How are uname/gname handled?

[aconz2]

In layer.md, it states that uname is secondary to uid and likewise for gname/gid. So far I've only run into tar layers which have empty uname/gname, but the spec seemingly supports it. I'm writing a tool that consumes layers and am wondering what you're supposed to do if you get a uname? And what exactly does it mean to be "secondary" to uid? AFAIK the tar header doesn't have a null/none/missing/blank value for uid, so it would always take precedence. And even if you wanted to use the uname, which /etc/passwd do you look it up in?

[sudo-bmitch]

Personally I avoid anything other than numeric uid/gid for portability and reproducibility (https://reproducible-builds.org/docs/archives/). I'll delegate to the major implementations for how uname and gname can be used. It could be that those fields are only valid on platforms that do not support a uid/gid (I'm not sure how Windows handles this), and otherwise uid/gid are only used.

Feedback from @stevvooe, @vbatts, or @mikebrow would be good.