Why would you have to error, or retain the offset? Block until that input exists, and/or return a higher offset to use for the next call - perhaps add the lengt

Why would you have to error, or retain the offset? Block until that input exists, and/or return a higher offset to use for the next call - perhaps add the length of error message so you essentially can't guess and have to show the error has been checked. It doesn't have to match the underlying bytes, it's just a progress marker.