val prefix = header.readByteString(PREFIX_CLEAN.size.toLong())
      if (prefix != PREFIX_CLEAN) throw IOException("unreadable cache file")
      val upstreamSize = header.readLong()
      val metadataSize = header.readLong()

      // Read the metadata.
      val metadataBuffer = Buffer()
      fileOperator.read(FILE_HEADER_SIZE + upstreamSize, metadataBuffer, metadataSize)

    metadata: ByteString?,
  ) {
    val source = file.source().buffer()
    assertThat(source.readByteString(prefix.size.toLong())).isEqualTo(prefix)
    assertThat(source.readLong()).isEqualTo(upstreamSize)
    assertThat(source.readLong()).isEqualTo(metadataSize.toLong())
    if (upstream != null) {
      assertThat(source.readUtf8(upstreamSize)).isEqualTo(upstream)
    }
    if (metadata != null) {

      name = "INTEGER",
      tagClass = DerHeader.TAG_CLASS_UNIVERSAL,
      tag = 2L,
      codec =
        object : BasicDerAdapter.Codec<Long> {
          override fun decode(reader: DerReader): Long = reader.readLong()

          override fun encode(
            writer: DerWriter,
            value: Long,
          ) = writer.writeLong(value)
        },
    )

  val INTEGER_AS_BIG_INTEGER =
    BasicDerAdapter(

    if (frameLength == PAYLOAD_SHORT.toLong()) {
      frameLength = (source.readShort() and 0xffff).toLong() // Value is unsigned.
    } else if (frameLength == PAYLOAD_LONG.toLong()) {
      frameLength = source.readLong()
      if (frameLength < 0L) {
        throw ProtocolException(
          "Frame length 0x${frameLength.toHexString()} > 0x7FFFFFFFFFFFFFFF",
        )
      }
    }

    if (bytesLeft == 0L) throw ProtocolException("unexpected length: $bytesLeft at $this")
    val byteArray = source.readByteArray(bytesLeft)
    return BigInteger(byteArray)
  }

  fun readLong(): Long {
    if (bytesLeft !in 1..8) throw ProtocolException("unexpected length: $bytesLeft at $this")

    var result = source.readByte().toLong() // No "and 0xff" because this is a signed value!