PaxExtensions fails to parse length-prefixed records when the value contains a newline

[Park-Jiyeonn]

Hi, I ran into a PAX parsing issue with a symlink whose linkpath value contains a newline.

Example:

let mut exts = tar::PaxExtensions::new(b"16 linkpath=a\nb\n");

let ext = exts.next().unwrap().unwrap();
assert_eq!(ext.key(), Ok("linkpath"));
assert_eq!(ext.value_bytes(), b"a\nb");

I expected this to parse as one PAX record, because the record is length-prefixed. But currently PaxExtensions splits the whole buffer on \n first.

From what I understand, PAX records should be parsed by consuming the decimal length prefix first.

This came up through linkpath on a symlink, but I think the same issue could affect any PAX keyword whose value contains a newline.

Would a patch that changes PaxExtensions to walk the buffer by record length be acceptable? I'd like to try it.

[Park-Jiyeonn]

@alexcrichton PTAL, thx!

[sleyva-jumptrading]

Ran into a similar issue, with the same proposed fix:
#428

I think it will be addressed in:
#457