I hope you enjoyed learning more about Ubuntu Core Desktop in our last post. This week we’ll be getting into some of the engineering work necessary to make this all work, specifically the Ubuntu Desktop Session as a snap.
Let’s talk a bit more about what the desktop session snap really is. This is the component that flavours or other derivatives will need to provide to build custom desktop experiences. For Ubuntu Desktop, the snap provides all the necessary desktop services to run a traditional GNOME desktop. An interesting point to make here is that the desktop session snap runs under confinement as well, isolated from other snaps. This isn’t some unconfined space for privileged services to run.
What does this really mean? We’ll start with a quick overview of how snaps use “plugs” and “slots”. Snapd leverages kernel level mediation for syscalls and resource access as well as providing a well defined set of interfaces that provide access to necessary resources and manages connections between the slot and plug. Users can disconnect these interfaces to prevent this access. All desktop snaps have a few common interfaces necessary to work in a desktop session, things like wayland and x11 are pretty self-explanatory. If you disconnect x11, the app will no longer be able to talk to an X server, for example. There’s another important interface for desktop snaps, which is the “desktop” interface. This interface provides access to things like fonts, XDG Portal APIs, App indicator, etc. For more information on these interfaces see the snapd documentation.
On classic Ubuntu systems, the slot side of interfaces like wayland, x11, and desktop are provided by the host system as these services are running unconfined on the host system. In the case of an Ubuntu Core based desktop, these slots are provided by the desktop session snap.
Interface Plug Slot Notes audio-playback firefox:audio-playback ubuntu-desktop-session:audio-playback - audio-record firefox:audio-record ubuntu-desktop-session:audio-record - desktop firefox:desktop ubuntu-desktop-session:desktop - wayland firefox:wayland ubuntu-desktop-session:wayland - x11 firefox:x11 ubuntu-desktop-session:x11 -
In the above example, firefox is allowed access to pipewire running inside the ubuntu-desktop-session for audio playback and recording. Disconnecting it from the “audio-record” slot will disable access to the microphone. The connection to the desktop slot allows it to access settings, fonts, and other DBus APIs that are running in ubuntu-desktop-session.
This is where we get into some of the engineering work that was necessary to build a desktop experience based on Ubuntu Core. Snapd has had the necessary mediation since the beginning, however running a desktop session requires the use of user session services. Snapd already had support for systemd system services, but we really need user session services and DBus activation in the user session to provide a full featured desktop.
These are features useful for classic desktop as well, but essential for desktop on Ubuntu Core. We set out to add support for user-daemon and dbus-activation in Snapd. More information can be found on the snapcraft forum.
This feature allows a snap to provide a systemd user daemon. Here’s an example of how we utilize this in the ubuntu-desktop-session snap to run pipewire:
apps: wireplumber: command: run.sh /usr/bin/wireplumber daemon: simple daemon-scope: user after: - pipewire
In this example, we run wireplumber via systemd in the user session, after pipewire has started.
DBus activation just extends the user-daemon functionality by registering a service to be launched when needed via DBus. Here’s an example of how we utilize this in the ubuntu-desktop-session snap to run xdg-desktop-portal-gnome:
slots: dbus-freedesktop-impl-portal-gnome: interface: dbus bus: system name: org.freedesktop.impl.portal.desktop.gnome apps: xdg-desktop-portal-gnome: command: run.sh /usr/libexec/xdg-desktop-portal-gnome daemon: simple activates-on: - dbus-freedesktop-impl-portal-gnome
Notice in this example we include a DBus slot definition and declare the user-daemon to activate on the slot.
Both the user-daemon and dbus-activation features landed in snapd a while back behind an experimental flag, enabled on classic Ubuntu Desktop since 22.04 for the snapd-desktop-integration snap which provides user services to improve integration with the desktop experience. We’ve enabled these on Ubuntu Core Desktop, as they are essential
Other Desktop Services
A full list of services running within confinement as part of the ubuntu-desktop-session snap:
- GNOME Shell
- Pipewire Pulse
- XDG Desktop Portal GNOME
- XDG Desktop Portal GTK
The desktop session snap also provides the wayland socket, surfaced through the wayland slot as well as X support via Xwayland surfaced through the x11 slot.
Stay tuned for our next post where we’ll talk about the desktop-launch interface and the challenges to enable application launching from within the desktop session snap.