Re: restarting s6-svscan (as pid 1)

[adam@westernsemico.com]

Quoting dan@telent.net (2023-11-17 14:20:32)
> I was thinking I could use the .svscan/finish script to check for the
> existence of the "maintenance mode" ramfs, remount it onto /
> and then `exec /bin/init` as its last action, though it seems a bit
> cheesy to have a file called `finish` that actually sometimes performs
> `single-user-mode` instead. Would that work?

I think your use case is *precisely* what .svscan/finish is for -- it's how you
get s6-svscan to exec() into some other process.  That other process can be an
instance of itself.

The fact that systemd has a special self-exec() mechanism always seemed weird
and bizzarre.  Just use the general mechanism.

> Perhaps a more general use case for re-execing pid 1 would be after OS
> upgrades as an alternative to rebooting

Sure, if you upgrade the libc or your compiler, and you want s6-svscan to use
those new libc/compiler, this is an easy way to do it.

> though other than wanting to preserve uptime for bragging rights I can't see
> any real advantage...

You can pass arbitrary large chunks of data to a re-exec()'ed pid1.  It's not
always easy to do that across a reboot, since you have to pass the data to the
bootloader and back.  Also "the data" could be open file descriptors.

  - a