Always communicate device OTK counts to clients

changelog.d/10485.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where Synapse would signal "no change" in device one-time key counts, whereas in reality the number of keys had been exhausted.

synapse/api/constants.py

@@ -127,6 +127,14 @@ class ToDeviceEventTypes:
     RoomKeyRequest = "m.room_key_request"
 
 
+class DeviceKeyAlgorithms:
+    """Spec'd algorithms for the generation of per-device keys"""
+
+    ED25519 = "ed25519"
+    CURVE25519 = "curve25519"
+    SIGNED_CURVE25519 = "signed_curve25519"
+
+
 class EduTypes:
     Presence = "m.presence"
 

synapse/handlers/sync.py

@@ -1093,6 +1093,9 @@ async def generate_sync_result(
         one_time_key_counts: JsonDict = {}
         unused_fallback_key_types: List[str] = []
         if device_id:
+            # TODO: If this is an incremental sync and there's been no change in a count
+            # since the provided since token, then simply omit that algorithm's entry
+            # altogether.
             one_time_key_counts = await self.store.count_e2e_one_time_keys(
                 user_id, device_id
             )

synapse/storage/databases/main/end_to_end_keys.py

@@ -21,6 +21,7 @@
 
 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 @@ def _count_e2e_one_time_keys(txn):
                 " 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(