bug#62955: Guix Home Breaks Some Foreign Distros

[Zacchaeus Scheffer <zaccysc@gmail.com>]

Hi all!

$HOME/PROFILE/setup-environment contains the following lines:

case $XDG_CONFIG_DIRS in
  *$HOME_ENVIRONMENT/profile/etc/xdg*) ;;
  *) export XDG_CONFIG_DIRS=$HOME_ENVIRONMENT/profile/etc/xdg:$XDG_CONFIG_DIRS ;;
esac

There are two bugs in this code.  Both bugs revolve around what happens
if XDG_CONFIG_DIRS is unset, as is the case in some foreign distros.

The first problem is if, XDG_CONFIG_DIRS is unset, and
$HOME_ENVIRONMENT/profile/etc/xdg does not exist, then the conditional
resolves to "is the empty string in the empty string", so
XDG_CONFIG_DIRS is prepended with $HOME_ENVIRONMENT/profile/etc/xdg,
despite it not existing.  I'm not sure if incuding a nonexistant path in
XDG_CONFIG_DIRS should cause problems, but this scenario is not
mentioned in the XDG specifications[1], so some programs are bound to
crash because of this.

The second problem (more important) relates to how XDG_CONFIG_DIRS is
interpreted if empty vs set to a value.  As per the xdg
specifications[1], if XDG_CONFIG_DIRS is unset, then a value of /etc/xdg
should be used.  When an empty XDG_CONFIG_DIRS is set by guix home,
programs find a non-empty value and ignore /etc/xdg.

The above problem means that when I do a guix home reconfigure on my
Librem 5 running PureOS, I get a black screen as Phosh fails to start as
it needs files in /etc/xdg.  I can fix it by adding:

XDG_CONFIG_DIRS="$XDG_CONFIG_DIRS:/etc/xdg"

to my .zlogin.  A similar issue can be seen with the lines regarding
XDG_DATA_DIRS in $HOME/PROFILE/setup-environment.  I think a good step
to resovling this issue is to set empty variables to their default value
first.  Maybe something like the following in gnu/home/services.scm:267

...
[ -f $PROFILE_FILE ] && . $PROFILE_FILE
[ -z "$XDG_DATA_DIRS"] && XDG_DATA_DIRS=/usr/local/share/:/usr/share/
[ -z "$XDG_CONFIG_DIRS"] && XDG_CONFIG_DIRS=/etc/xdg
case $XDG_DATA_DIRS in
...

This addresses the second problem, but not the first.  Someone may find
a more elegant way to deal with both issues.

-Zacchae

[1] https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html