val result = Relay(randomAccessFile, upstream, 0L, metadata, bufferMaxSize)

      // Write a dirty header. That way if we crash we won't attempt to recover this.
      randomAccessFile.setLength(0L)
      result.writeHeader(PREFIX_DIRTY, -1L, -1L)

      return result
    }

    /**
     * Creates a relay that reads a recorded stream from [file].
     *

    /** Returns the unbuffered stream with the value for [index]. */
    fun getSource(index: Int): Source = sources[index]

    /** Returns the byte length of the value for [index]. */
    fun getLength(index: Int): Long = lengths[index]

    override fun close() {
      for (source in sources) {
        source.closeQuietly()
      }
    }
  }

  /** Edits the values for an entry. */

  }

  private fun Snapshot.assertValue(
    index: Int,
    value: String,
  ) {
    getSource(index).use { source ->
      assertThat(sourceAsString(source)).isEqualTo(value)
      assertThat(getLength(index)).isEqualTo(value.length.toLong())
    }
  }

  private fun sourceAsString(source: Source) = source.buffer().readUtf8()

  private fun Editor.assertInoperable() {