bug#29631: 25.3; Unable to use custom fontset as frame default font

[Thomas Morgan]

I started Emacs with `emacs -Q', entered the following expression
in *scratch*, and evaluated it with C-M-x:

  (progn
    ;; Create a new fontset called fontset-liberation.
    (create-fontset-from-fontset-spec
     "-*-Liberation Mono-normal-normal-normal-*-*-*-*-*-m-0-fontset-liberation")

    ;; Set its primary font to Liberation Mono.
    (set-fontset-font "fontset-liberation" 'unicode-bmp "Liberation Mono")

    ;; Add a fallback to Freemono for characters that Liberation Mono lacks.
    (set-fontset-font "fontset-liberation" 'unicode-bmp "Freemono:size=40"
                      nil 'append)

    ;; Set the frame's default font to the new fontset.

    (set-face-font 'default "fontset-liberation")

    ;; Return font objects for "a", an ASCII character that Liberation Mono has,
    ;; and "ȷ" (LATIN SMALL LETTER DOTLESS J), a character in Freemono but
    ;; not in Liberation Mono.
    (list (font-at 0 nil "a") (font-at 0 nil "ȷ")))

The result was this:

  (#<font-object "-1ASC-Liberation 
Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1">
   #<font-object "-1ASC-Liberation 
Serif-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1">)

The font for ASCII character "a" was Liberation Mono as expected, but
I expected the second font to be Freemono and it was Liberation Serif.

I checked which fontset is being used as default:

  (face-attribute 'default :fontset)

It's fontset-auto1, not fontset-liberation:

  "-1ASC-Liberation Mono-normal-normal-normal-*-15-*-*-*-m-0-fontset-auto1"

I typed `M-x describe-fontset RET fontset-auto1 RET'.

  Fontset: -1ASC-Liberation 
Mono-normal-normal-normal-*-15-*-*-*-m-0-fontset-auto1
  CHAR RANGE (CODE RANGE)
      FONT NAME (REQUESTED and [OPENED])
  C-@ .. Ÿ (#x43 .. #x9F)
      -*-*-*-*-*-*-*-*-*-*-*-*-iso10646-1
    .. ɏ (#xA0 .. #x24F)
      -*-*-*-*-*-*-*-*-*-*-*-*-iso10646-1
          [-1ASC-Liberation Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1]
          [-1ASC-Liberation 
Serif-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1]
  ɐ .. [#x3FFF7F] (#x250 .. #x3FFF7F)
      -*-*-*-*-*-*-*-*-*-*-*-*-iso10646-1
  [\200] .. [\377] (#x3FFF80 .. #x3FFFFF)
      -*-*-*-*-*-*-*-*-*-*-*-*-iso10646-1

    ---<fallback to the default fontset>---
  [...]

(Non-UTF8 characters are replaced with representations in brackets.)

So fontset-auto1 includes Liberation Serif but not Freemono.
fontset-auto1 seems to be generated from fontset-liberation's primary
font without regard for its fallback fonts.

To test whether changing the fallback font in fontset-auto1 has
an effect, I restarted Emacs with `emacs -Q' and evaluated the
following expression, which modifies fontset-auto1 instead of
fontset-liberation.

  (progn
    ;; Create a new fontset called fontset-liberation.
    (create-fontset-from-fontset-spec
     "-*-Liberation Mono-normal-normal-normal-*-*-*-*-*-m-0-fontset-liberation")

    ;; Set the frame's default font to the new fontset.
    (set-face-font 'default "fontset-liberation")

    ;; Set the frame's primary font to Liberation Mono.
    (set-fontset-font "fontset-auto1" 'unicode-bmp "Liberation Mono")

    ;; Add a fallback to Freemono for characters that Liberation Mono lacks.
    (set-fontset-font "fontset-auto1" 'unicode-bmp "Freemono:size=40"
                      nil 'append)

    ;; Return font objects for "a", an ASCII character that Liberation Mono has,
    ;; and "ȷ" (LATIN SMALL LETTER DOTLESS J), a character in Freemono but
    ;; not in Liberation Mono.
    (list (font-at 0 nil "a") (font-at 0 nil "ȷ")))

This changed the font for the non-ASCII character to Freemono
and returned what I expected:

  (#<font-object "-1ASC-Liberation 
Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1">
   #<font-object "-GNU 
-FreeMono-normal-normal-normal-*-40-*-*-*-m-0-iso10646-1">)

(Incidentally, evaluating the above progn without restarting Emacs
causes a core dump, but I have a smaller test case for that and I'll
make another report about it.)

It looks as if fontset-auto1 is generated by fontset_from_font
in fontset.c, but I don't understand why the specified fontset
(fontset-liberation) is not used and I haven't been able to find
anything in the manual that clarifies this behavior.  I'd like
to know how to use a custom fontset for a frame's default face
(and for other faces), but if that's not supported, this may be
a documentation bug.

Information from report-emacs-bug:

In GNU Emacs 25.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.21)
 of 2017-11-05 built on localhost
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
Configured using:
 'configure --disable-dependency-tracking
 --prefix=/nix/store/7px74nmmy9wnl594jsk0lcgz1ygsbwfj-emacs-25.3
 --with-modules --with-x-toolkit=gtk3 --with-xft'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS GSETTINGS NOTIFY LIBSELINUX GNUTLS
LIBXML2 FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

Important settings:
  value of $LC_ALL: en_US.utf8
  value of $LC_COLLATE: C
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  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
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
(#<font-object "-1ASC-Liberation 
Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1"> #<font-object "-GNU 
-FreeMono-normal-normal-normal-*-40-*-*-*-m-0-iso10646-1">)

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
cl-loaddefs pcase cl-lib mail-prsvr mail-utils time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel x-win term/common-win x-dnd 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 cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded 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 dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 87168 5967)
 (symbols 48 19960 0)
 (miscs 40 56 121)
 (strings 32 14566 4561)
 (string-bytes 1 435478)
 (vectors 16 11914)
 (vector-slots 8 439170 4779)
 (floats 8 166 74)
 (intervals 56 242 21)
 (buffers 976 18))