execution_service.py: Handle client not supporting multi-instances

buildgrid/server/execution/execution_service.py

@@ -86,6 +86,11 @@ class ExecutionService(remote_execution_pb2_grpc.ExecutionServicer):
             yield operations_pb2.Operation()
 
     def _get_instance(self, name):
+        # If client does not support multiple instances, it may omit the
+        # instance name request parameter, so better map our default:
+        if not name and len(self._instances) == 1:
+            name = 'main'
+
         try:
             return self._instances[name]