Subject: bug#53294: 29.0.50; Indirect font changes incorrectly affecting original buffer

From: Andrew Hyatt <[email-masked]>
Date: Sun, 16 Jan 2022 00:13:26 -0500

Hi all,

I noticed a bug recently where if I scale up fonts in an org 
capture buffer, it affects the original buffer, which keeps 
getting bigger and bigger every time org-capture is run.

However, you don't need org to reproduce this.  Here's a quick way 
to reproduce, which works with emacs -Q:

(require 'face-remap) (defun ash/big-font () 
  "Creates a font that is big enough for about 20 lines of text." 
  (interactive) (let ((text-scale-mode-amount (/ (frame-height) 
  20))) 
    (text-scale-mode 1))) 
 
(defun ash/reproduce-with-indirect-buffer () 
  (interactive) (let ((buf (get-buffer-create "*Orig buffer*"))) 
    (set-buffer buf) (variable-pitch-mode 1) ;; same way org mode 
    creates indirect buffer (set-buffer (make-indirect-buffer buf 
    "*Indirect buffer*" 'clone)) (ash/big-font-new) (kill-buffer 
    (current-buffer)))) 

Running ash/reproduce-with-indirect-buffer will increase the 
indirect buffer in size each time.  If you look at 
face-remapping-alist, it's clear that the original buffer's value 
is being altered by the indirect buffer.

I traced this down to the setcdr in face-remove-add-relative, 
which is incorrectly alterting a buffer local variable, by setting 
parts of a variable in a way that will affect the original 
variable.  Evidently making something buffer local doesn't clone 
it, it just copies the value, and so you can alter the original by 
operations such as setcdr. 

Assuming that the fixing how variables becomes buffer local is too 
much, I've fixed this in face-remap.el.  The patch is attached.

In GNU Emacs 29.0.50 (build 1, aarch64-apple-darwin21.2.0, NS 
appkit-2113.20 Version 12.1 (Build 21C52)) 
 of 2021-12-26 built on andrews-mbp.lan 
Repository revision: 978987f7ad58cd66fe51cefde53ba4771b189aeb 
Repository branch: master Windowing system distributor 'Apple', 
version 10.3.2113 System Description:  macOS 12.1
 
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