Fix systemd.Apply() to check for DBus error before waiting on a channel.

The channel was introduced in #1683 to work around a race condition.
However, the check for error in StartTransientUnit ignores the error for
an already existing unit, and in that case there will be no notification
from DBus (so waiting on the channel will make it hang.)

Later PR #1754 added a timeout, which worked around the issue, but we
can fix this correctly by only waiting on the channel when there is no
error. Fix the code to do so.

The timeout handling was kept, since there might be other cases where
this situation occurs (https://bugzilla.redhat.com/show_bug.cgi?id=1548358
mentions calling this code from inside a container, it's unclear whether
an existing container was in use or not, so not sure whether this would
have fixed that bug as well.)

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

libcontainer/cgroups/systemd/apply_systemd.go

@@ -297,16 +297,16 @@ func (m *Manager) Apply(pid int) error {
 	}
 
 	statusChan := make(chan string)
-	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, statusChan); err != nil && !isUnitExists(err) {
+	if _, err := theConn.StartTransientUnit(unitName, "replace", properties, statusChan); err == nil {
+		select {
+		case <-statusChan:
+		case <-time.After(time.Second):
+			logrus.Warnf("Timed out while waiting for StartTransientUnit(%s) completion signal from dbus. Continuing...", unitName)
+		}
+	} else if !isUnitExists(err) {
 		return err
 	}
 
-	select {
-	case <-statusChan:
-	case <-time.After(time.Second):
-		logrus.Warnf("Timed out while waiting for StartTransientUnit completion signal from dbus. Continuing...")
-	}
-
 	if err := joinCgroups(c, pid); err != nil {
 		return err
 	}