From eb6ac6ab6836247463e02fc129fe3f278bcefead Mon Sep 17 00:00:00 2001
From: Jan Tojnar <jtojnar@gmail.com>
Date: Tue, 8 Sep 2020 12:59:32 +0200
Subject: [PATCH] nixos/fontconfig: Fix compatibility with unstable apps

Fontconfig 2.14 from unstable reverted back to using /etc/fonts/fonts.conf
for its configuration. Unfortunately, on NixOS 20.03, this still points
to configuration for Fontconfig 2.10, with cache version 3.

When an app linked against Fontconfig 2.14 reads the config and does not
find a compatible cache, it writes a new cache entries to ~/.cache/fontconfig.
Unfortunately, the fontconfig 2.14 uses the same cache version as 2.12 in 20.03 (7)
so when the apps from 20.03 later read the cache, they cannot make much sense
of it and are unable to find any fonts.

I added a new fonts.fontconfig.disableVersionedFontConfiguration option, which,
when enabled, makes /etc/fonts/fonts.conf point to the configuration file
for the latest fontconfig, instead of the ancient 2.10 version.
This is necessary to prevent packages from Nixpkgs unstable breaking apps
from 20.03.

Enabling this should not cause any issues as there are no programs
using the legacy fontconfig version since NixOS 15.03.

Unfortunately, if a person already ran an app from unstable
before applying this patch, they will need to delete ~/.cache/fontconfig manually.
---
 nixos/modules/config/fonts/fontconfig.nix | 24 ++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/nixos/modules/config/fonts/fontconfig.nix b/nixos/modules/config/fonts/fontconfig.nix
index 3bfa1893a8ba..e5e0a6915a79 100644
--- a/nixos/modules/config/fonts/fontconfig.nix
+++ b/nixos/modules/config/fonts/fontconfig.nix
@@ -195,7 +195,7 @@ let
   confPkg = pkgs.runCommand "fontconfig-conf" {
     preferLocalBuild = true;
   } ''
-    support_folder=$out/etc/fonts/conf.d
+    support_folder=$out/etc/fonts/${lib.optionalString cfg.disableVersionedFontConfiguration "2.10/"}conf.d
     latest_folder=$out/etc/fonts/${latestVersion}/conf.d
 
     mkdir -p $support_folder
@@ -206,6 +206,11 @@ let
     ln -s ${latestPkg.out}/etc/fonts/fonts.conf \
           $latest_folder/../fonts.conf
 
+    ${lib.optionalString cfg.disableVersionedFontConfiguration ''
+    ln -s $latest_folder/../fonts.conf \
+          $latest_folder/../../fonts.conf
+    ''}
+
     # fontconfig default config files
     ln -s ${supportPkg.out}/etc/fonts/conf.d/*.conf \
           $support_folder/
@@ -292,6 +297,23 @@ in
           '';
         };
 
+        disableVersionedFontConfiguration = mkOption {
+          type = types.bool;
+          default = true;
+          description = ''
+            If enabled, /etc/fonts/fonts.conf will contain configuration file
+            for the latest fontconfig, instead of the ancient 2.10 version.
+            This is necessary for using packages from Nixpkgs unstable.
+
+            Without it, running programs from unstable will populate
+            ~/.cache/fontconfig with values incompatible with
+            programs from NixOS 20.03.
+
+            Enabling this should not cause any issues as there are no programs
+            using the legacy fontconfig version since NixOS 15.03.
+          '';
+        };
+
         confPackages = mkOption {
           internal = true;
           type     = with types; listOf path;