Minor Description
DataFileReader.blockStart is hidden from the caller, but exposing it (as readonly) would allow callers to discover sync points in a file by watching it for changes.
seek(long) only takes an exact block boundary, while sync(long) moves to the next block boundary. There does not appear to be a way to rediscover points that you can seek() to, which would be very useful for building or recovering an index for a file.