* building a new relay with [.read].
   */
  fun newSource(): Source? {
    synchronized(this@Relay) {
      if (file == null) return null
      sourceCount++
    }

    return RelaySource()
  }

  internal inner class RelaySource : Source {
    private val timeout = Timeout()

    /** The operator to read and write the shared file. Null if this source is closed. */