IBus and different GTK versions

[Ricardo Wurmus]

Hi Guix,

I’m having some problems using IBus as an input method for all
applications on GuixSD.  Usually, it is enough to specify the following
variables:

    export GTK_IM_MODULE=ibus
    export address@hidden
    export QT_IM_MODULE=ibus

    export IBUS_COMPONENT_PATH="$HOME/.guix-profile/share/ibus/component"

The last variable is actually suggested by Guix via the search paths
feature (specified on the “ibus” package), while the other three are
suggested when running “ibus-setup”.

Even when these environment variables are set, IBus won’t work right
away.  After installation there is one more issue which can be solved
easily.  IBus uses dconf to store its settings, so changing the settings
with “ibus-setup” requires the dconf-service to be running (I’m planning
to write a dmd service for this):

    $(guix build dconf)/libexec/dconf-service &
    ibus-setup

Okay, now we can actually select input methods and our changes will be
kept.  I installed the “ibus-libpinyin” package for inputting Chinese
characters via pinyin, and selected it from the menu in “ibus-setup”.

Unfortunately, things still don’t work, because the GTK_IM_MODULE “ibus”
cannot be found.  GTK does not know where to look for it.  It looks up
input method modules in a file called “immodules.cache”, which is
generated with “gtk-query-immodules-3.0” for GTK3 and
“gtk-query-immodules-2.0” for GTK2.

Our GTK packages come with “immodules.cache” files at

    $out/lib/gtk-3.0/3.0.0/immodules.cache

and

    $out/lib/gtk-2.0/2.10.0/immodules.cache

for the two different versions of GTK.  Of course, neither of these
files contain a reference to the IBus input module, which is located at

    $out/lib/gtk-3.0/3.0.0/immodules/im-ibus.so
    $out/lib/gtk-2.0/2.10.0/immodules/im-ibus.so

in the output of the “ibus” package.  I can generate an entry for the
immodules.cache file with

    gtk-query-immodules-3.0 
~/.guix-profile/lib/gtk-3.0/3.0.0/immodules/im-ibus.so

and append this to a copy of the GTK immodules.cache file.  To make GTK
use it I need to point to it with the GTK_IM_MODULE_FILE variable.

However, it is not advisable to mix input method modules for different
versions of GTK.  When I did the above and listed both the ‘im-ibus.so’
for GTK2 and GTK3 in the immodules.cache file and opened the graphical
file dialogue in Emacs (which uses GTK3), Emacs would crash with a
segfault if the GTK2 input method module for IBus was listed first.  I
get a similar problem when the GTK3 module is listed first and I try to
run any of the XFCE tools, e.g. ‘xfce4-keyboard-settings’, which all use
GTK2.  They all fail to start when the GTK3 module is listed first.

This means that we cannot just use one cache file and we cannot let
users set GTK_IM_MODULE_FILE for the whole session.  In fact, the GTK
documentation warns that this variable should not be used session-wide.

It seems that GTK3 only looks for
‘$libdir/gtk-3.0/3.0.0/immodules.cache’ and GTK2 only looks for
‘$libdir/gtk-2.0/2.10.0/immodules.cache’ by default, where ‘$libdir’ is
‘$out/lib’ of the GTK package.  (According to
https://developer.gnome.org/gtk3/stable/gtk-running.html)

To make IBus work we need to find a way to make the different major
versions of GTK respect different ‘immodules.cache’ files that are *not*
located in their ‘$libdir’.  In that case we could generate a fresh
‘immodules.cache’ file for each major GTK version when building a
profile.  Unfortunately, I haven’t been able to find any documented
means to achieve this.  It might require patching the GTKs to check an
additional path or environment variable.

If this was the only way to achieve this behaviour I would suggest to
alter the GTKs to respect two variables in addition to the flawed and
dangerous ‘GTK_IM_MODULE_FILE’:

    ‘GTK2_IM_MODULE_FILE’ for GTK2
    ‘GTK3_IM_MODULE_FILE’ for GTK3

Right now, the only way to make IBus work for all GTK applications (no
matter the version) is to edit the ‘immodules.cache’ files in the GTK
packages’ output directory so that each contains a reference to the
correct IBus input method module for the particular GTK version.  This
involves changing stuff in ‘/gnu/store’ and I’m just not comfortable
doing this after each update.

What do you think would be the best approach here?

~~ Ricardo