Handle systray absence and restarts gracefully

Move registration into a goroutine so that we can handle the case when
org.kde.StatusNotifierWatcher is absent at startup or when it is
restarted later. (Restarting the systray is common with, eg, i3 or
sway config reload.)

In the goroutine, we setup a signal listener for NameOwnerChanged on
org.kde.StatusNotifierWatcher so we can handle it coming and going
appropriately.

The goroutine will exit when quitChan is closed so it shouldn't leak.

Author: bdwalton

systray_unix.go

@@ -227,10 +227,51 @@ func nativeStart() {
 	instance.menuProps = menuProps
 	instance.lock.Unlock()
 
-	obj := conn.Object("org.kde.StatusNotifierWatcher", "/StatusNotifierWatcher")
+	go stayRegistered()
+}
+
+func register() bool {
+	obj := instance.conn.Object("org.kde.StatusNotifierWatcher", "/StatusNotifierWatcher")
 	call := obj.Call("org.kde.StatusNotifierWatcher.RegisterStatusNotifierItem", 0, path)
 	if call.Err != nil {
-		log.Printf("systray error: failed to register our icon with the notifier watcher (maybe no tray is running?): %s\n", call.Err)
+		log.Printf("systray error: failed to register: %v\n", call.Err)
+		return false
+	}
+
+	return true
+}
+
+func stayRegistered() {
+	register()
+
+	conn := instance.conn
+	if err := conn.AddMatchSignal(
+		dbus.WithMatchObjectPath("/org/freedesktop/DBus"),
+		dbus.WithMatchInterface("org.freedesktop.DBus"),
+		dbus.WithMatchSender("org.freedesktop.DBus"),
+		dbus.WithMatchMember("NameOwnerChanged"),
+		dbus.WithMatchArg(0, "org.kde.StatusNotifierWatcher"),
+	); err != nil {
+		log.Printf("systray error: failed to register signal matching: %v\n", err)
+		// If we can't monitor signals, there is no point in
+		// us being here. we're either registered or not (per
+		// above) and will roll the dice from here...
+		return
+	}
+
+	sc := make(chan *dbus.Signal, 10)
+	conn.Signal(sc)
+
+	for {
+		select {
+		case sig := <-sc:
+			// sig.Body has the args, which are [name old_owner new_owner]
+			if sig.Body[2] != "" {
+				register()
+			}
+		case <-quitChan:
+			return
+		}
 	}
 }