engine: resources: svc: Use dbus session bus for user session svc

This patch adds a util function, SessionBusUsable, that makes and returns
a new usable dbus session bus. If the svc bool session is true, the resource
will use a bus created with that function.

engine/resources/svc.go

@@ -86,6 +86,7 @@ func (obj *SvcRes) Watch() error {
 	}
 
 	var conn *systemd.Conn
+	var bus *dbus.Conn
 	var err error
 	if obj.Session {
 		conn, err = systemd.NewUserConnection() // user session
@@ -99,7 +100,11 @@ func (obj *SvcRes) Watch() error {
 	defer conn.Close()
 
 	// if we share the bus with others, we will get each others messages!!
-	bus, err := util.SystemBusPrivateUsable() // don't share the bus connection!
+	if obj.Session {
+		bus, err = util.SessionBusPrivateUsable()
+	} else {
+		bus, err = util.SystemBusPrivateUsable()
+	}
 	if err != nil {
 		return errwrap.Wrapf(err, "failed to connect to bus")
 	}

util/util.go

@@ -359,7 +359,7 @@ func TimeAfterOrBlock(t int) <-chan time.Time {
 	return time.After(time.Duration(t) * time.Second)
 }
 
-// SystemBusPrivateUsable makes using the private bus usable
+// SystemBusPrivateUsable makes using the private bus usable.
 // TODO: should be upstream: https://github.com/godbus/dbus/issues/15
 func SystemBusPrivateUsable() (conn *dbus.Conn, err error) {
 	conn, err = dbus.SystemBusPrivate()
@@ -378,6 +378,25 @@ func SystemBusPrivateUsable() (conn *dbus.Conn, err error) {
 	return conn, nil // success
 }
 
+// SessionBusPrivateUsable makes using the private bus usable.
+// TODO: should be upstream: https://github.com/godbus/dbus/issues/15
+func SessionBusPrivateUsable() (conn *dbus.Conn, err error) {
+	conn, err = dbus.SessionBusPrivate()
+	if err != nil {
+		return nil, err
+	}
+	if err = conn.Auth(nil); err != nil {
+		conn.Close()
+		conn = nil
+		return
+	}
+	if err = conn.Hello(); err != nil {
+		conn.Close()
+		conn = nil
+	}
+	return conn, nil // success
+}
+
 // SortedStrSliceCompare takes two lists of strings and returns whether or not
 // they are equivalent. It will return nil if both sets contain the same
 // elements, regardless of order, and an error if they do not.