Implement watch/unwatch elected_as_leader for redis driver

Change-Id: Idd5143801cfe0e095f31c04e9268887605268b62

tooz/drivers/redis.py

@@ -245,7 +245,6 @@ def __init__(self, member_id, parsed_url, options):
         namespace = options.get('namespace', self.DEFAULT_NAMESPACE)
         self._namespace = self._to_binary(namespace)
         self._group_prefix = self._namespace + b"_group"
-        self._leader_prefix = self._namespace + b"_leader"
         self._beat_prefix = self._namespace + b"_beats"
         self._groups = self._namespace + b"_groups"
         self._client = None
@@ -385,6 +384,10 @@ def _encode_member_id(self, member_id):
     def _decode_member_id(self, member_id):
         return self._to_binary(member_id)
 
+    def _encode_group_leader(self, group_id):
+        group_id = self._to_binary(group_id)
+        return b"leader_of_" + group_id
+
     def _encode_group_id(self, group_id, apply_namespace=True):
         group_id = self._to_binary(group_id)
         if not apply_namespace:
@@ -649,13 +652,27 @@ def watch_leave_group(self, group_id, callback):
     def unwatch_leave_group(self, group_id, callback):
         return super(RedisDriver, self).unwatch_leave_group(group_id, callback)
 
-    @staticmethod
-    def watch_elected_as_leader(group_id, callback):
-        raise tooz.NotImplemented
+    def watch_elected_as_leader(self, group_id, callback):
+        return super(RedisDriver, self).watch_elected_as_leader(
+            group_id, callback)
+
+    def unwatch_elected_as_leader(self, group_id, callback):
+        return super(RedisDriver, self).unwatch_elected_as_leader(
+            group_id, callback)
+
+    def _get_leader_lock(self, group_id):
+        name = self._encode_group_leader(group_id)
+        return self.get_lock(name)
 
-    @staticmethod
-    def unwatch_elected_as_leader(group_id, callback):
-        raise tooz.NotImplemented
+    def _run_leadership(self, watch):
+        for group_id, hooks in six.iteritems(self._hooks_elected_leader):
+            if watch.expired():
+                return
+            leader_lock = self._get_leader_lock(group_id)
+            if leader_lock.acquire(blocking=False):
+                # We got the lock
+                hooks.run(coordination.LeaderElected(group_id,
+                                                     self._member_id))
 
     def run_watchers(self, timeout=None):
         w = timeutils.StopWatch(duration=timeout)
@@ -693,6 +710,7 @@ def run_watchers(self, timeout=None):
                         coordination.MemberLeftGroup(group_id,
                                                      member_id)))
             self._group_members[group_id] = group_members
+        self._run_leadership(w)
         return result