Fix poor buffering case for MultipartReader

[yschimke]

100mb Body is fully written, reader wants 1MB chunks, but get 8kb at a time. Unfortunately internally we do indexOf over the full 100mb.

outer reading 1048576
indexOf over 104857611 with maxResult 8192
inner 8192
outer read 8192
outer reading 1048576
indexOf over 104849419 with maxResult 8192
inner 8192
outer read 8192

[yschimke]

@swankjesse any suggestions for the right way to test and fix this?

[yschimke]

I'm not sure if this is actually a bug in Okio RealBufferedSource that it assumes that checking segment size is optimal?

internal inline fun RealBufferedSource.commonRead(sink: Buffer, byteCount: Long): Long {
  require(byteCount >= 0L) { "byteCount < 0: $byteCount" }
  check(!closed) { "closed" }

  if (buffer.size == 0L) {
    if (byteCount == 0L) return 0L
    val read = source.read(buffer, Segment.SIZE.toLong())
    if (read == -1L) return -1L
  }

  val toRead = minOf(byteCount, buffer.size)
  return buffer.read(sink, toRead)
}

Also if we do want to scan 8kb at a time, how can we limit the indexOf?

    private fun currentPartBytesRemaining(maxResult: Long): Long {
      source.require(crlfDashDashBoundary.size.toLong())

      return when (val delimiterIndex = source.buffer.indexOf(crlfDashDashBoundary)) {
        -1L -> minOf(maxResult, source.buffer.size - crlfDashDashBoundary.size + 1)
        else -> minOf(maxResult, delimiterIndex)
      }
    }

Do we need this added to Buffer?

  override fun indexOf(bytes: ByteString, fromIndex: Long, toIndex: Long): Long

[yschimke]

500ms with the fix
I stopped after 2 minutes without the fix

[yschimke]

@swankjesse requesting post review (as agreed)