
other issues?

Personally I have solved it by using an advice:

(defun +append-buffer-undo-list-to-buffer-local-variables-advice
(orig-fn &rest args)
       "Override `buffer-local-variables' and call ORIG-FN with ARGS.
     There is a bug in Emacs where the `buffer-undo-list' data is
     missing from the output of `buffer-local-variables'. This
     advice temporarily overrides the function and appends the
     missing data."
       (let ((orig-buffer-local-variables-fn (symbol-function
'buffer-local-variables)))
         (cl-letf (((symbol-function 'buffer-local-variables)
                    #'(lambda () (append (funcall
orig-buffer-local-variables-fn)
                                    `(,(cons 'buffer-undo-list
buffer-undo-list))))))
           (apply orig-fn args))))
(advice-add #'desktop-buffer-info :around
#'+append-buffer-undo-list-to-buffer-local-variables-advice)

I hope this "long-standing" bug would allow more people to utilize a
persistent undo without
jumping through hoops.

Thanks!

In GNU Emacs 27.0.50 (build 2, x86_64-apple-darwin18.0.0, NS
appkit-1671.00 Version 10.14 (Build 18A391))
of 2018-11-07 built on C02V91MHHV2Q.local
Repository revision: 811d9291fcfb12d87bad277d4e8b25152129d73d
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14.1

Recent messages:
Desktop: 1 frame, 1 buffer restored.
You can run the command ‘desktop-read’ with M-x d-rea RET
Desktop: 1 frame, 1 buffer restored.
user-error: No further undo information [2 times]
user-error: Beginning of history; no preceding item
user-error: End of history; no default available

Configured using:
'configure --disable-dependency-tracking --disable-silent-rules
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Memory information:
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