constructor(rawSource: Source) {
      rawSource.use {
        val source = rawSource.buffer()
        val urlLine = source.readUtf8LineStrict()
        // Choice here is between failing with a correct RuntimeException
        // or mostly silently with an IOException
        url = urlLine.toHttpUrlOrNull() ?: throw IOException("Cache corruption for $urlLine").also {
          Platform.get().log("cache corruption", WARN, it)