code: def seek(self, loc):
          
          assert loc == 0
  
          # rewind progress bar
          if self.progressbar:
              self.progressbar.update(-self._tell)
  
          self._fp_left.seek(loc)
          self._fp_right.seek(loc)
          self._tell = loc
          self._buf = Buffer()
docs: Called if upload fails and must be retried.