Fix a crash on node migration.

After an unsuccessful initial cluster sync it may happen that the
cluster statefulset is empty. This has been made more likely since
88d6a7b, since it has introduced syncing volumes before statefulsets,
and the volume sync mail fail for different reasons (i.e. the volume has
been shrinked, or too many calls to Amazon).

pkg/cluster/pod.go

@@ -203,6 +203,15 @@ func (c *Cluster) MigrateMasterPod(podName spec.NamespacedName) error {
 		c.logger.Warningf("pod %q is not a master", podName)
 		return nil
 	}
+	// we must have a statefulset in the cluster for the migration to work
+	if c.Statefulset == nil {
+		sset, err := c.KubeClient.StatefulSets(c.Namespace).Get(c.statefulSetName(), metav1.GetOptions{})
+		if err != nil {
+			return fmt.Errorf("could not retrieve cluster statefulset: %v", err)
+		}
+		c.Statefulset = sset
+	}
+	// We may not have a cached statefulset if the initial cluster sync has aborted, revert to the spec in that case.
 	if *c.Statefulset.Spec.Replicas == 1 {
 		c.logger.Warningf("single master pod for cluster %q, migration will cause longer downtime of the master instance", c.clusterName())
 	} else {