Yes. Apparently everyone re-implementing daemontools does something like this. So that brings me back to my original question: is there consensus that the historical behaviour is a bug? Or are there valid use casesĀ¹?
I don't think the historical behaviour is a *bug*, because the historical behaviour is documented and conforms to its documentation. It also comes from a time when supplementary groups weren't used as much as they are today. It's just that not having supplementary groups can defeat intuitive expectations when performing a group permissions check. That does not happen every day, but it does happen sometimes. s6-setuidgid had the same behaviour as setuidgid until I got bitten by that very problem, at which point I realized that "user identity" is not only uid and gid as it is for files, but also supplementary groups, and so I added supplementary groups support to s6-*uidgid. But it had been years until I found it necessary. So, YMMV. I'd say supplementary groups support is useful and allows the tool to better match user intuition, so it has value. But is it *mandatory* for correctness? You decide. -- Laurent