Fix MSC3983 support: Use the unstable /keys/claim federation endpoint if multiple keys are requested (#15755)

Author: clokep

changelog.d/15755.misc

@@ -0,0 +1 @@
+Fix requesting multiple keys at once over federation, related to [MSC3983](https://github.com/matrix-org/matrix-spec-proposals/pull/3983).

synapse/federation/federation_client.py

@@ -260,7 +260,9 @@ async def claim_client_keys(
         use_unstable = False
         for user_id, one_time_keys in query.items():
             for device_id, algorithms in one_time_keys.items():
-                if any(count > 1 for count in algorithms.values()):
+                # If more than one algorithm is requested, attempt to use the unstable
+                # endpoint.
+                if sum(algorithms.values()) > 1:
                     use_unstable = True
                 if algorithms:
                     # For the stable query, choose only the first algorithm.
@@ -296,6 +298,7 @@ async def claim_client_keys(
         else:
             logger.debug("Skipping unstable claim client keys API")
 
+        # TODO Potentially attempt multiple queries and combine the results?
         return await self.transport_layer.claim_client_keys(
             user, destination, content, timeout
         )