Stop synapse from saving messages in device_inbox for hidden devices.

changelog.d/10097.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug which allowed hidden devices to receive to-device messages, resulting in unnecessary database bloat.

synapse/storage/databases/main/deviceinbox.py

@@ -485,10 +485,12 @@ def _add_messages_to_local_device_inbox_txn(
             devices = list(messages_by_device.keys())
             if len(devices) == 1 and devices[0] == "*":
                 # Handle wildcard device_ids.
+                # We exclude hidden devices (such as cross-signing keys) here as they are
+                # not expected to receive to-device messages.
                 devices = self.db_pool.simple_select_onecol_txn(
                     txn,
                     table="devices",
-                    keyvalues={"user_id": user_id},
+                    keyvalues={"user_id": user_id, "hidden": False},
                     retcol="device_id",
                 )
 
@@ -501,10 +503,12 @@ def _add_messages_to_local_device_inbox_txn(
                 if not devices:
                     continue
 
+                # We exclude hidden devices (such as cross-signing keys) here as they are
+                # not expected to receive to-device messages.
                 rows = self.db_pool.simple_select_many_txn(
                     txn,
                     table="devices",
-                    keyvalues={"user_id": user_id},
+                    keyvalues={"user_id": user_id, "hidden": False},
                     column="device_id",
                     iterable=devices,
                     retcols=("device_id",),