Cannot override fontconfig

[taku0]

There is no way to add high-priority, system-wide fontconfig configuration file.

In conventional Linux systems, we can write /etc/fonts/conf.d/99-overrideall to override font configurations. (e.g. overriding 60-latain.conf or 65-nonlatain.conf to modify font priorities.)
We cannot utilize local.conf since it is loaded from 51-local.conf and is overridden by 60-latain and others.

/etc/fonts/fonts.conf includes both /etc/fonts/conf.d and ${fontconfig}/etc/fonts/conf.d, but /etc/fonts/conf.d is included before ${fontconfig}/etc/fonts/conf.d, so that all configurations in /etc/fonts/conf.d are overridden by configurations in ${fontconfig}/etc/fonts/conf.d.

Possible fixes

• Add a NixOS option that generates ${fontconfig}/etc/fonts/conf.d/99-overrideall
• Add a NixOS option that add/remove arbitrary files to/from ${fontconfig}/etc/fonts/conf.d/
• Include /etc/fonts/conf.d after ${fontconfig}/etc/fonts/conf.d so that administrators can generate configuration files using environment.etc

[shlevy]

IMO in general .d directories should be merged after the base config. @edolstra Any objections to making that switch here?

[7c6f434c]

I'm also for the last option. Maybe we should do this already?

[vcunat]

I thought all fontconfig *.d directories are processed in parallel, so adding 99-* into either should be able to override anything. I recently read the docs, but I'm not sure about this.

[vcunat]

Hmm, if not, then it might be best to do some kind of buildEnv on the two config trees and use that one.

[taku0]

@vcunat Unfortunately, *.d directories are not processed in parallel. Try FC_DEBUG=65535 fc-list or see FcConfigParseAndLoadDir @ src/fcxml.c

[vcunat]

The overriding of parts of config is no easy problem. For example, specified directories (fonts, cache) are used in the order they were specified, so one will typically want to add stuff before the defaults and not after them (or one may even want to remove/replace them).

[vcunat]

I think this is fine now in staging. The following are loaded after the defaults:

• system-wide /etc/fonts/{fontconfig.configVersion}/conf.d/* (you can add files in there by environment.etc.fonts...);
• per-user XDG path ~/.config/fontconfig/conf.d/.