Watch for change of failover key.

If the value is empty and leader didn't changed, this probably means
that failover failed.

After 15 seconds timeout we will consider failover status = unknown

patroni/api.py

@@ -129,9 +129,11 @@ class RestApiHandler(BaseHTTPRequestHandler):
                 cluster = self.server.patroni.dcs.get_cluster()
                 if cluster.leader and cluster.leader.name != leader:
                     return 200, ('Successfully failed over to ' + cluster.leader.name).encode('utf-8')
+                if not cluster.failover:
+                    return 503, b'Failover failed'
             except:
                 pass
-        return 503, b'Failover failed'
+        return 503, b'Failover status unknown'
 
     def is_failover_possible(self, cluster, leader, member):
         if leader and not cluster.leader or cluster.leader.name != leader:

tests/test_api.py

@@ -134,6 +134,10 @@ class TestRestApiHandler(unittest.TestCase):
         cluster.members = [Member(0, 'postgresql0', 30, {'api_url': 'http'})]
         MockRestApiServer(RestApiHandler, request)
         with patch.object(MockPatroni, 'dcs') as d:
+            cluster = d.get_cluster.return_value
+            cluster.leader.name = 'postgresql1'
+            cluster.failover = None
+            MockRestApiServer(RestApiHandler, request)
             d.get_cluster = Mock(side_effect=Exception())
             MockRestApiServer(RestApiHandler, request)
             d.manual_failover.return_value = False