code: def _complete_execution(self, g):
          
  
          # Triggered via completion callback.
          #
          # Runs in its own greenlet, so take care to forward the
          # exception, if any, to fail the entire transfer in event of
          # trouble.
          assert g.ready()
          self.greenlets.remove(g)
  
          placed = UserCritical(msg='placeholder bogus exception',
                                hint='report a bug')
  
          if g.successful():
              try:
                  segment = g.get()
  
                  if not segment.explicit:
                      segment.mark_done()
              except BaseException as e:
                  # Absorb and forward exceptions across the channel.
                  placed = e
              else:
                  placed = None
          else:
              placed = g.exception
  
          self.wait_change.put(placed)
docs: Forward any raised exceptions across a channel.