Bug#1036568: Design Defect in systemd-poweroff.service (Race Condition)

[Luca Boccassi]

Control: retitle -1 conflict between mdadm.shutdown and nutshutdown
Control: reassign -1 nut-server 2.8.0-7

On Mon, 22 May 2023 12:24:15 -0700 Alex Meyer <debbug523 at zezax.com>
wrote:
> Package: systemd
> Version: 247.3-7+deb11u2
> 
> I'm running Debian bullseye with systemd.  I also have installed
mdadm 
> and nut-server.
> 
> Looking in /lib/systemd/system-shutdown, I see two drop-in files with
> contents as follows:
> 
> mdadm.shutdown:
> 
> #!/bin/sh
> # We need to ensure all md arrays with external metadata
> # (e.g. IMSM, DDF) are clean before completing the shutdown.
> /sbin/mdadm --wait-clean --scan
> 
> nutshutdown:
> 
> #!/bin/sh
> /sbin/upsmon -K >/dev/null 2>&1 && /sbin/upsdrvctl shutdown
> 
> The man page for systemd-poweroff.service explains that:
> 
> All executables in this directory are executed in parallel
> 
> This is the problem.
> 
> mdadm.shutdown is trying to delay the shutdown action until data 
> integrity can be guaranteed (for arrays with external metadata). 
> Meanwhile, nutshutdown will immediately turn off the power.  These
are 
> obviously incompatible.
> 
> If execution were changed from parallel to sequential with a 
> deterministic ordering, this race condition could be avoided.
> 
> There may also be other ways to address the problem, such as
relocating 
> the mdadm stuff to some other spot in the systemd world.  This is 
> speculation on my part, as I'm no expert on systemd.
> 
> I should note that I do not personally have any md arrays with
external 
> metadata.  I do, however, have a UPS powering my Debian box and I can
> verify that orderly shutdown causes it to de-energize immediately at 
> this point.  I think it's self-evident that waiting for anything is 
> doomed to fail when run in parallel with powering off the host.
> 
> So, to be clear, this is a design defect I noticed, not a problem I'm
> experiencing.  Still, it looks like a doozy, and I feel compelled to 
> report it.
> 
> I see this as a structural problem.  It's clearly documented as the 
> intended behavior.  It doesn't depend on specific contents of my
mdadm 
> or nut configuration files, nor on my specific kernel.  Nevertheless:

This is nothing to do with src:systemd, both of these are shipped by
the respective packages, if they do not interact well then they need to
be fixed there.

Reassigning to nutserver as cutting power during shutdown seems the
slightly worst of the two, but will let the two maintainers figure out
if that's the right destination.

-- 
Kind regards,
Luca Boccassi
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