Address code review

- Add a new param to the abstract DCS attempt_to_take_leader
- Make sure the cluster is wiped-out properly if we created the
  initialize key, but failed to populate it with leader and member.
  This actually means that we may wipe out the running cluster without
  the intialization key, but that is a very unlikely case in practice.

patroni/ctl.py

@@ -687,13 +687,19 @@ def scaffold(cluster_name, config_file, dcs, sysid):
     config, dcs, cluster = ctl_load_config(cluster_name, config_file, dcs)
     if cluster and cluster.initialize:
         raise PatroniCtlException("This cluster is already initialized")
-    dcs.initialize(create_new=True, sysid=sysid)
+
+    if not dcs.initialize(create_new=True, sysid=sysid):
+        # initialize key already exists, don't touch this cluster
+        raise PatroniCtlException("Initialize key for cluster {0} already exists".format(cluster_name))
 
     set_defaults(config, cluster_name)
 
-    # make sure the leader key will never expire
-    if not (touch_member(config, dcs) and dcs.attempt_to_acquire_leader(permanent=True)):
-        dcs.delete_leader()
-        dcs.cancel_initialization()
-        return 1
-    return 0
+    try:
+        # make sure the leader keys will never expire
+        if not (touch_member(config, dcs) and dcs.attempt_to_acquire_leader(permanent=True)):
+            # we did initialize this cluster, but failed to write the leader or member keys, wipe it down completely.
+            raise PatroniCtlException("Unable to install permanent leader for cluster {0}".format(cluster_name))
+    except:
+        dcs.delete_cluster()
+        raise
+    click.echo("Cluster {0} has been created successfully".format(cluster_name, sysid))

patroni/dcs/__init__.py

@@ -313,9 +313,10 @@ class AbstractDCS(object):
         for example for etcd `prevValue` parameter must be used."""
 
     @abc.abstractmethod
-    def attempt_to_acquire_leader(self):
+    def attempt_to_acquire_leader(self, permanent=False):
         """Attempt to acquire leader lock
         This method should create `/leader` key with value=`~self._name`
+        :param permanent: if set to `!True`, the leader key will never expie. Used in patronictl for the external master
         :returns: `!True` if key has been created successfully.
 
         Key must be created atomically. In case if key already exists it should not be