doc: convert GNOME platform docs to markdown

2020-12-07 23:49:56 +01:00

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GNOME

Packaging GNOME applications

Programs in the GNOME universe are written in various languages but they all use GObject-based libraries like GLib, GTK or GStreamer. These libraries are often modular, relying on looking into certain directories to find their modules. However, due to Nix’s specific file system organization, this will fail without our intervention. Fortunately, the libraries usually allow overriding the directories through environment variables, either natively or thanks to a patch in nixpkgs. Wrapping the executables to ensure correct paths are available to the application constitutes a significant part of packaging a modern desktop application. In this section, we will describe various modules needed by such applications, environment variables needed to make the modules load, and finally a script that will do the work for us.

Settings

GSettings API is often used for storing settings. GSettings schemas are required, to know the type and other metadata of the stored values. GLib looks for glib-2.0/schemas/gschemas.compiled files inside the directories of XDG_DATA_DIRS.

On Linux, GSettings API is implemented using dconf backend. You will need to add dconf GIO module to GIO_EXTRA_MODULES variable, otherwise the memory backend will be used and the saved settings will not be persistent.

Last you will need the dconf database D-Bus service itself. You can enable it using programs.dconf.enable.

Some applications will also require gsettings-desktop-schemas for things like reading proxy configuration or user interface customization. This dependency is often not mentioned by upstream, you should grep for org.gnome.desktop and org.gnome.system to see if the schemas are needed.

GdkPixbuf loaders

GTK applications typically use GdkPixbuf to load images. But gdk-pixbuf package only supports basic bitmap formats like JPEG, PNG or TIFF, requiring to use third-party loader modules for other formats. This is especially painful since GTK itself includes SVG icons, which cannot be rendered without a loader provided by librsvg.

Unlike other libraries mentioned in this section, GdkPixbuf only supports a single value in its controlling environment variable GDK_PIXBUF_MODULE_FILE. It is supposed to point to a cache file containing information about the available loaders. Each loader package will contain a lib/gdk-pixbuf-2.0/2.10.0/loaders.cache file describing the default loaders in gdk-pixbuf package plus the loader contained in the package itself. If you want to use multiple third-party loaders, you will need to create your own cache file manually. Fortunately, this is pretty rare as not many loaders exist.

gdk-pixbuf contains a setup hook that sets GDK_PIXBUF_MODULE_FILE from dependencies but as mentioned in further section, it is pretty limited. Loaders should propagate this setup hook.

Icons

When an application uses icons, an icon theme should be available in XDG_DATA_DIRS during runtime. The package for the default, icon-less hicolor-icon-theme (should be propagated by every icon theme) contains a setup hook that will pick up icon themes from buildInputs and pass it to our wrapper. Unfortunately, relying on that would mean every user has to download the theme included in the package expression no matter their preference. For that reason, we leave the installation of icon theme on the user. If you use one of the desktop environments, you probably already have an icon theme installed.

To avoid costly file system access when locating icons, GTK, as well as Qt, can rely on icon-theme.cache files from the themes' top-level directories. These files are generated using gtk-update-icon-cache, which is expected to be run whenever an icon is added or removed to an icon theme (typically an application icon into hicolor theme) and some programs do indeed run this after icon installation. However, since packages are installed into their own prefix by Nix, this would lead to conflicts. For that reason, gtk3 provides a setup hook that will clean the file from installation. Since most applications only ship their own icon that will be loaded on start-up, it should not affect them too much. On the other hand, icon themes are much larger and more widely used so we need to cache them. Because we recommend installing icon themes globally, we will generate the cache files from all packages in a profile using a NixOS module. You can enable the cache generation using gtk.iconCache.enable option if your desktop environment does not already do that.

Packaging icon themes

Icon themes may inherit from other icon themes. The inheritance is specified using the Inherits key in the index.theme file distributed with the icon theme. According to the icon theme specification, icons not provided by the theme are looked for in its parent icon themes. Therefore the parent themes should be installed as dependencies for a more complete experience regarding the icon sets used.

The package hicolor-icon-theme provides a setup hook which makes symbolic links for the parent themes into the directory share/icons of the current theme directory in the nix store, making sure they can be found at runtime. For that to work the packages providing parent icon themes should be listed as propagated build dependencies, together with hicolor-icon-theme.

Also make sure that icon-theme.cache is installed for each theme provided by the package, and set dontDropIconThemeCache to true so that the cache file is not removed by the gtk3 setup hook.

GTK Themes

Previously, a GTK theme needed to be in XDG_DATA_DIRS. This is no longer necessary for most programs since GTK incorporated Adwaita theme. Some programs (for example, those designed for elementary HIG) might require a special theme like pantheon.elementary-gtk-theme.

GObject introspection typelibs

GObject introspection allows applications to use C libraries in other languages easily. It does this through typelib files searched in GI_TYPELIB_PATH.

Various plug-ins

If your application uses GStreamer or Grilo, you should set GST_PLUGIN_SYSTEM_PATH_1_0 and GRL_PLUGIN_PATH, respectively.

Onto `wrapGAppsHook`

Given the requirements above, the package expression would become messy quickly:

preFixup = ''
  for f in $(find $out/bin/ $out/libexec/ -type f -executable); do
    wrapProgram "$f" \
      --prefix GIO_EXTRA_MODULES : "${getLib dconf}/lib/gio/modules" \
      --prefix XDG_DATA_DIRS : "$out/share" \
      --prefix XDG_DATA_DIRS : "$out/share/gsettings-schemas/${name}" \
      --prefix XDG_DATA_DIRS : "${gsettings-desktop-schemas}/share/gsettings-schemas/${gsettings-desktop-schemas.name}" \
      --prefix XDG_DATA_DIRS : "${hicolor-icon-theme}/share" \
      --prefix GI_TYPELIB_PATH : "${lib.makeSearchPath "lib/girepository-1.0" [ pango json-glib ]}"
  done
'';

Fortunately, there is [wrapGAppsHook]{#ssec-gnome-hooks-wrapgappshook}. It works in conjunction with other setup hooks that populate environment variables, and it will then wrap all executables in bin and libexec directories using said variables.

For convenience, it also adds dconf.lib for a GIO module implementing a GSettings backend using dconf, gtk3 for GSettings schemas, and librsvg for GdkPixbuf loader to the closure. In case you are packaging a program without a graphical interface, you might want to use [wrapGAppsNoGuiHook]{#ssec-gnome-hooks-wrapgappsnoguihook}, which runs the same script as wrapGAppsHook but does not bring gtk3 and librsvg into the closure.

wrapGAppsHook itself will add the package’s share directory to XDG_DATA_DIRS.
[]{#ssec-gnome-hooks-glib} glib setup hook will populate GSETTINGS_SCHEMAS_PATH and then wrapGAppsHook will prepend it to XDG_DATA_DIRS.
[]{#ssec-gnome-hooks-gdk-pixbuf} gdk-pixbuf setup hook will populate GDK_PIXBUF_MODULE_FILE with the path to biggest loaders.cache file from the dependencies containing GdkPixbuf loaders. This works fine when there are only two packages containing loaders (gdk-pixbuf and e.g. librsvg) – it will choose the second one, reasonably expecting that it will be bigger since it describes extra loader in addition to the default ones. But when there are more than two loader packages, this logic will break. One possible solution would be constructing a custom cache file for each package containing a program like services/x11/gdk-pixbuf.nix NixOS module does. wrapGAppsHook copies the GDK_PIXBUF_MODULE_FILE environment variable into the produced wrapper.
[]{#ssec-gnome-hooks-gtk-drop-icon-theme-cache} One of gtk3’s setup hooks will remove icon-theme.cache files from package’s icon theme directories to avoid conflicts. Icon theme packages should prevent this with dontDropIconThemeCache = true;.
[]{#ssec-gnome-hooks-dconf} dconf.lib is a dependency of wrapGAppsHook, which then also adds it to the GIO_EXTRA_MODULES variable.
[]{#ssec-gnome-hooks-hicolor-icon-theme} hicolor-icon-theme’s setup hook will add icon themes to XDG_ICON_DIRS which is prepended to XDG_DATA_DIRS by wrapGAppsHook.
[]{#ssec-gnome-hooks-gobject-introspection} gobject-introspection setup hook populates GI_TYPELIB_PATH variable with lib/girepository-1.0 directories of dependencies, which is then added to wrapper by wrapGAppsHook. It also adds share directories of dependencies to XDG_DATA_DIRS, which is intended to promote GIR files but it also pollutes the closures of packages using wrapGAppsHook.

::: warning The setup hook currently does not work in expressions with strictDeps enabled, like Python packages. In those cases, you will need to disable it with strictDeps = false;. :::
[]{#ssec-gnome-hooks-gst-grl-plugins} Setup hooks of gst_all_1.gstreamer and gnome3.grilo will populate the GST_PLUGIN_SYSTEM_PATH_1_0 and GRL_PLUGIN_PATH variables, respectively, which will then be added to the wrapper by wrapGAppsHook.

You can also pass additional arguments to makeWrapper using gappsWrapperArgs in preFixup hook:

preFixup = ''
  gappsWrapperArgs+=(
    # Thumbnailers
    --prefix XDG_DATA_DIRS : "${gdk-pixbuf}/share"
    --prefix XDG_DATA_DIRS : "${librsvg}/share"
    --prefix XDG_DATA_DIRS : "${shared-mime-info}/share"
  )
'';

Updating GNOME packages

Most GNOME package offer updateScript, it is therefore possible to update to latest source tarball by running nix-shell maintainers/scripts/update.nix --argstr package gnome3.nautilus or even en masse with nix-shell maintainers/scripts/update.nix --argstr path gnome3. Read the package’s NEWS file to see what changed.

Frequently encountered issues

`GLib-GIO-ERROR **: 06:04:50.903: No GSettings schemas are installed on the system`

There are no schemas available in XDG_DATA_DIRS. Temporarily add a random package containing schemas like gsettings-desktop-schemas to buildInputs. glib and wrapGAppsHook setup hooks will take care of making the schemas available to application and you will see the actual missing schemas with the next error. Or you can try looking through the source code for the actual schemas used.

`GLib-GIO-ERROR **: 06:04:50.903: Settings schema ‘org.gnome.foo’ is not installed`

Package is missing some GSettings schemas. You can find out the package containing the schema with nix-locate org.gnome.foo.gschema.xml and let the hooks handle the wrapping as above.

When using `wrapGAppsHook` with special derivers you can end up with double wrapped binaries.

This is because derivers like python.pkgs.buildPythonApplication or qt5.mkDerivation have setup-hooks automatically added that produce wrappers with makeWrapper. The simplest way to workaround that is to disable the wrapGAppsHook automatic wrapping with dontWrapGApps = true; and pass the arguments it intended to pass to makeWrapper to another.

In the case of a Python application it could look like:

python3.pkgs.buildPythonApplication {
  pname = "gnome-music";
  version = "3.32.2";

  nativeBuildInputs = [
    wrapGAppsHook
    gobject-introspection
    ...
  ];

  dontWrapGApps = true;

  # Arguments to be passed to `makeWrapper`, only used by buildPython*
  preFixup = ''
    makeWrapperArgs+=("''${gappsWrapperArgs[@]}")
  '';
}

And for a QT app like:

mkDerivation {
  pname = "calibre";
  version = "3.47.0";

  nativeBuildInputs = [
    wrapGAppsHook
    qmake
    ...
  ];

  dontWrapGApps = true;

  # Arguments to be passed to `makeWrapper`, only used by qt5’s mkDerivation
  preFixup = ''
    qtWrapperArgs+=("''${gappsWrapperArgs[@]}")
  '';
}

I am packaging a project that cannot be wrapped, like a library or GNOME Shell extension.

You can rely on applications depending on the library setting the necessary environment variables but that is often easy to miss. Instead we recommend to patch the paths in the source code whenever possible. Here are some examples:

[]{#ssec-gnome-common-issues-unwrappable-package-gnome-shell-ext} Replacing a GI_TYPELIB_PATH in GNOME Shell extension – we are using substituteAll to include the path to a typelib into a patch.
[]{#ssec-gnome-common-issues-unwrappable-package-gsettings} The following examples are hardcoding GSettings schema paths. To get the schema paths we use the functions
- glib.getSchemaPath Takes a nix package attribute as an argument.
- glib.makeSchemaPath Takes a package output like $out and a derivation name. You should use this if the schemas you need to hardcode are in the same derivation.
[]{#ssec-gnome-common-issues-unwrappable-package-gsettings-vala} Hard-coding GSettings schema path in Vala plug-in (dynamically loaded library) – here, substituteAll cannot be used since the schema comes from the same package preventing us from pass its path to the function, probably due to a Nix bug.

[]{#ssec-gnome-common-issues-unwrappable-package-gsettings-c} Hard-coding GSettings schema path in C library – nothing special other than using Coccinelle patch to generate the patch itself.

I need to wrap a binary outside `bin` and `libexec` directories.

You can manually trigger the wrapping with wrapGApp in preFixup phase. It takes a path to a program as a first argument; the remaining arguments are passed directly to wrapProgram function.

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