fontconfig: Only read versioned config dirs

Falling back to unversioned `/etc/fonts/conf.d` when versioned one does not exist
is problematic since it only occurs on non-NixOS systems and those are likely
to have a different version of fontconfig. When those versions use incompatible
elements in the config, apps using fontconfig will crash.

Instead, we are now falling back to the in-package `fonts.conf` file that loads
both the versioned global `conf.d` directory and the in-package `conf.d` since using
upstream settings on non-NixOS is preferable to not being able to use apps there.

In fact, we would not even need to link `fonts.conf`, as the in-package `fonts.conf`
will be always used unless someone creates the global one manually (the option is still
retained if one wants to write a custom NixOS module and to avoid unnecessary stat call on NixOS).

Additionally, since the `fonts.conf` will always load `conf.d` from the package, we no longer
need to install them to sytem `/etc` in the module. This needed some mucking with `50-user.conf`
which disables configs in user directories (a good thing IMO, NixOS module will turn it back on)
but otherwise, it is cleaner. The files are still prioritized by their name, regardless of their location.

See https://github.com/NixOS/nixpkgs/pull/73795#issuecomment-634370125 for more information.

nixos/modules/config/fonts/fontconfig.nix

@@ -214,15 +214,7 @@ let
     # fontconfig default config files
     ln -s ${supportPkg.out}/etc/fonts/conf.d/*.conf \
           $support_folder/
-    ln -s ${latestPkg.out}/etc/fonts/conf.d/*.conf \
-          $latest_folder/
-
-    # update latest 51-local.conf path to look at the latest local.conf
-    rm    $latest_folder/51-local.conf
-
-    substitute ${latestPkg.out}/etc/fonts/conf.d/51-local.conf \
-               $latest_folder/51-local.conf \
-               --replace local.conf /etc/fonts/${latestVersion}/local.conf
+    # Latest fontconfig is configured to look for the upstream defaults inside the package.
 
     # 00-nixos-cache.conf
     ln -s ${cacheConfSupport} \
@@ -236,7 +228,11 @@ let
     # 50-user.conf
     ${optionalString (!cfg.includeUserConf) ''
     rm $support_folder/50-user.conf
-    rm $latest_folder/50-user.conf
+    ''}
+    # Since latest fontconfig looks for default files inside the package,
+    # we had to move this one elsewhere to be able to exclude it here.
+    ${optionalString cfg.includeUserConf ''
+    ln -s ${latestPkg.out}/etc/fonts/conf.d.bak/50-user.conf $latest_folder/50-user.conf
     ''}
 
     # local.conf (indirect priority 51)

pkgs/development/libraries/fontconfig/config-compat.patch

@@ -1,17 +1,22 @@
-commit 05c6adf8104b4321d3a3716a7b9feb6bf223ed0c (HEAD, nixpkgs)
-Author: Vladimír Čunát <vcunat@gmail.com>
-Date:   Tue Nov 4 12:24:25 2014 +0100
+From 2ff9b53ce755be183ef9274f7dd3f9ac537173f6 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Vladim=C3=ADr=20=C4=8Cun=C3=A1t?= <vcunat@gmail.com>
+Date: Tue, 4 Nov 2014 12:24:25 +0100
+Subject: [PATCH] add check for /etc/fonts/@configVersion@/fonts.conf
 
-    add check for /etc/fonts/@configVersion@/fonts.conf
-    
-    It's checked between FONTCONFIG_FILE and the usual /etc/fonts/fonts.conf.
-    Also, hardcode /etc/fonts/fonts.conf to prevent accidental override.
+It's checked between FONTCONFIG_FILE and the in-package etc/fonts/fonts.conf.
+The latter is used so that on non-NixOS distributions, fontconfig works at least
+with upstream defaults, even when the global config is incompatible.
+
+Co-Authored-By: Jan Tojnar <jtojnar@gmail.com>
+---
+ src/fccfg.c | 7 ++++++-
+ 1 file changed, 6 insertions(+), 1 deletion(-)
 
 diff --git a/src/fccfg.c b/src/fccfg.c
-index 6377fd7..e9eb10a 100644
+index 342c996..98a1324 100644
 --- a/src/fccfg.c
 +++ b/src/fccfg.c
-@@ -2070,8 +2070,13 @@ FcConfigFilename (const FcChar8 *url)
+@@ -2391,8 +2391,13 @@ FcConfigGetFilename (FcConfig      *config,
      if (!url || !*url)
      {
  	url = (FcChar8 *) getenv ("FONTCONFIG_FILE");
@@ -22,7 +27,10 @@ index 6377fd7..e9eb10a 100644
 +	}
  	if (!url)
 -	    url = (FcChar8 *) FONTCONFIG_FILE;
-+	    url = (FcChar8 *) "/etc/fonts/fonts.conf";
++	    url = (FcChar8 *) FONTCONFIG_PATH "/" FONTCONFIG_FILE;
      }
      file = 0;
  
+-- 
+2.26.2
+

pkgs/development/libraries/fontconfig/default.nix

@@ -13,9 +13,10 @@
 
 /** Font configuration scheme
  - ./config-compat.patch makes fontconfig try the following root configs, in order:
-    $FONTCONFIG_FILE, /etc/fonts/${configVersion}/fonts.conf, /etc/fonts/fonts.conf
+    $FONTCONFIG_FILE, /etc/fonts/${configVersion}/fonts.conf, ${fontconfig.out}/etc/fonts/fonts.conf
     This is done not to override config of pre-2.11 versions (which just blow up)
-    and still use *global* font configuration at both NixOS or non-NixOS.
+    and still use *global* font configuration at NixOS,
+    falling back to upstream defaults on non-NixOS.
  - NixOS creates /etc/fonts/${configVersion}/fonts.conf link to $out/etc/fonts/fonts.conf,
     and other modifications should go to /etc/fonts/${configVersion}/conf.d
  - See ./make-fonts-conf.xsl for config details.
@@ -99,11 +100,20 @@ stdenv.mkDerivation rec {
   postInstall = ''
     cd "$out/etc/fonts"
     xsltproc --stringparam fontDirectories "${dejavu_fonts.minimal}" \
+      --stringparam fontconfig "$out" \
       --stringparam fontconfigConfigVersion "${configVersion}" \
       --path $out/share/xml/fontconfig \
       ${./make-fonts-conf.xsl} $out/etc/fonts/fonts.conf \
       > fonts.conf.tmp
     mv fonts.conf.tmp $out/etc/fonts/fonts.conf
+
+    # Make it easier to remove user config in NixOS module.
+    mkdir -p $out/etc/fonts/conf.d.bak
+    mv $out/etc/fonts/conf.d/50-user.conf $out/etc/fonts/conf.d.bak
+
+    # update latest 51-local.conf path to look at the latest local.conf
+    substituteInPlace $out/etc/fonts/conf.d/51-local.conf \
+      --replace local.conf /etc/fonts/${configVersion}/local.conf
   '';
 
   passthru = {

pkgs/development/libraries/fontconfig/make-fonts-conf.xsl

@@ -31,6 +31,9 @@
       <!-- versioned system-wide config -->
       <include ignore_missing="yes">/etc/fonts/<xsl:value-of select="$fontconfigConfigVersion" />/conf.d</include>
 
+      <!-- upstream config -->
+      <include><xsl:value-of select="$fontconfig" />/etc/fonts/conf.d</include>
+
       <dir prefix="xdg">fonts</dir>
       <xsl:for-each select="str:tokenize($fontDirectories)">
         <dir><xsl:value-of select="." /></dir>