code: func (e *endpoint) Readiness(mask waiter.EventMask) waiter.EventMask {
  	// The endpoint is always writable.
  	result := waiter.EventOut & mask
  
  	// Determine if the endpoint is readable if requested.
  	if (mask & waiter.EventIn) != 0 {
  		e.rcvMu.Lock()
  		if !e.rcvList.Empty() || e.rcvClosed {
  			result |= waiter.EventIn
  		}
  		e.rcvMu.Unlock()
  	}
  
  	return result
  }
docs: // Readiness returns the current readiness of the endpoint. For example, if
  // waiter.EventIn is set, the endpoint is immediately readable.