<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _wait(self):
 """Block until an upload finishes Raise an exception if that tar volume failed with an error. """

        val = self.wait_change.get()

        if isinstance(val, Exception):
            # Don't other uncharging, because execution is going to stop
            raise val
        else:
            # Uncharge for resources.
            self.member_burden -= len(val)
            self.concurrency_burden -= 1