Removing conflicts of init system

[Guillem Jover]

On Sat, 2018-12-22 at 20:11:13 +0100, Adam Borowski wrote:
> On Sat, Dec 22, 2018 at 05:54:26PM +0100, Guillem Jover wrote:
> > On Fri, 2018-12-21 at 23:57:38 +0100, Thorsten Glaser wrote:
> > > No. update-alternatives is too fragile to handle things like
> > > /bin/sh and init(8).
> > 
> > While this certainly used to be true, I don't it has been the case for
> > a long time now? It seems like oral tradition to me now. :) There is I
> > think a single non-wishlist bug report against u-a in the BTS. So if
> > there are still problems I'd be more than happy to hear about them.
> 
> I believe this is a misconception based on an actual problem: the
> alternatives system cannot be used for /bin/sh symlinks as

> 1. it uses shell scripts itself (making it impossible to recover) and

I don't think u-a has used any shell in any of its recent history. The
only external thing u-a itself it currently still uses is mv(1) via an
execvp(3), *only* in case a rename(3) failed for example due to a
cross-device move.

> 2. is not atomic, leaving a brief time window when there's no /bin/sh.

But it should! It uses a temporary file first and then does the usual
atomic rename(3) dance over it. If there is any place where this is
not the case, I'm also happy to hear. :)

> Nothing of that issue applies to /bin/init which is ever executed only
> during the boot process then at controlled moment during package updates.

It's still as important, as you might suffer from a sudden crash and
be left w/o a /bin/init program.

Thanks,
Guillem