bug#37355: 26.1; CTL rendering for some indic characters fails

[Joshua Immanuel]

After upgrading to Debian Buster, CTL rendering seems to be broken for
some characters in Tamil. I have installed fonts-lohit-taml package for
font support.

To see this problem, we can do a "C-x 8 RET" and try auto completing
"TAMIL VOWEL SIGN" in mini buffer. In this, we can notice empty
characters instead of actuals for "TAMIL VOWEL SIGN AU", "TAMIL VOWEL
SIGN O" and "TAMIL VOWEL SIGN OO". So, while typing these characters it
makes the previous character disappear.

I thought, this could be a m17n-db issue and tried building
emacs-27.0.50 from [1]. But, that too had the same issue inspite of the
fact that it uses HaffBuzz. 

However, this problem is not present in the latest HEAD. So, doing a
"git bisect" on the repository, I found that the following commit is
the one which actually fixed the problem.

"64767008f70af88d193d36eb6d2c27e4bc95ea8e Support
font driver supersession"

Is this fix relevant to emacs-26. Can this be backported to fix this
issue?


1: https://alpha.gnu.org/gnu/emacs/pretest/windows/emacs-27/


In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.4)
 of 2019-02-03, modified by Debian built on zam904
Windowing system distributor 'The X.Org Foundation', version
11.0.12004000
System Description:     Debian GNU/Linux 10 (buster)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Configured using:
 'configure --build x86_64-linux-gnu --prefix=/usr
 --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var/lib --infodir=/usr/share/info
 --mandir=/usr/share/man --enable-libsystemd --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-
lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-
lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --build
 x86_64-linux-gnu --prefix=/usr --sharedstatedir=/var/lib
 --libexecdir=/usr/lib --localstatedir=/var/lib
 --infodir=/usr/share/info --mandir=/usr/share/man --enable-libsystemd
 --with-pop=yes
 --enable-locallisppath=/etc/emacs:/usr/local/share/emacs/26.1/site-
lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/26.1/site-
lisp:/usr/share/emacs/site-lisp
 --with-sound=alsa --without-gconf --with-mailutils --with-x=yes
 --with-x-toolkit=gtk3 --with-toolkit-scroll-bars 'CFLAGS=-g -O2
 -fdebug-prefix-map=/build/emacs-26.1+1=. -fstack-protector-strong
 -Wformat -Werror=format-security -Wall' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' LDFLAGS=-Wl,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 THREADS LIBSYSTEMD LCMS2

Important settings:
  value of $LANG: en_IN
  locale-coding-system: iso-latin-1-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
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  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
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Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
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mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors 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 composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray 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 lcms2
dynamic-setting system-font-setting font-render-setting move-toolbar
gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 95517 5617)
 (symbols 48 20366 1)
 (miscs 40 46 94)
 (strings 32 28331 1460)
 (string-bytes 1 743064)
 (vectors 16 14638)
 (vector-slots 8 496162 7960)
 (floats 8 50 67)
 (intervals 56 362 243)
 (buffers 992 12))

-- 
Joshua Immanuel
HiPro IT Solutions Private Limited
http://hipro.co.in

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