def _wait(self):
val = self.wait_change.get()<EOL>if isinstance(val, Exception):<EOL><INDENT>raise val<EOL><DEDENT>else:<EOL><INDENT>self.member_burden -= len(val)<EOL>self.concurrency_burden -= <NUM_LIT:1><EOL><DEDENT>
Block until an upload finishes

        Raise an exception if that tar volume failed with an error.