Fix battery status querying on GNU/Linux

The code was looking at the wrong bus (why on Earth does D-Bus have two
distinct buses, anyway?), and failed to unpack the return value from a
variant.

It seems to now work correctly, although UPower seems to categorize my
desktop PC's power source as "unknown" rather than the more accurate
"line power". No harm done because our code only shows the battery
indicator if the device is detected to have a battery.

src/desktop/dbus_features.cpp

@@ -98,7 +98,7 @@ DBusHandlerResult filter_dbus_signal(DBusConnection *, DBusMessage *buf, void *)
 	return DBUS_HANDLER_RESULT_HANDLED;
 }
 
-DBusConnection *get_dbus_connection()
+DBusConnection *get_dbus_session_bus()
 {
 	static bool initted = false;
 	static DBusConnection *connection = nullptr;
@@ -127,6 +127,21 @@ DBusConnection *get_dbus_connection()
 	return connection;
 }
 
+DBusConnection *get_dbus_system_bus()
+{
+	static DBusConnection *connection = nullptr;
+
+	if (connection == nullptr)
+	{
+		DBusError err;
+		dbus_error_init(&err);
+		connection = dbus_bus_get(DBUS_BUS_SYSTEM, &err);
+		dbus_error_free(&err);
+	}
+
+	return connection;
+}
+
 uint32_t send_dbus_notification(DBusConnection *connection, uint32_t replaces_id,
 	const std::string &owner, const std::string &message)
 {
@@ -199,9 +214,9 @@ uint32_t send_dbus_notification(DBusConnection *connection, uint32_t replaces_id
 }
 
 template<typename T>
-T get_power_source_property(const std::string &name, int type, T fallback)
+T get_power_source_property(const std::string &name, T fallback)
 {
-	DBusConnection *connection = get_dbus_connection();
+	DBusConnection *connection = get_dbus_system_bus();
 	if (!connection) return fallback;
 
 	std::unique_ptr<DBusMessage, std::function<void(DBusMessage*)>> msg(dbus_message_new_method_call(
@@ -229,10 +244,13 @@ T get_power_source_property(const std::string &name, int type, T fallback)
 		return fallback;
 	}
 
+	// The value is returned as a variant. We need to recurse into it to read the underlying value.
+	DBusMessageIter iter;
+	dbus_message_iter_init(ret.get(), &iter);
+	DBusMessageIter sub_iter;
+	dbus_message_iter_recurse(&iter, &sub_iter);
 	T value;
-	dbus_message_get_args(ret.get(), nullptr,
-		type, &value,
-		DBUS_TYPE_INVALID);
+	dbus_message_iter_get_basic(&sub_iter, &value);
 
 	return value;
 }
@@ -245,7 +263,7 @@ const std::size_t MAX_MSG_LINES = 5;
 
 void send_notification(const std::string & owner, const std::string & message, bool with_history)
 {
-	DBusConnection *connection = get_dbus_connection();
+	DBusConnection *connection = get_dbus_session_bus();
 	if (!connection) return;
 
 	wnotify_by_owner & noticias = notifications.get<by_owner>();
@@ -286,12 +304,12 @@ void send_notification(const std::string & owner, const std::string & message, b
 
 bool does_device_have_battery()
 {
-	return get_power_source_property<uint32_t>("Type", DBUS_TYPE_UINT32, 0) == 2;
+	return get_power_source_property<uint32_t>("Type", 0) == 2;
 }
 
 double get_battery_percentage()
 {
-	return get_power_source_property<double>("Percentage", DBUS_TYPE_DOUBLE, 0.0);
+	return get_power_source_property<double>("Percentage", 0.0);
 }
 
 } // end namespace dbus