bug#19117: 25.0.50; emacs on x11 chooses different fonts for the same face sometimes

[Dima Kogan]

Hi. I'm observing an issue when running the latest GNU Emacs built from
source using the lucid X toolkit; this is on a Debian machine running
X11 (build details at the end).

Starting a few months ago (this is a regression) I'm seeing that
sometimes Emacs renders some faces with an unrequested (and ugly) font.
I can usually make it do the right thing by moving Emacs windows around
or resizing them, or maybe opening a new Emacs frame. It's not
consistent, and I don't know exactly what triggers it breaking or how I
can reliably fix it.

I'm attaching two screenshots of 'M-x describe-face italic' when it's
broken and right after I moved the windows around a bit, and re-invoked
describe-face. Note the very different font. Using what-cursor-position
I can see that in the wrong case the font is

 -urw-nimbus mono l-regular-o-normal--11-79-100-100-p-70-iso8859-1 (#x70)

and in the right case the font is

 -adobe-courier-medium-o-normal--11-80-100-100-m-60-iso8859-1 (#x6D)

The latter is correct since I customized my default-frame-alist to contain

 (font . "-adobe-courier-medium-r-*-*-*-80-*-*-m-*-iso8859-1")

Can somebody give me some pointers on how to debug this to make this bug
report useful? I see the breakage many times a day every day.

Thanks.

good.png
Description: PNG image

bad.png
Description: PNG image

In GNU Emacs 25.0.50.1 (x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2014-11-02 on lilbastard, modified by Debian
 (emacs-snapshot package, version 2:20141102+emacs-24.4-1596-gb8f900f-1)
Windowing system distributor `The X.Org Foundation', version 11.0.11600000
System Description:     Debian GNU/Linux unstable (sid)

Configured using:
 `configure --build x86_64-linux-gnu --host x86_64-linux-gnu
 --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var --infodir=/usr/share/info/emacs-snapshot
 --mandir=/usr/share/man --with-pop=yes
 
--enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/25.0.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/25.0.50/site-lisp:/usr/share/emacs/site-lisp
 --with-crt-dir=/usr/lib/x86_64-linux-gnu/ --with-x=yes
 --with-x-toolkit=lucid 'CFLAGS=-DDEBIAN -DSITELOAD_PURESIZE_EXTRA=5000
 -g -O2' CPPFLAGS=-D_FORTIFY_SOURCE=2 'LDFLAGS=-g -Wl,--as-needed
 -znocombreloc''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB

Important settings:
  value of $LC_ALL: C
  value of $LANG: C
  locale-coding-system: nil