/** The operator to read and write the shared file. Null if this source is closed. */
    private var fileOperator: FileOperator? = FileOperator(file!!.channel)

    /** The next byte to read. This is always less than or equal to [upstreamPos]. */
    private var sourcePos = 0L

    /**
     * Selects where to find the bytes for a read and read them. This is one of three sources.
     *
     * ## Upstream
     *