fontconfig: workaround KDE overwriting symlink

[daniel-sampliner]

Description

Changing KDE font settings using GUI causes it to convert $XDG_CONFIG_HOME/fontconfig/conf.d/10-hm-fonts.conf to a plain file and then insert its own settings.

A bug was filed upstream¹, however upstream does not appear receptive to making any changes.

Instead of patching KDE as suggested by upstream, we can simply work around the behavior by providing a mutable file for KDE to do what it wants with. Since it appears that KDE will always use the first file it finds², we create the mutable file 00-aaa-local.conf.

Fixes: #5162

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

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  nix-shell --pure tests -A run.all.

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Footnotes

1. https://bugs.kde.org/show_bug.cgi?id=498694 ↩

2. https://github.com/KDE/plasma-workspace/blob/83bebc7896986eb27e0e1a8c62edaa98f567e2ce/kcms/fonts/kxftconfig.cpp#L192 ↩

[bmrips]

Looks overall good to me! Can't we even remove the enableMutablePlaceholder guard, since the change is backwards compatible and does not have any downside? Or am I overlooking something?

However, it was failing with an error I couldn't determine the cause of:

In the test environment, the pkgs set is stubbed, i.e., the outPath of all packages is @<name>@. This also holds for systemd, such that xdg.configFile."...".source = "@systemd@/..." is not an absolute path anymore and the type-check fails. To fix this error, you have to modify the stub such that outPath is not stubbed, which can be achieved by setting test.stubs.systemd.outPath = null;.

[daniel-sampliner]

My reasoning for enableMutablePlaceholder guard is because the change introduces "impurity". Even though the change is backwards incompatible, it still may be better to make it explicitly opt-in (especially since the issue only affects certain Desktop Managers that are being configured imperatively or via UI).

Good to know regarding the test stub! I will give that a shot and update the tests.

[bmrips]

Yes, I get that. In case that #8084 is accepted, there also is the option to change the rule type from C to L+$, such that the placeholder is linked, overwritten on login, and deleted when fontconfig gets disabled. The only downside is that, in case that KDE picks up on the placeholder, the changes done through the GUI will be overwritten.

[daniel-sampliner]

Perhaps the best way to handle the mutability would be under the DE's submodule? For example in KDE's module, when enabled it would enable the fontconfig mutable placeholder. Additionally KDE's module could expose a toggle whether to wipe out GUI changes or not?

Although I don't use home-manager to configure my DE at all so I don't know if this fits into the current model of things.