Do not exit when encountering invalid system ID. (#669)

Do not exit when the cluster system ID is empty or the one that doesn't pass the validation check. In that case, the cluster most likely needs a reinit; mention it in the result message.
Avoid terminating Patroni, as otherwise reinit cannot happen.

patroni/ha.py

@@ -1101,6 +1101,11 @@ class Ha(object):
                 self.dcs.initialize(create_new=(self.cluster.initialize is None), sysid=self.state_handler.sysid)
             else:
                 # check if we are allowed to join
+                data_sysid = self.state_handler.sysid
+                if not self.sysid_valid(data_sysid):
+                    # data directory is not empty, but no valid sysid, cluster must be broken, suggest reinit
+                    return "data dir for the cluster is not empty, but system ID is invalid; consider doing reinitalize"
+
                 if self.sysid_valid(self.cluster.initialize) and self.cluster.initialize != self.state_handler.sysid:
                     logger.fatal("system ID mismatch, node %s belongs to a different cluster: %s != %s",
                                  self.state_handler.name, self.cluster.initialize, self.state_handler.sysid)

tests/test_ha.py

@@ -18,7 +18,6 @@ from test_postgresql import psycopg2_connect, MockPostmaster
 
 SYSID = '12345678901'
 
-
 def true(*args, **kwargs):
     return True