sway: spawn bar with posix_spawn instead of fork + exec

[WavyEbuilder]

No description provided.

[emersion]

What is the motivation for this?

Seems like this is incompatible with pthread_atfork() either way.

[WavyEbuilder]

What is the motivation for this?

fork + exec is slower, and just worse compared to posix_spawn (which as per the name is also in posix). I don't see a reason to not use posix_spawn where possible, unlike fork it doesn't dup the entire parent. Not a huge deal, but still wasting memory.

Seems like this is incompatible with pthread_atfork() either way.

I didn't realise they were there, but in the cases for sched and signals posix_spawn can do that:
https://man7.org/linux/man-pages/man3/posix_spawnattr_setsigdefault.3p.html
https://man7.org/linux/man-pages/man3/posix_spawnattr_setschedpolicy.3p.html

[WavyEbuilder]

If we end up doing it though, we may as well do it everywhere, except exec (as in sway's exec, not the syscall) currently. We need FreeBSD to support setsid() for posix spawn. There is POSIX_SPAWN_SETSID in glibc, but sadly this is not in FreeBSD yet. It is new in posix 2024 though, so it should get it at some point!

C-f for POSIX_SPAWN_SETSID on https://pubs.opengroup.org/onlinepubs/9799919799/functions/posix_spawn.html to see it in posix 2024 (I would link to the section directly but I can't seem to do that, so C-f will have to suffice, sorry).

Also https://austingroupbugs.net/view.php?id=1044