netutil: helpers for querying if current network connection is metered

Add a helper for querying if current default connection is metered. The helper talks only to NetworkManager, and fails gracefully if NM is not available. Signed-off-by: Maciej Borzecki <maciej.zenon.borzecki@canonical.com> 

1 files changed, 65 insertions, 0 deletions

diff --git a/netutil/metered.go b/netutil/metered.go
new file mode 100644
index 0000000000..720d17bb0f
--- /dev/null
+++ b/netutil/metered.go
@@ -0,0 +1,65 @@
+// -*- Mode: Go; indent-tabs-mode: t -*-
+
+/*
+ * Copyright (C) 2018 Canonical Ltd
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+package netutil
+
+import (
+	"fmt"
+
+	"github.com/godbus/dbus"
+
+	"github.com/snapcore/snapd/logger"
+)
+
+const (
+	// https://developer.gnome.org/NetworkManager/stable/nm-dbus-types.html#NMMetered
+	NetworkManagerMeteredUnknown = 0
+	NetworkManagerMeteredYes = 1
+	NetworkManagerMeteredNo = 2
+	NetworkManagerMeteredGuessYes = 3
+	NetworkManagerMeteredGuessNo = 4
+)
+
+// IsOnMeteredConnection checks whether the current default network connection
+// is metered. If the state can not be determined, returns false and an error.
+func IsOnMeteredConnection() (bool, error) {
+	// obtain a shared connection to system bus, no need to close it
+	conn, err := dbus.SystemBus()
+	if err != nil {
+	return false, fmt.Errorf("cannot connect to system bus: %v", err)
+	}
+
+	return isNMOnMetered(conn)
+}
+
+func isNMOnMetered(conn *dbus.Conn) (bool, error) {
+	nmObj := conn.Object("org.freedesktop.NetworkManager", "/org/freedesktop/NetworkManager")
+	// https://developer.gnome.org/NetworkManager/stable/gdbus-org.freedesktop.NetworkManager.html
+	dbusV, err := nmObj.GetProperty("org.freedesktop.NetworkManager.Metered")
+	if err != nil {
+	return false, err
+	}
+	v, ok := dbusV.Value().(uint32)
+	if !ok {
+	return false, fmt.Errorf("unexpected Metered property signature: %v", dbusV.String())
+	}
+	logger.Debugf("metered state reported by NetworkManager: %s", dbusV.String())
+
+	return v == NetworkManagerMeteredGuessYes || v == NetworkManagerMeteredYes, nil
+}