* resources are released and all following calls to [.newSource] return null. Guarded by this.
   */
  var sourceCount = 0

  val isClosed: Boolean
    get() = file == null

  @Throws(IOException::class)
  private fun writeHeader(
    prefix: ByteString,
    upstreamSize: Long,
    metadataSize: Long,
  ) {
    val header =