>A pidfd does not let you hold a reference to an individual thread, only to a process

I think that's outdated: https://github.com/torvalds/linux/commit/64bef697d33b75fc06c...

Probably still up to date for most distros. That shows as being in the 6.9 tree only, but Ubuntu 24.04 just shipped with 6.8

It would be nice to see these APIs wrapped usably. Even something as basic as FreeBSD's similar `pdfork()` and `pdkill()` would go a long way. (`pidfd_send_signal` exists, but glibc does not provide an ergonomic wrapper. Nor is there an ergonomic wrapper around clone w/ CLONE_PIDFD.)