Why does systemd not send unit properties changed notifcations on session bus? - python-3.x

Question

I implemented a dbus systemd listener in Python (3.7) that shall monitor property changes of a systemd unit. On the session dbus it does not receive any notifications. Running on the system dbus the code does what is expected.
Is there a way to also receive unit changed notifications on the session bus?
My system: A Raspberry 4 running latest version of Raspberry PI OS.
This is the service I created.
[Unit]
Description = A dummy service
[Service]
Type = simple
ExecStart = /bin/true
RemainAfterExit=yes
I installed the service to /etc/systemd/system and ~/.config/systemd/user and executed daemon-reload for the system and the user session. After doing, the service is known as user and as system service.
This is the dummy_listener.py code
#!/usr/bin/env python3
# Python version required: >= 3.7 (because of used asyncio API)
"""A simple subscriber/listener for systemd unit signals"""
import sys
import asyncio
from dbus_next.aio import MessageBus
from dbus_next import BusType
class DbusDummyService(): # pylint: disable=no-self-use
"""Asyncio based dummy.service listener"""
async def init(self, bus_type=BusType.SESSION):
"""Register listener callback with dbus bus_type"""
bus = await MessageBus(bus_type=bus_type).connect()
# Get introspection XML
introspection = await bus.introspect('org.freedesktop.systemd1',
'/org/freedesktop/systemd1/unit/dummy_2eservice')
# Select systemd service object
obj = bus.get_proxy_object('org.freedesktop.systemd1',
'/org/freedesktop/systemd1/unit/dummy_2eservice', introspection)
# Get required interfaces
properties_if = obj.get_interface('org.freedesktop.DBus.Properties')
# Monitor service status changes
properties_if.on_properties_changed(self.on_properties_changed_cb)
def on_properties_changed_cb(self, interface_name, changed_props, invalidated_props):
"""Callback expected to be called on unit property changes"""
print(f"Callback invoked for interface {interface_name}:")
print(f" Properties updated")
for prop, val in changed_props.items():
print(f" {prop} set to {val.value}")
print(f" Properties invalidated")
for prop in invalidated_props:
print(f" {prop} invalidated")
async def main(bus_type):
"""Asyncio main"""
# Initialize dbus listener
await DbusDummyService().init(bus_type)
# Run loop forever (waiting for dbus signals)
await asyncio.get_running_loop().create_future()
if __name__ == "__main__":
try:
BUS_TYPE = BusType.SYSTEM if 'sys' in sys.argv[1] else BusType.SESSION
except BaseException:
BUS_TYPE = BusType.SESSION
asyncio.run(main(BUS_TYPE))
The listener is ran like this on system dbus
sudo python3 dummy_lister.py sys
For session bus it is ran with
python3 dummy_lister.py
In a separate window I now restart the dummy services and expect the listener to output the prints.
For the session dbus:
systemctl --user restart dummy
For the system dbus:
sudo systemctl restart dummy
On the session dbus the listener just prints nothing. On the system dbus, I receive a bunch of messages.
Any ideas?

Answer 1

systemd doesn't send PropertiesChanged signals unless at least one client is subscribed to it. You need to call the Subscribe() method from the org.freedesktop.systemd1.Manager interface on the /org/freedesktop/systemd1 object.