gnome._gdkPixbufCacheBuilder_DO_NOT_USE: Extract from nixos/gdk-pixbuf

Unlike previously, we now fail loudly when a package not containing a gdk-pixbuf modules is passed.

nixos/modules/services/x11/gdk-pixbuf.nix

···
 { config, lib, pkgs, ... }:
 
-with lib;
-
 let
   cfg = config.services.xserver.gdk-pixbuf;
 
-  # Get packages to generate the cache for. We always include gdk-pixbuf.
-  effectivePackages = unique ([pkgs.gdk-pixbuf] ++ cfg.modulePackages);
-
-  # Generate the cache file by running gdk-pixbuf-query-loaders for each
-  # package and concatenating the results.
-  loadersCache = pkgs.runCommand "gdk-pixbuf-loaders.cache" { preferLocalBuild = true; } ''
-    (
-      for package in ${concatStringsSep " " effectivePackages}; do
-        module_dir="$package/${pkgs.gdk-pixbuf.moduleDir}"
-        if [[ ! -d $module_dir ]]; then
-          echo "Warning (services.xserver.gdk-pixbuf): missing module directory $module_dir" 1>&2
-          continue
-        fi
-        GDK_PIXBUF_MODULEDIR="$module_dir" \
-          ${pkgs.stdenv.hostPlatform.emulator pkgs.buildPackages} ${pkgs.gdk-pixbuf.dev}/bin/gdk-pixbuf-query-loaders
-      done
-    ) > "$out"
-  '';
+  loadersCache = pkgs.gnome._gdkPixbufCacheBuilder_DO_NOT_USE {
+    extraLoaders = lib.unique (cfg.modulePackages);
+  };
 in
 
 {
   options = {
-    services.xserver.gdk-pixbuf.modulePackages = mkOption {
-      type = types.listOf types.package;
+    services.xserver.gdk-pixbuf.modulePackages = lib.mkOption {
+      type = lib.types.listOf lib.types.package;
       default = [ ];
       description = lib.mdDoc "Packages providing GDK-Pixbuf modules, for cache generation.";
     };
···
   # If there is any package configured in modulePackages, we generate the
   # loaders.cache based on that and set the environment variable
   # GDK_PIXBUF_MODULE_FILE to point to it.
-  config = mkIf (cfg.modulePackages != []) {
+  config = lib.mkIf (cfg.modulePackages != []) {
     environment.variables = {
       GDK_PIXBUF_MODULE_FILE = "${loadersCache}";
     };

pkgs/desktops/gnome/default.nix

···
 lib.makeScope pkgs.newScope (self: with self; {
   updateScript = callPackage ./update.nix { };
 
+  # Temporary helper until gdk-pixbuf supports multiple cache files.
+  # This will go away, do not use outside Nixpkgs.
+  _gdkPixbufCacheBuilder_DO_NOT_USE = callPackage ./gdk-pixbuf-cache-builder.nix { };
+
   libsoup = pkgs.libsoup.override { gnomeSupport = true; };
   libchamplain = pkgs.libchamplain.override { libsoup = libsoup; };
 

pkgs/desktops/gnome/gdk-pixbuf-cache-builder.nix

···
+{
+  runCommand,
+  pkg-config,
+  gdk-pixbuf,
+  lib,
+  stdenv,
+  buildPackages,
+}:
+
+{
+  extraLoaders,
+}:
+
+let
+  # Get packages to generate the cache for. We always include gdk-pixbuf.
+  loaderPackages = [
+    gdk-pixbuf
+  ] ++ extraLoaders;
+in
+
+# Generate the cache file by running gdk-pixbuf-query-loaders for each
+# package and concatenating the results.
+runCommand "gdk-pixbuf-loaders.cache" {
+  preferLocalBuild = true;
+} ''
+  (
+    for package in ${lib.escapeShellArgs loaderPackages}; do
+      module_dir="$package/${gdk-pixbuf.moduleDir}"
+      if [[ ! -d "$module_dir" ]]; then
+        echo "Error: gdkPixbufCacheBuilder: Passed package “''${package}” does not contain GdkPixbuf loaders in “${gdk-pixbuf.moduleDir}”." 1>&2
+        exit 1
+      fi
+      GDK_PIXBUF_MODULEDIR="$module_dir" \
+        ${stdenv.hostPlatform.emulator buildPackages} ${gdk-pixbuf.dev}/bin/gdk-pixbuf-query-loaders
+    done
+  ) > "$out"
+''