Re: s6 daemon restart feature enhancement suggestion

[adam@westernsemico.com]

Quoting Laurent Bercot (2024-05-26 12:53:39)
> If you are building a distribution aimed at supporting several kinds
> of hardware, I suggest adding flexibility at the *source database*
> level, and building the compiled database at system configuration time

If at all possible, do this.

> (or, in extreme cases, at boot time, though I do not recommend that if
> you can avoid it, since you lose the static bootability guarantee).

I believe there is also a third approach which is sort of a middle-ground: all
oneshots and longruns are compiled statically, but the contents of bundles are
determined at boot time.  Think of it by analogy to the Linux kernel: distros
nowadays build every single module under the sun -- no matter how obscure -- but
only a few ever get loaded.

I believe that s6-rc-bundle is the tool used to edit the bundles:

  https://www.skarnet.org/software/s6-rc/s6-rc-bundle.html

I haven't actually tried this myself, but perhaps it might help.  Changing the
contents of bundles seems like a much less-violent action to take at boot time,
with fewer possible failure modes.

If you go this route, there is a fork in the road.  Assuming you're using
s6-linux-init,

- The easy route: in a perfect world you'd do this boot-time detection from
  within `rc.init`, before invoking `s6-rc start $DEFAULT_BUNDLE`.
  Unfortunately at that point you probably don't have anything mounted
  read-write (except /run), haven't started your hotplug daemon (e.g. mdevd),
  and haven't done a coldplug yet.  So the easy route might not work.

- The hard route: remember that s6-rc's locks are *not* reentrant!  You will
  have to think very carefully about what locks are held with s6-rc-bundle is
  called, and whether or not to use its `-b` flag.  Don't try to use s6-rc tools
  from within a oneshot.

  - a