bug#19408: 25.0.50; Incorrect handling of face-font-rescale-alist on startup mangles font settings

[Kirill Ignatiev <kirill.ignatiev@gmail.com>]

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Take the following .emacs file:

(custom-set-faces
 '(default ((t (:inherit nil :height 110 :foundry "nil" :family
"Source Code Pro")))))

(add-to-list 'face-font-rescale-alist (cons (font-spec :family
"STIXGeneral") 0.95) t)

Run emacs with no other customizations, using -Q -l <file.el>. The
font in the initial frame in the scratch buffer will be Source Code
Pro, as expected. Open a new frame, the font in that frame will be
some other font (in my case Helvetica), not the default one.

During startup, if face-font-rescale-alist changes, emacs will mangle
the default font setting, causing it to be wrong for all non-initial
frames.

I found the following workaround:

;; Workaround for emacs/lisp/startup.el:670
(defadvice frame-notice-user-settings (before my:rescale-alist)
  ;; (message "Set face-font-rescale-alist")
  (add-to-list 'face-font-rescale-alist
      (cons (font-spec :family "STIXGeneral") 0.95) t))
(ad-activate 'frame-notice-user-settings)

for this particular bit of lisp/startup.el (line ~660)

 ;; FIXME: The user's init file may change
 ;; face-font-rescale-alist.  However, the default face
 ;; already has an assigned font object, which does not take
 ;; face-font-rescale-alist into account.  For such
 ;; situations, we ought to have a way to find all font
 ;; objects and regenerate them; currently we do not.  As a
 ;; workaround, we specifically reset te default face's :font
 ;; attribute here.  See bug#1785.
 (unless (eq face-font-rescale-alist
     old-face-font-rescale-alist)
   (set-face-attribute 'default nil :font (font-spec)))

I am not sure how the default font should be regenerated in case it's
affected by face-font-rescale-alist, but using (font-spec) is wrong
because it resets user's own settings.

I attached the output of report-emacs-bug.

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In GNU Emacs 25.0.50.32 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version 10.9.5 (Build 13F34))
 of 2014-12-16 on dhcp1-38.red.sandbox
Repository revision: d08f4f8fc5219324ac352c6d89f0db5c6a18ce32
Windowing system distributor `Apple', version 10.3.1265
Configured using:
 `configure --with-ns --without-sound CPPFLAGS=-I/sw/include
 LDFLAGS=-L/sw/lib'

Configured features:
ACL LIBXML2 ZLIB

Important settings:
  value of $LANG: en_IE.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  diff-auto-refine-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Type C-x 1 to delete the help window.
Making completion list...
Mark set
"25.0.50.32"

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr emacsbug message dired format-spec
rfc822 mml mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils vc-git diff-mode easy-mmode help-mode
easymenu advice help-fns time-date tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel ns-win tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese case-table epa-hook jka-cmpr-hook help simple abbrev
minibuffer nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote make-network-process
cocoa ns multi-tty emacs)

Memory information:
((conses 16 84880 8004)
 (symbols 48 19497 1)
 (miscs 40 99 214)
 (strings 32 15805 4810)
 (string-bytes 1 387443)
 (vectors 16 10556)
 (vector-slots 8 399657 6002)
 (floats 8 63 93)
 (intervals 56 572 0)
 (buffers 976 15))