On Mon, May 27, 2013 at 10:36:41AM +, Duncan wrote
Here's an idea I've not seen proposed yet.
Make the wrapper function something like a cross between a simple
bootloader and traditional single-user-mode.
Normal mode, like the bootloader for many users, would be a default
choice with (configurable) either a timeout of a couple seconds, or (say
shift) key-held-down detection, thus no boot delay as if the key isn't
down at the moment of detection, boot proceeds using the existing config.
In the event of an init switchup, the user either sets an option before
shutdown (much like the run-once defaults of grub, etc, set pre-
shutdown), or selects switchup mode with the appropriate boot-time
trigger (using the above mentioned timeout or key-down sensing).
That is still adding unnecessary complexity to the systems, and an
additional possible point of failure.
The special switchup mode would then operate much like single-user in
terms of available functionality and the fact that it's a special mode
used for system maintenance, but would (normally, with a drop-to-shell
option as well) be rather more guided, presumably using a menu, again
much like a bootloader, except that it's PID-1 instead of pre-kernel.
What does this accomplish that could not be accomplished by...
* placing a switcher script in /sbin
* booting to single-user mode, and running the switcher script
Critical point #1 is that much like single-user mode, switchup mode
would NOT run normally, but would be specifically selected, with
either a reboot or simply starting the new init system at switchup
mode exit.
It waddles like single-user mode
It quacks like single-user mode
It flies like single-user mode
It *IS* single-user mode
Why bother re-inventing the wheel. Use single-user mode, already.
Critical point #2 is that the actual normal-mode wrapper would be very
small both in terms of boot-time and code, thus both easy to debug, and
not increasing boot time by much at all, particularly in key-down-
detection mode where there'd be no timeout delay to tick down. A small
binary that simply either runs the timout or detects key-down, before
execing the normal init, is all that would run in a normal bootup. The
complex functionality would only be invoked if switchup mode is chosen,
as entirely separate functionality execed place of the normal init it
would have otherwise execed.
Meanwhile, switchup mode (again, a separate binary execed only if chosen
from the tiny one designed to run inline at each boot) would have a menu
with options to invoke scripts for each of the init-system choices
offered, and a further fall-back to shell option.
Each init-system package would then depend (perhaps conditionally based
on an init-switcher USE flag) on the init-switcher package, and would
ship one gentoo specific file, the switcher script, thus putting each
switcher script under the control of the gentoo maintainers for that init-
system package, who could set it up to be as simple or as complex as
necessary for their init system. Those who needed a rw root to switch
out various files could arrange for their switcher script to handle that,
while those who could do without, possibly handling things later with
their own native init-service, could do without the rw root bit.
Similarly, switchup mode exit-time behavior, presumably either simply
triggering a reboot, or starting the selected init-system directly, would
be entirely under the control of the individual init-system package
maintainers, via the switch-script they maintain.
I repeat, all this can be handled in single-user mode right now.
As a first bonus, even people who aren't interested in more than one init-
system might find setting the init-switcher USE flag useful, especially
on EFI systems, since it would give them the advantages of switchup mode,
namely a drop to shell option as yet another alternative to single user
mode,
Oh boy... a second single-user mode.
AND perhaps even MORE importantly, access to a more or less automated
init-system restore option, triggered via selection of the same
switcher script that would otherwise be run to switch between init-
systems. Again, the contents of that init-system-specific switcher-
script would be entirely under the control of the gentoo maintainers
for that package, so they could do whatever fancy working-condition
checks and restores from backup that they wanted.
What you're talking about is rescue-CD functionality. I don't know if
it's possible, but it would be very nice as a standalone rescue CD
(actually USB stick nowadays). If so, I would much rather prefer it on
as rescue CD/USB-stick than as a boot option. If a system is really
screwed up, you can't even assume that it'll boot to single-mode from
the hard drive.
As a second bonus, switchup mode would be extremely flexible and
extensible via
