Annotate log_function decorator (#10943)

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

synapse/federation/federation_client.py

@@ -227,7 +227,7 @@ class FederationClient(FederationBase):
         )
 
     async def backfill(
-        self, dest: str, room_id: str, limit: int, extremities: Iterable[str]
+        self, dest: str, room_id: str, limit: int, extremities: Collection[str]
     ) -> Optional[List[EventBase]]:
         """Requests some more historic PDUs for the given room from the
         given destination server.
@@ -237,6 +237,8 @@ class FederationClient(FederationBase):
             room_id: The room_id to backfill.
             limit: The maximum number of events to return.
             extremities: our current backwards extremities, to backfill from
+                Must be a Collection that is falsy when empty.
+                (Iterable is not enough here!)
         """
         logger.debug("backfill extrem=%s", extremities)
 
@@ -250,11 +252,22 @@ class FederationClient(FederationBase):
 
         logger.debug("backfill transaction_data=%r", transaction_data)
 
+        if not isinstance(transaction_data, dict):
+            # TODO we probably want an exception type specific to federation
+            # client validation.
+            raise TypeError("Backfill transaction_data is not a dict.")
+
+        transaction_data_pdus = transaction_data.get("pdus")
+        if not isinstance(transaction_data_pdus, list):
+            # TODO we probably want an exception type specific to federation
+            # client validation.
+            raise TypeError("transaction_data.pdus is not a list.")
+
         room_version = await self.store.get_room_version(room_id)
 
         pdus = [
             event_from_pdu_json(p, room_version, outlier=False)
-            for p in transaction_data["pdus"]
+            for p in transaction_data_pdus
         ]
 
         # Check signatures and hash of pdus, removing any from the list that fail checks