[bug#56050] [PATCH] home: services: environment-variables: Fix XDG base directories.

[Philip McGrath]

On Wednesday, June 22, 2022 4:38:11 PM EDT Ludovic Courtès wrote:
> Philip McGrath <philip@philipmcgrath.com> skribis:
> > When the environment initialization script is run, XDG_DATA_DIRS and/or
> > XDG_CONFIG_DIRS may be empty or unset, in which case we must use their
> > respective defaults from the specification, rather than ending the value
> > with a trailing ":". For further discussion, see
> > <https://lists.gnu.org/archive/html/help-guix/2022-05/msg00147.html>.
> > 
> > * gnu/home/services.scm
> > (environment-variables->setup-environment-script): Use conditional
> > parameter expansion for XDG_DATA_DIRS and XDG_CONFIG_DIRS.
> 
> [...]
> 
> > +++ b/gnu/home/services.scm
> > @@ -208,7 +208,7 @@ (define
> > (environment-variables->setup-environment-script vars)> 
> >  case $XDG_DATA_DIRS in
> >  
> >    *$HOME_ENVIRONMENT/profile/share*) ;;
> > 
> > -  *) export XDG_DATA_DIRS=$HOME_ENVIRONMENT/profile/share:$XDG_DATA_DIRS
> > ;; +  *) export
> > XDG_DATA_DIRS=$HOME_ENVIRONMENT/profile/share:${XDG_DATA_DIRS:-/usr/local
> > /share/:/usr/share/} ;;
> What about doing it this way:
> 
>   export
> "XDG_DATA_DIRS=$HOME_ENVIRONMENT/profile/share${XDG_DATA_DIRS:+:}$XDG_DATA_
> DIRS"
> 
> That would avoid adding /usr, which makes little sense for Guix and
> could lead to bad surprises on foreign distros, such as loading
> incompatible data from the host distro.
> 
> WDYT?
> 
> >  case $XDG_CONFIG_DIRS in
> >  
> >    *$HOME_ENVIRONMENT/profile/etc/xdg*) ;;
> > 
> > -  *) export
> > XDG_CONFIG_DIRS=$HOME_ENVIRONMENT/profile/etc/xdg:$XDG_CONFIG_DIRS ;; + 
> > *) export
> > XDG_CONFIG_DIRS=$HOME_ENVIRONMENT/profile/etc/xdg:${XDG_CONFIG_DIRS:-/etc
> > /xdg} ;;
> Same question here, though /etc/xdg is a bit less problematic as it
> could exist on Guix System too.
> 

Unfortunately this doesn't work: on a foreign distro (concretely, for me, 
Kubuntu 22.04) when XDG_CONFIG_DIRS isn't set globally, this would expand 
equivalent to just:

    XDG_CONFIG_DIRS=$HOME_ENVIRONMENT/profile/etc/xdg

Arguably, that's slightly better than ending with a trailing ":", since the 
semantics of an empty element in or at the end of the path list doesn't seem to 
be defined by the specification. However, it leaves me with the same problem: 
SDDM can't find the configuration it needs to start my KDE Plasma session 
successfully.

I do see how /usr would not make a lot of sense for Guix System. Since those 
paths are written into the spec for empty or unset variables, it seems like 
Guix System probably should arrange for all of the XDG variables to be set to 
something non-empty very early, maybe in /etc/profile itself.

I based this patch on the file that gets installed as /etc/profile.d/guix.sh (I 
haven't found its source yet). Maybe we could leave this code alone if we 
change that file, instead? Then, both on Guix System and on foreign distros, 
some system-wide code would be responsible for initializing these search paths, 
and we could assume in contexts like this that they are explicitly set and not 
empty.

One catch is that, right now, this is what was installed for me into 
/etc/profile.d/guix.sh:

```
# _GUIX_PROFILE: `guix pull` profile
_GUIX_PROFILE="$HOME/.config/guix/current"
if [ -L $_GUIX_PROFILE ]; then
  export PATH="$_GUIX_PROFILE/bin${PATH:+:}$PATH"
  # Export INFOPATH so that the updated info pages can be found
  # and read by both /usr/bin/info and/or $GUIX_PROFILE/bin/info
  # When INFOPATH is unset, add a trailing colon so that Emacs
  # searches 'Info-default-directory-list'.
  export INFOPATH="$_GUIX_PROFILE/share/info:$INFOPATH"
fi

# GUIX_PROFILE: User's default profile
GUIX_PROFILE="$HOME/.guix-profile"
[ -L $GUIX_PROFILE ] || return
GUIX_LOCPATH="$GUIX_PROFILE/lib/locale"
export GUIX_PROFILE GUIX_LOCPATH

[ -f "$GUIX_PROFILE/etc/profile" ] && . "$GUIX_PROFILE/etc/profile"

# set XDG_DATA_DIRS to include Guix installations
export 
XDG_DATA_DIRS="$GUIX_PROFILE/share:${XDG_DATA_DIRS:-/usr/local/share/:/usr/share/}"
```

but $HOME/.guix-profile doesn't exist for me (only $HOME/.guix-home), so this 
script exits without setting GUIX_LOCPATH or XDG_DATA_DIRS. Maybe something 
about that should change?

-Philip

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