[Nut-upsuser] [systemd-devel] Trying to turn off a UPS with home-made service unit

[Roger Price]

On Wed Sep 11 10:45:48 PDT 2013 Lennart Poettering wrote:

> So, it's a really bad idea to turn off UPS from userspace, simply 
> because you will race against the kernel's fs sync() code, and you never 
> know what will finish first: your UPS shutdown timeout or your fs 
> sync(). Doing this from userspace is hence racy.

> If you want to use a hack like this then at least do it via the 
> /usr/lib/systemd/system-shutdown/ drop-in directory which is called very 
> late during shutdown. It will shorten the race window, but not erase it, 
> since the kernel will flush various buffers only after the reboot() 
> system call is invoked.

Here is the effect on a simple openSUSE 12.3 test rig of replacing my 
home-made service unit /lib/systemd/system/ups-delayed-shutdown.service 
with a script in the /usr/lib/systemd/system-shutdown/ drop-in directory. 
I used the default UPS offdelay of 20 secs. The X's indicate the race 
window. (Sorry for the ascii art.)

1) My "bad idea" ups-delayed-shutdown.service

systemctl               system
poweroff                 halt
    |     |XXXXXXXXXXXXXXXX|
    0   2 | 4   6   8  10  12  14  16  18  20  22  24  26  28  30  32 secs
          |                                       |
       upsdrvctl                                 UPS
       shutdown                                shutdown

2) Replaced by script in /usr/lib/systemd/system-shutdown/

systemctl               system
poweroff                 halt
    |                 |XXXX|
    0   2   4   6   8 |10  12  14  16  18  20  22  24  26  28  30  32 secs
                      |                                       |
                  upsdrvctl                                  UPS
                  shutdown                                 shutdown

The script solution has the advantage of reducing the race window from 9 
to 3 secs.  Note that the same security can also be achieved by the 
service unit by setting offdelay = 26 in ups.conf.  My script is

    #! /bin/bash
    # Delayed turn off for the UPS unit.
    # Needed for automatic system restart when wall power returns.
    UPSDRVCTL_BIN=/usr/lib/ups/driver/upsdrvctl
    $UPSDRVCTL_BIN shutdown

I tried adding

    echo `date -I` `date +%T` "$0 calls $UPSDRVCTL_BIN shutdown" >> /var/log/UPS.log

to get a trace of the activity, but nothing is written in 
/var/log/UPS.log.

Roger