Booting, s6-rc services and controlling terminal

From: Guillermo <gdiazhartusch_at_gmail.com>
Date: Sat, 2 Apr 2016 12:56:02 -0300

Hello,

Excuse me if this is a basic POSIX question.

Consider a machine with an s6-linux-init-maker-style setup, that also
uses s6-rc in the stage2 and stage2_finish scripts. Longruns managed
by s6-rc without a 'producer-for' file in their definition directory
have their stdout and stderr redirected to the catch-all logger, and
so do oneshots via the s6rc-oneshot-runner service. So if one wants to
have a service output to the console instead (e.g. an early one in the
boot process), its definition would have to do explicit redirections
to /dev/console. But should the service also be turned into a session
leader and set a controlling terminal, or only in special cases?

I thought that this was needed only in special cases, like in
processes that end up spawning a shell, but I see other init systems
sometimes do it automatically. For example, with nosh, a service
configured to output to a terminal AND defined using a systemd unit
file, when converted to a bundle directory (nosh's version of a
service definition directory) with the converter tool, has a 'run'
script that calls open-controlling-tty (nosh's chainloading utility to
open a device as a controlling terminal). And sysvinit's code has
setsid() and TIOCSCTTY ioctl() calls for child processes, depending on
the 'runlevel' and 'action' fields of the corresponding line in
/etc/inittab.

What those two cases have in common is that the mechanism involved
also unconditionally redirects stdin to a terminal device; in the nosh
case, I'm not sure if because of a limitation of the unit
file-to-bundle directory converter. But I don't know why the
"controlling terminal maneuver" is done there, so I now wonder if
s6-rc service definitions would also require it in more cases than I
thought, instead of simple FD redirections.

Thanks,
G.
Received on Sat Apr 02 2016 - 15:56:02 UTC