feat(dbus): Add avahi-daemon configuration to system services

Introduce a `configure_avahi_daemon` function to manage the avahi-daemon
service and socket with proper post actions. Update the interface
configuration to enable avahi management using the new function.

src/revpi_middleware/dbus_middleware1/system_config/interface_config.py

@@ -5,7 +5,7 @@
 from collections import namedtuple
 from logging import getLogger
 
-from .revpi_config import ConfigActions, configure_gui, simple_systemd
+from .revpi_config import ConfigActions, configure_avahi_daemon, configure_gui, simple_systemd
 from ..dbus_helper import DbusInterface
 
 log = getLogger(__name__)
@@ -85,6 +85,6 @@ AVAILABLE_FEATURES = {
     "revpipyload": FeatureFunction(simple_systemd, ["revpipyload.service"]),
     "bluetooth": False,
     "ieee80211": False,
-    "avahi": False,
+    "avahi": FeatureFunction(configure_avahi_daemon, []),
     "external-antenna": False,
 }

src/revpi_middleware/dbus_middleware1/system_config/revpi_config.py

@@ -93,6 +93,18 @@ class RevPiConfig:
         return self._cm_with_wifi
 
 
+def configure_avahi_daemon(action: ConfigActions):
+    return_value = simple_systemd(action, "avahi-daemon.service")
+
+    # Post actions for avahi-daemon
+    if action in (ConfigActions.ENABLE, ConfigActions.DISABLE):
+        # Apply the enable/disable action to the avahi socket AFTER the service
+        # unit, because a connected socket could interrupt stop
+        simple_systemd(action, "avahi-daemon.socket")
+
+    return return_value
+
+
 def configure_gui(action: ConfigActions):
     gui_available = access("/usr/bin/startx", X_OK)