disabling sleep when shutting down

[Ian Smith]

On Sat, 26 Sep 2015 00:26:31 -0700, Colin Percival wrote:
 > Hi all,
 > 
 > I think there is consensus for:
 > * Using a sysctl (simpler than a device node),
 > * Setting this sysctl on all architectures,
 > * Calling the sysctl kern.suspend_blocked,
 > * Consulting the sysctl from the ACPI code (for now) and possibly from
 > other platform-specific forms of sleeping (in the indefinite future).

Sounds all good to me, FWTW.

 > Points without consensus:
 > * jkim thinks we should prevent suspend when we're dropping to single-user
 > mode; I'm not sure I see the point, but I don't think there's any harm in
 > doing that too.

We should prevent suspending _during_ shutdown to SU, of course.  Once 
happily in SU, is there any reason to disallow suspend?  I've done it.

 > * Ian Smith would like to have suspend blocked for the last 5 minutes before
 > shutdown(8) signals init to shut the system down.  I don't think anyone else
 > has expressed a desire for this, and some people have raised concerns about
 > blocking suspend for too long in case a system is running out of battery; so
 > I'm inclined to leave this out at this point.  (It would be easy enough to
 > add the sysctl-frobbing to shutdown(8) if desired later.)

Sorry, but that rather misrepresents my position; I was trying to deal 
specifically with the LID foot-shooting potential, in the case of user- 
initiated shutdown, so looking at possible mechanisms.  Not to worry, I 
clearly didn't express myself clearly :^\

As jkim pointed out, code speaks for itself, and C is read-only here at 
best .. though after 30-odd years I can usually follow it around ..

 > With the above in mind, I've written (but not yet actually compiled or tested,
 > so beware of typos) a patch which I think makes sense.  If nobody is violently
 > opposed to this I'll make sure I got the details right and then commit this in
 > a few days.

+static void
+block_suspend(int block)
+{
+
+       sysctlbyname("kern.suspend_blocked", NULL, NULL, &block, 
sizeof(block));
+}

This doesn't appear to get called?

Any idea if any of this may not be straightforward to MFC, to 9 maybe?

Thanks for going for the generic solution.  Hope it wins that race :)

cheers, Ian
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