add device signatures to device key query results

changelog.d/6844.bugfix

@@ -0,0 +1 @@
+Fix an issue with cross-signing where device signatures were not sent to remote servers.

synapse/storage/data_stores/main/devices.py

@@ -320,6 +320,11 @@ def _get_device_update_edus_by_remote(self, destination, from_stream_id, query_m
                     device_display_name = device.get("device_display_name", None)
                     if device_display_name:
                         result["device_display_name"] = device_display_name
+                    if "signatures" in device:
+                        for sig_user_id, sigs in device["signatures"].items():
+                            result["keys"].setdefault("signatures", {}).setdefault(
+                                sig_user_id, {}
+                            ).update(sigs)
                 else:
                     result["deleted"] = True
 
@@ -524,6 +529,11 @@ def _get_devices_with_keys_by_user_txn(self, txn, user_id):
                 device_display_name = device.get("device_display_name", None)
                 if device_display_name:
                     result["device_display_name"] = device_display_name
+                if "signatures" in device:
+                    for sig_user_id, sigs in device["signatures"].items():
+                        result["keys"].setdefault("signatures", {}).setdefault(
+                            sig_user_id, {}
+                        ).update(sigs)
 
                 results.append(result)