Use _check_sigs_and_hash_and_fetch to validate backfill requests

changelog.d/8350.bugfix

@@ -0,0 +1 @@
+Partially mitigate bug where newly joined servers couldn't get past events in a room when there is a malformed event.

synapse/federation/federation_client.py

@@ -217,11 +217,9 @@ async def backfill(
             for p in transaction_data["pdus"]
         ]
 
-        # FIXME: We should handle signature failures more gracefully.
-        pdus[:] = await make_deferred_yieldable(
-            defer.gatherResults(
-                self._check_sigs_and_hashes(room_version, pdus), consumeErrors=True,
-            ).addErrback(unwrapFirstError)
+        # 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, check_db=False
         )
 
         return pdus
@@ -350,6 +348,7 @@ async def _check_sigs_and_hash_and_fetch(
         room_version: RoomVersion,
         outlier: bool = False,
         include_none: bool = False,
+        check_db: bool = True,
     ) -> List[EventBase]:
         """Takes a list of PDUs and checks the signatures and hashes of each
         one. If a PDU fails its signature check then we check if we have it in