gtk: new service giving support to GTK+ applications

nixos/modules/config/gtk/gtk-icon-cache.nix

@@ -0,0 +1,86 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+{
+  options = {
+    gtk.iconCache.enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to build icon theme caches for GTK+ applications.
+      '';
+    };
+  };
+
+  config = mkIf config.gtk.iconCache.enable {
+
+    # (Re)build icon theme caches
+    # ---------------------------
+    # Each icon theme has its own cache. The difficult is that many
+    # packages may contribute with icons to the same theme by installing
+    # some icons.
+    #
+    # For instance, on my current NixOS system, the following packages
+    # (among many others) have icons installed into the hicolor icon
+    # theme: hicolor-icon-theme, psensor, wpa_gui, caja, etc.
+    #
+    # As another example, the mate icon theme has icons installed by the
+    # packages mate-icon-theme, mate-settings-daemon, and libmateweather.
+    #
+    # The HighContrast icon theme also has icons from different packages,
+    # like gnome-theme-extras and meld.
+
+    # When the cache is built all of its icons has to be known. How to
+    # implement this?
+    #
+    # I think that most themes have all icons installed by only one
+    # package. On my system there are 71 themes installed. Only 3 of them
+    # have icons installed from more than one package.
+    #
+    # If the main package of the theme provides a cache, presumably most
+    # of its icons will be available to applications without running this
+    # module. But additional icons offered by other packages will not be
+    # available. Therefore I think that it is good that the main theme
+    # package installs a cache (although it does not completely fixes the
+    # situation for packages installed with nix-env).
+    #
+    # The module solution presented here keeps the cache when there is
+    # only one package contributing with icons to the theme. Otherwise it
+    # rebuilds the cache taking into account the icons provided all
+    # packages.
+
+    environment.extraSetup = ''
+      # For each icon theme directory ...
+
+      find $out/share/icons -mindepth 1 -maxdepth 1 -print0 | while read -d $'\0' themedir
+      do
+
+        # In order to build the cache, the theme dir should be
+        # writable. When the theme dir is a symbolic link to somewhere
+        # in the nix store it is not writable and it means that only
+        # one package is contributing to the theme. If it already has
+        # a cache, no rebuild is needed. Otherwise a cache has to be
+        # built, and to be able to do that we first remove the
+        # symbolic link and make a directory, and then make symbolic
+        # links from the original directory into the new one.
+
+        if [ ! -w "$themedir" -a -L "$themedir" -a ! -r "$themedir"/icon-theme.cache ]; then
+          name=$(basename "$themedir")
+          path=$(readlink -f "$themedir")
+          rm "$themedir"
+          mkdir -p "$themedir"
+          ln -s "$path"/* "$themedir"/
+        fi
+
+        # (Re)build the cache if the theme dir is writable, replacing any
+        # existing cache for the theme
+
+        if [ -w "$themedir" ]; then
+          rm -f "$themedir"/icon-theme.cache
+          ${pkgs.gtk3.out}/bin/gtk-update-icon-cache --ignore-theme-index "$themedir"
+        fi
+      done
+    '';
+  };
+
+}

nixos/modules/config/system-path.nix

@@ -135,10 +135,6 @@ in
       # outputs TODO: note that the tools will often not be linked by default
       postBuild =
         ''
-          if [ -x $out/bin/gtk-update-icon-cache -a -f $out/share/icons/hicolor/index.theme ]; then
-              $out/bin/gtk-update-icon-cache $out/share/icons/hicolor
-          fi
-
           if [ -x $out/bin/glib-compile-schemas -a -w $out/share/glib-2.0/schemas ]; then
               $out/bin/glib-compile-schemas $out/share/glib-2.0/schemas
           fi

nixos/modules/module-list.nix

@@ -11,6 +11,7 @@
   ./config/xdg/icons.nix
   ./config/xdg/menus.nix
   ./config/xdg/mime.nix
+  ./config/gtk/gtk-icon-cache.nix
   ./config/gnu.nix
   ./config/i18n.nix
   ./config/iproute2.nix