Use OrderedDict to gurantee order returned is the same as we were building the list

We are using a Dict over a list to gurantee we don't duplicate the event if it's already in there.
I assume this is why we were using a Set before.

synapse/handlers/federation_event.py

@@ -416,6 +416,18 @@ class FederationEventHandler:
         events = await self._federation_client.backfill(
             dest, room_id, limit=limit, extremities=extremities
         )
+        logger.info(
+            "from remote server: got backfill response events=%s",
+            [
+                {
+                    "event_id": ev.event_id,
+                    "type": ev["type"],
+                    "depth": ev["depth"],
+                    "content": ev["content"].get("body", None),
+                }
+                for ev in events
+            ],
+        )
 
         if not events:
             return

synapse/storage/databases/main/event_federation.py

@@ -14,7 +14,7 @@
 import itertools
 import logging
 from queue import Empty, PriorityQueue
-from typing import Collection, Dict, Iterable, List, Optional, Set, Tuple
+from typing import Collection, Dict, Iterable, List, Optional, Set, OrderedDict, Tuple
 
 from prometheus_client import Counter, Gauge
 
@@ -1007,7 +1007,7 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
     def _get_backfill_events(self, txn, room_id, event_list, limit):
         logger.debug("_get_backfill_events: %s, %r, %s", room_id, event_list, limit)
 
-        event_results = set()
+        event_results = OrderedDict()
 
         # We want to make sure that we do a breadth-first, "depth" ordered
         # search.
@@ -1120,7 +1120,7 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
                     event_id,
                 )
 
-            event_results.add(event_id)
+            event_results[event_id] = event_id
 
             # Try and find any potential historical batches of message history.
             #
@@ -1181,7 +1181,7 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
                 if row[2] not in event_results:
                     queue.put((-row[0], -row[1], row[2]))
 
-        return event_results
+        return event_results.values()
 
     async def get_missing_events(self, room_id, earliest_events, latest_events, limit):
         ids = await self.db_pool.runInteraction(