Fix bug in the Cluster class (#2794)

The `workers` attribute when not passed explicitly was set to the same mutable `dict` object every time.

Problem was introduced in #2652

patroni/dcs/__init__.py

@@ -555,36 +555,50 @@ class TimelineHistory(NamedTuple):
         return TimelineHistory(version, value, lines)
 
 
-class Cluster(NamedTuple):
-    """Immutable object (namedtuple) which represents PostgreSQL cluster.
+class Cluster(NamedTuple('Cluster',
+                         [('initialize', Optional[str]),
+                          ('config', Optional[ClusterConfig]),
+                          ('leader', Optional[Leader]),
+                          ('last_lsn', int),
+                          ('members', List[Member]),
+                          ('failover', Optional[Failover]),
+                          ('sync', SyncState),
+                          ('history', Optional[TimelineHistory]),
+                          ('slots', Optional[Dict[str, int]]),
+                          ('failsafe', Optional[Dict[str, str]]),
+                          ('workers', Dict[int, 'Cluster'])])):
+    """Immutable object (namedtuple) which represents PostgreSQL or Citus cluster.
+
+    .. note::
+        We are using an old-style attribute declaration here because otherwise it is not possible to override `__new__`
+        method. Without it the *workers* by default gets always the same :class:`dict` object that could be mutated.
+
     Consists of the following fields:
-    :param initialize: shows whether this cluster has initialization key stored in DC or not.
-    :param config: global dynamic configuration, reference to `ClusterConfig` object
-    :param leader: `Leader` object which represents current leader of the cluster
-    :param last_lsn: int or long object containing position of last known leader LSN.
-        This value is stored in the `/status` key or `/optime/leader` (legacy) key
-    :param members: list of Member object, all PostgreSQL cluster members including leader
-    :param failover: reference to `Failover` object
-    :param sync: reference to `SyncState` object, last observed synchronous replication state.
-    :param history: reference to `TimelineHistory` object
-    :param slots: state of permanent logical replication slots on the primary in the format: {"slot_name": int}
-    :param failsafe: failsafe topology. Node is allowed to become the leader only if its name is found in this list.
-    :param workers: workers of the Citus cluster, optional. Format: {int(group): Cluster()}
+
+    :ivar initialize: shows whether this cluster has initialization key stored in DC or not.
+    :ivar config: global dynamic configuration, reference to `ClusterConfig` object.
+    :ivar leader: :class:`Leader` object which represents current leader of the cluster.
+    :ivar last_lsn: :class:int object containing position of last known leader LSN.
+                     This value is stored in the `/status` key or `/optime/leader` (legacy) key.
+    :ivar members: list of:class:` Member` objects, all PostgreSQL cluster members including leader
+    :ivar failover: reference to :class:`Failover` object.
+    :ivar sync: reference to :class:`SyncState` object, last observed synchronous replication state.
+    :ivar history: reference to `TimelineHistory` object.
+    :ivar slots: state of permanent logical replication slots on the primary in the format: {"slot_name": int}.
+    :ivar failsafe: failsafe topology. Node is allowed to become the leader only if its name is found in this list.
+    :ivar workers: dictionary of workers of the Citus cluster, optional. Each key is an :class:`int` representing
+                   the group, and the corresponding value is a :class:`Cluster` instance.
     """
-    initialize: Optional[str]
-    config: Optional[ClusterConfig]
-    leader: Optional[Leader]
-    last_lsn: int
-    members: List[Member]
-    failover: Optional[Failover]
-    sync: SyncState
-    history: Optional[TimelineHistory]
-    slots: Optional[Dict[str, int]]
-    failsafe: Optional[Dict[str, str]]
-    workers: Dict[int, 'Cluster'] = {}
+
+    def __new__(cls, *args: Any, **kwargs: Any):
+        """Make workers argument optional and set it to an empty dict object."""
+        if len(args) < len(cls._fields) and 'workers' not in kwargs:
+            kwargs['workers'] = {}
+        return super(Cluster, cls).__new__(cls, *args, **kwargs)
 
     @staticmethod
     def empty() -> 'Cluster':
+        """Produce an empty :class:`Cluster` instance."""
         return Cluster(None, None, None, 0, [], None, SyncState.empty(), None, None, None)
 
     def is_empty(self):