Error if starting without quorum

If collective mode is present, but no candidates worked, still error
out.

confluent_server/confluent/collective/manager.py

@@ -251,6 +251,8 @@ def start_collective():
     for member in cfm.list_collective():
         if member == myname:
             continue
+        if cfm.cfgleader is None:
+            cfm.cfgleader = True
         ldrcandidate = cfm.get_collective_member(member)['address']
         connect_to_leader(name=myname, leader=ldrcandidate)
 

confluent_server/confluent/config/configmanager.py

@@ -212,6 +212,8 @@ def _rpc_set_group_attributes(tenant, attribmap, autocreate):
     ConfigManager(tenant)._true_set_group_attributes(attribmap, autocreate)
 
 def exec_on_leader(function, *args):
+    if isinstance(cfgleader, bool):
+        raise exc.DegradedCollective()
     xid = os.urandom(8)
     while xid in _pendingchangesets:
         xid = os.urandom(8)

confluent_server/confluent/exceptions.py

@@ -68,6 +68,11 @@ class LockedCredentials(ConfluentException):
     _apierrorstr = 'Credential store locked'
 
 
+class DegradedCollective(ConfluentException):
+    # We are in a collective with at least half of the member nodes missing
+    _apierrorstr = 'Collective does not have quorum'
+
+
 class ForbiddenRequest(ConfluentException):
     # The client request is not allowed by authorization engine
     apierrorcode = 403