I was using this module and this module helped me alot. Thank you.

As per GNU Tar Manual: Storing Sparse Files, sparse infomation might span multiple blocks.
GNU LongName, etc is handled by archive/tar therefore not appear to users explicitly.

However, this line only assumes headers are always one block long... Am I right?

I maybe open a PR for this. But considering the fact that no one has ever reported about this, there's no such tar files on the wild I guess?