FederationClient.backfill: stop flagging events as outliers (#11632)

Events returned by `backfill` should not be flagged as outliers.

Fixes:

```
AssertionError: null
  File "synapse/handlers/federation.py", line 313, in try_backfill
    dom, room_id, limit=100, extremities=extremities
  File "synapse/handlers/federation_event.py", line 517, in backfill
    await self._process_pulled_events(dest, events, backfilled=True)
  File "synapse/handlers/federation_event.py", line 642, in _process_pulled_events
    await self._process_pulled_event(origin, ev, backfilled=backfilled)
  File "synapse/handlers/federation_event.py", line 669, in _process_pulled_event
    assert not event.internal_metadata.is_outlier()
```

See https://sentry.matrix.org/sentry/synapse-matrixorg/issues/231992

Fixes #8894.

changelog.d/11632.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.19.3 which could sometimes cause `AssertionError`s when backfilling rooms over federation.

synapse/federation/federation_client.py

@@ -272,7 +272,7 @@ async def backfill(
 
         # Check signatures and hash of pdus, removing any from the list that fail checks
         pdus[:] = await self._check_sigs_and_hash_and_fetch(
-            dest, pdus, outlier=True, room_version=room_version
+            dest, pdus, room_version=room_version
         )
 
         return pdus

synapse/handlers/federation_event.py

@@ -666,7 +666,9 @@ async def _process_pulled_event(
         logger.info("Processing pulled event %s", event)
 
         # these should not be outliers.
-        assert not event.internal_metadata.is_outlier()
+        assert (
+            not event.internal_metadata.is_outlier()
+        ), "pulled event unexpectedly flagged as outlier"
 
         event_id = event.event_id