bug#26877: building fonts: fontcache must be updated regulary

[Mark H Weaver <mhw@netris.org>]

ng0 <ng0@pragmatique.xyz> writes:

> The problem: Right now after just installing fonts, they do
> not become available. This can lead to some applications
> appearing "broken", etc. Other operating systems solve this
> by having a post-install hook which roughly does just this.
>
> A possible solution:
> Our fictional, not yet existing, font-build-system should
> run "fc-cache --force --really-force",

This cannot work, because the build system code is run within the build
container, where it won't have access to anything in your home
directory.  Furthermore, for most users, the packages are built on
another machine, namely one of our build farm slaves.

Even the build system did somehow have access to your home directory,
building the package is not the right time to run "fc-cache".  On a
multiuser system, if two users request the same derivation, it is only
built for the first user, and the second user simply uses the one that
the first user built.  The relevant time to run "fc-cache" is when the
font is _installed_ in your user profile.

The usual Guix way to handle things like this is to arrange for the font
cache to be part of the profile, and to create a profile hook that
creates the cache from all of the built packages included within.

       Mark