code: func (fo *FileWriterOutput) OnHup() {
  	if fo.WriterOutput != nil {
  		wc, ok := fo.WriterOutput.w.(io.Closer)
  		if ok {
  			err := wc.Close()
  			if err != nil {
  				fo.fallbackLog("Closing %#v failed: %s", fo.path, err)
  			}
  		}
  		fo.WriterOutput = nil
  	}
  }
docs: // Throw away any references/handles to the output file. This probably
  // means the admin wants to rotate the file out and have this process
  // open a new one. Close the underlying io.Writer if that is a thing
  // that it knows how to do.