Autostart gtk application with systemd

Question

I am writing a small daemon which logs the active window name. The relevant part goes like:

import wnck
import gtk

from plugins import IReporter


class ActiveWindowNameReporter(IReporter):

    export_as = 'active_window_name'

    def report(self):
        name = None

        screen = wnck.screen_get_default()
        screen.force_update()
        while gtk.events_pending():
                gtk.main_iteration()

        if screen:
            active_window = screen.get_active_window()

            if active_window:
                name = active_window.get_name()

        return name

I wrote a simple systemd user service file for this:

[Unit]
Description=Activity Monitor Service

[Service]
ExecStart=/usr/bin/python /home/me/Projects/monitor/monitor.py

[Install]
WantedBy=graphical.target

However, running the service fails. In journalctl -r I can see:

Unit actor.service entered failed state.
actor.service: main process exited, code=exited, status=1/FAILURE
ImportError: could not import gtk
import wnck
File "/home/me/Projects/monitor/reporters/active_window_name.py", line 1, in <mod
from reporters import *
File "/home/me/Projects/monitor/monitor.py", line 16, in <module>
Traceback (most recent call last):
detected unhandled Python exception in '/home/tbabej/Projects/actor/actor.py'      
Started Activity Monitor Service.                                                  
Starting Activity Monitor Service...

Seems like the import of gtk module fails because the session is not available? Setting the XDG_SESSION_ID variable in the script did not help though.

Answer 1

System services cannot and should not interact with user sessions. The assumption that there's always exactly one graphical session has never been true on Unix (X11 was multi-user since the beginning), and isn't even true on Windows (after NT 4.0 introduced Terminal Services and XP added Fast User Switching).

In other words, a system service doesn't know which X11 display it should use; doesn't have the right credentials even if you hardcode :0 (only the session's owner knows the right $XAUTHORITY value – being root does not grant any magic powers); and very often will be auto-started before any X11 displays appear, even if you hardcode /home/me/.Xauthority.

This means that scripts like yours must be started inside the graphical session – either through ~/.config/autostart/*.desktop (the XDG Autostart specification), or through ~/.xprofile (a regular shell script).

# ~/.config/autostart/monitor.desktop

[Desktop Entry]
Name=Activity monitor
Type=Application
Exec=nm-applet