val bufferMaxSize: Long,
) {
  /** The thread that currently has access to upstream. Possibly null. Guarded by this. */
  var upstreamReader: Thread? = null

  /**
   * A buffer for [upstreamReader] to use when pulling bytes from upstream. Only the
   * [upstreamReader] thread may access this buffer.
   */
  val upstreamBuffer = Buffer()

  /** True if there are no further bytes to read from [upstream]. Guarded by this. */