camoz/sway-systemd

Systemd integration for Sway session

Systemd integration for Sway

Goals and requirements

The goal of this project is to provide a minimal set of configuration files and scripts required for running Sway in a systemd environment. This includes several areas of integration:

• Propagate required variables to the systemd user session environment.
• Define sway-session.target for starting user services.
• Place GUI applications into a systemd scopes for systemd-oomd compatibility.

Non-goals

• Running the compositor itself as a user service. sway-services already exists and does exactly that.

• Managing sway environment. It's hard, opinionated and depends on the way user starts sway, so I don't have a solution that works for everyone and is acceptable for default configuration. See also #6.
  The common solutions are ~/.profile (if your display manager supports that), ~/.pam_environment, or a wrapper script that sets the variables before starting sway.

• Supporting multiple concurrent sway sessions for the same user. It's uncommon and doing so would cause problems for which there are no easy solutions: To make this work, we set WAYLAND_DISPLAY with a systemd user session. This environment variable only accurate per-session, while systemd user sessions are per-user. If the user starts a second Sway instance on the same machine, the new instance would overwrite this value, which could cause some services to break in the first session if they were to read the variable after the second session starts.

Components

Session target

Systemd forbids starting the graphical-session.target directly and encourages use of an environment-specific target units. Thus, the package here defines sway-session.target that binds to graphical-session.target and starts user services enabled for a graphical session. sway-session.target should be started when the compositor is ready and the user-session environment is set, and stopped before the compositor exits.

A systemd user service may depend on or reference sway-session.target only if it is specific for sway. Otherwise, it's recommended to use graphical-session.target.

Session script

The session.sh script is responsible for importing variables into systemd and dbus activation environments and starting session target. When the --with-cleanup argument is specified, it also waits in the background until the compositor exits, stops the session target and unsets variables for systemd user session.

The script itself does not set any variables except XDG_CURRENT_DESKTOP; it simply passes the values received from sway. The list of variables and the name of the session target are currently hardcoded and could be changed by editing the script.

For a better description see comments in the code.

Cgroups assignment script

The assign-cgroups.py script subscribes to a new window i3 ipc event and automatically creates a transient scope unit (with path app.slice/app-${app_id}.slice/app-${app_id}-${pid}.scope) for each GUI application launched in the same cgroup as the compositor. Existing child processes of the application are assigned to the same scope.

The script is necessary to overcome a limitation of systemd-oomd: it only tracks resource usage by cgroups and kills the whole group when a single application misbehaves and exceeds resource usage limits. By placing individual apps into isolated cgroups we are decreasing the chance that oomd killer would target the group with the compositor and accidentally terminate the session.

It can also be used to impose resource usage limits on a specific application, because transient units are still loading override configs.
For example, by creating $XDG_CONFIG_HOME/systemd/user/app-firefox.slice.d/override.conf with content

[Slice]
MemoryHigh=2G

you can tell systemd that all Firefox processes combined are not allowed to use more than 2 Gb of memory. See systemd.resource-control(5) for other available resource control options.

Installation

Dependencies

Session script calls these commands: swaymsg, systemctl, dbus-update-activation-environment.

Cgroups script uses following python packages: dbus-next, i3ipc, psutil, tenacity, python-xlib

Installing with meson

meson build
sudo ninja -C build install

Only the session part is installed by default. Pass -Dcgroups=enabled to the meson build command to install cgroups assignment script as well.

The command will install configuration files from config.d to the /etc/sway/config.d/ directory which is included from the default sway config. If you are using custom sway configuration file and already removed the include /etc/sway/config.d/* line you may need to edit your config and include the installed files.

Installing manually/using directly from git checkout

1. Clone repository.
2. Copy sway-session.target to the systemd user unit directory (/usr/lib/systemd/user/, $XDG_CONFIG_HOME/systemd/user/ or ~/.config/systemd/user are common locations).
3. Run systemctl --user daemon-reload to make systemd rescan the service files.
4. Add exec /path/to/cloned/repo/src/session.sh --with-cleanup to your sway config for environment and session configuration.
5. Add exec /path/to/cloned/repo/src/assign-cgroups.py to your sway config to enable cgroup assignment script.
6. Restart your sway session or run swaymsg with the commands above. Simple config reload is insufficient as it does not execute exec commands.