Chack if candidate is the same as the leader specified in api

patroni/api.py

@@ -1038,6 +1038,9 @@ class RestApiHandler(BaseHTTPRequestHandler):
             logger.warning('received failover request with leader specifed - performing switchover')
             action = 'switchover'
 
+        if not data and leader == candidate:
+            data = 'Switchover target and source are the same'
+
         if not data and not scheduled_at:
             data = self.is_failover_possible(cluster, leader, candidate, action)
             if data:

tests/test_api.py

@@ -526,7 +526,6 @@ class TestRestApiHandler(unittest.TestCase):
             response_mock.assert_called_with(
                 400, 'Switchover is possible only to a specific candidate in a paused state')
 
-        # Switchover without a candidate specified
         # No healthy nodes to promote in both sync and async mode
         for is_synchronous_mode, response in (
                 (True, 'switchover is not possible: can not find sync_standby'),
@@ -538,6 +537,12 @@ class TestRestApiHandler(unittest.TestCase):
 
         # [Switchover to the candidate specified]
 
+        # Candidate to promote is the same as the leader specified
+        with patch.object(RestApiHandler, 'write_response') as response_mock:
+            request = post + '53\n\n{"leader": "postgresql2", "candidate": "postgresql2"}'
+            MockRestApiServer(RestApiHandler, request)
+            response_mock.assert_called_with(400, 'Switchover target and source are the same')
+
         # Current leader is different from the one specified
         with patch.object(RestApiHandler, 'write_response') as response_mock:
             cluster.leader.name = 'postgresql2'