code: func (e *endpoint) zeroReceiveWindow(scale uint8) bool {
  	if e.rcvBufUsed >= e.rcvBufSize {
  		return true
  	}
  
  	return ((e.rcvBufSize - e.rcvBufUsed) >> scale) == 0
  }
docs: // zeroReceiveWindow checks if the receive window to be announced now would be
  // zero, based on the amount of available buffer and the receive window scaling.
  //
  // It must be called with rcvListMu held.