Make get_auth_chain_ids return a Set

It has a set internally, and a set is often useful where it gets used, so let's
avoid converting to an intermediate list.

synapse/federation/federation_server.py

@@ -571,7 +571,7 @@ async def _on_state_ids_request_compute(
     ) -> JsonDict:
         state_ids = await self.handler.get_state_ids_for_pdu(room_id, event_id)
         auth_chain_ids = await self.store.get_auth_chain_ids(room_id, state_ids)
-        return {"pdu_ids": state_ids, "auth_chain_ids": auth_chain_ids}
+        return {"pdu_ids": state_ids, "auth_chain_ids": list(auth_chain_ids)}
 
     async def _on_context_state_request_compute(
         self, room_id: str, event_id: Optional[str]

synapse/storage/databases/main/event_federation.py

@@ -121,7 +121,7 @@ async def get_auth_chain_ids(
         room_id: str,
         event_ids: Collection[str],
         include_given: bool = False,
-    ) -> List[str]:
+    ) -> Set[str]:
         """Get auth events for given event_ids. The events *must* be state events.
 
         Args:
@@ -130,7 +130,7 @@ async def get_auth_chain_ids(
             include_given: include the given events in result
 
         Returns:
-            list of event_ids
+            set of event_ids
         """
 
         # Check if we have indexed the room so we can use the chain cover
@@ -159,7 +159,7 @@ async def get_auth_chain_ids(
 
     def _get_auth_chain_ids_using_cover_index_txn(
         self, txn: Cursor, room_id: str, event_ids: Collection[str], include_given: bool
-    ) -> List[str]:
+    ) -> Set[str]:
         """Calculates the auth chain IDs using the chain index."""
 
         # First we look up the chain ID/sequence numbers for the given events.
@@ -272,11 +272,11 @@ def _get_auth_chain_ids_using_cover_index_txn(
                 txn.execute(sql, (chain_id, max_no))
                 results.update(r for r, in txn)
 
-        return list(results)
+        return results
 
     def _get_auth_chain_ids_txn(
         self, txn: LoggingTransaction, event_ids: Collection[str], include_given: bool
-    ) -> List[str]:
+    ) -> Set[str]:
         """Calculates the auth chain IDs.
 
         This is used when we don't have a cover index for the room.
@@ -331,7 +331,7 @@ def _get_auth_chain_ids_txn(
             front = new_front
             results.update(front)
 
-        return list(results)
+        return results
 
     async def get_auth_chain_difference(
         self, room_id: str, state_sets: List[Set[str]]

tests/storage/test_event_federation.py

@@ -260,16 +260,16 @@ def test_auth_chain_ids(self, use_chain_cover_index: bool):
         self.assertCountEqual(auth_chain_ids, ["h", "i", "j", "k"])
 
         auth_chain_ids = self.get_success(self.store.get_auth_chain_ids(room_id, ["h"]))
-        self.assertEqual(auth_chain_ids, ["k"])
+        self.assertEqual(auth_chain_ids, {"k"})
 
         auth_chain_ids = self.get_success(self.store.get_auth_chain_ids(room_id, ["i"]))
-        self.assertEqual(auth_chain_ids, ["j"])
+        self.assertEqual(auth_chain_ids, {"j"})
 
         # j and k have no parents.
         auth_chain_ids = self.get_success(self.store.get_auth_chain_ids(room_id, ["j"]))
-        self.assertEqual(auth_chain_ids, [])
+        self.assertEqual(auth_chain_ids, set())
         auth_chain_ids = self.get_success(self.store.get_auth_chain_ids(room_id, ["k"]))
-        self.assertEqual(auth_chain_ids, [])
+        self.assertEqual(auth_chain_ids, set())
 
         # More complex input sequences.
         auth_chain_ids = self.get_success(