Add a check to make sure that during state conflict res we only request a PDU we don't have.

synapse/state.py

@@ -191,10 +191,18 @@ class StateHandler(object):
                 key=lambda x: x.depth
             )
 
+            pdu_id = missing_prev.prev_state_id
+            origin = missing_prev.prev_state_origin
+
+            is_missing = yield self.store.get_pdu(pdu_id, origin) is None
+
+            if not is_missing:
+                raise Exception("Conflict resolution failed.")
+
             yield self._replication.get_pdu(
                 destination=missing_prev.origin,
-                pdu_origin=missing_prev.prev_state_origin,
-                pdu_id=missing_prev.prev_state_id,
+                pdu_origin=origin,
+                pdu_id=pdu_id,
                 outlier=True
             )
 

tests/test_state.py

@@ -37,6 +37,7 @@ class StateTestCase(unittest.TestCase):
             "update_current_state",
             "get_latest_pdus_in_context",
             "get_current_state",
+            "get_pdu",
         ])
         self.replication = Mock(spec=["get_pdu"])
 
@@ -220,6 +221,8 @@ class StateTestCase(unittest.TestCase):
 
         self.replication.get_pdu.side_effect = set_return_tree
 
+        self.persistence.get_pdu.return_value = None
+
         is_new = yield self.state.handle_new_state(new_pdu)
 
         self.assertTrue(is_new)