Make it possible to schedule failover in less then 10 seconds

But only when API request was posted to the leader

features/patroni_api.feature

@@ -44,7 +44,7 @@ Scenario: check the failover via the API
 	And replication works from postgres1 to postgres0 after 15 seconds
 
 Scenario: check the scheduled failover
-	Given I issue a scheduled failover at http://127.0.0.1:8009 from postgres1 to postgres0 in 10 seconds
+	Given I issue a scheduled failover at http://127.0.0.1:8009 from postgres1 to postgres0 in 1 seconds
 	Then I receive a response code 200
 	And postgres0 is a leader after 20 seconds
         And postgres0 role is the primary after 5 seconds

patroni/api.py

@@ -203,8 +203,12 @@ class RestApiHandler(BaseHTTPRequestHandler):
                         data = b'Cannot schedule failover in the past'
                         status_code = 422
                     elif self.server.patroni.dcs.manual_failover(leader, candidate, scheduled_at=scheduled_at):
+                        self.server.patroni.dcs.event.set()
                         data = b'Failover scheduled'
                         status_code = 200
+                    else:
+                        data = b'failed to write failover key into DCS'
+                        status_code = 503
                 except (ValueError, TypeError):
                     logger.exception('Invalid scheduled failover time: %s', request['scheduled_at'])
                     data = b'Unable to parse scheduled timestamp. It should be in an unambiguous format, e.g. ISO 8601'
@@ -212,12 +216,12 @@ class RestApiHandler(BaseHTTPRequestHandler):
             else:
                 data = self.is_failover_possible(cluster, leader, candidate)
                 if not data:
-                    if not self.server.patroni.dcs.manual_failover(leader, candidate):
-                        data = b'failed to write failover key into DCS'
-                        status_code = 503
-                    else:
+                    if self.server.patroni.dcs.manual_failover(leader, candidate):
                         self.server.patroni.dcs.event.set()
                         status_code, data = self.poll_failover_result(cluster.leader and cluster.leader.name, candidate)
+                    else:
+                        data = b'failed to write failover key into DCS'
+                        status_code = 503
         else:
             status_code = 400
             data = b'No values given for required parameters leader and candidate'

tests/test_api.py

@@ -188,6 +188,9 @@ class TestRestApiHandler(unittest.TestCase):
         request = b'POST /failover HTTP/1.0\nAuthorization: Basic dGVzdDp0ZXN0\nContent-Length: 103\n\n{"leader": ' +\
                   b'"postgresql1", "member": "postgresql2", "scheduled_at": "6016-02-15T18:13:30.568224+01:00"}'
         MockRestApiServer(RestApiHandler, request)
+        with patch.object(MockPatroni, 'dcs') as d:
+            d.manual_failover.return_value = False
+            MockRestApiServer(RestApiHandler, request)
 
         # Exception: No timezone specified
         request = b'POST /failover HTTP/1.0\nAuthorization: Basic dGVzdDp0ZXN0\nContent-Length: 97\n\n{"leader": ' +\