Always communicate device OTK counts to clients (#10485)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

synapse/storage/databases/main/end_to_end_keys.py

@@ -21,6 +21,7 @@ from canonicaljson import encode_canonical_json
 
 from twisted.enterprise.adbapi import Connection
 
+from synapse.api.constants import DeviceKeyAlgorithms
 from synapse.logging.opentracing import log_kv, set_tag, trace
 from synapse.storage._base import SQLBaseStore, db_to_json
 from synapse.storage.database import DatabasePool, make_in_list_sql_clause
@@ -381,9 +382,15 @@ class EndToEndKeyWorkerStore(EndToEndKeyBackgroundStore):
                 " GROUP BY algorithm"
             )
             txn.execute(sql, (user_id, device_id))
-            result = {}
+
+            # Initially set the key count to 0. This ensures that the client will always
+            # receive *some count*, even if it's 0.
+            result = {DeviceKeyAlgorithms.SIGNED_CURVE25519: 0}
+
+            # Override entries with the count of any keys we pulled from the database
             for algorithm, key_count in txn:
                 result[algorithm] = key_count
+
             return result
 
         return await self.db_pool.runInteraction(