Comment 18 for bug 1669564

On Tue, May 2, 2017 at 8:54 AM, Dimitri John Ledkov <email address hidden>
wrote:

> Ideally, the following should happen:
> * boot
> * Created link configuration context
> <netplan creates/changes .link files to have a new MTU setting>
> * Check if link configuration needs reloading -> appears in the debug logs
> * New MTU is successfully applied
>

Let me put in the cloud-init sequence and see if we can figure out what may
be missing.

* boot
* systemd-udev* sockets, trigger, settle, udevd , cloud-init-local : run
* cloud-init writes a netplan.yaml
* cloud-init invokes netplan-generate
* system reaches sysinit.target

Looking at the deps on the systemd-udev units; there's no strict ordering
in cloud-init
that says it would run before, or strictly after udev related units.

If you note about the ignoring updated files that are less than 3 seconds
old as well
as systemd's variability w.r.t unit ordering due to missing explicit
Before/After directives
might explain the race.

I'd prefer not to wait for 3 seconds just because; so it maybe use useful
to see if we can
run cloud-init before udev; however it's not clear if invoking udev settle
in cloudinit (which
is done at various places directly (or indirectly through calling programs
like blkid or other
system programs) would end up waiting.

We could also explicitly order cloud-init-local after the udevd service
which depends on
all other udev units; however, if cloud-init runs immediately after it,
the 3 seconds may not
have passed.

I like running after udevd, but I would like to see if we can
force/configure udev to not
wait that 3 seconds;

> If the 'Check ...' is missing from the debug logs, after netplan has
> run, udevd will not reload the configs.
>
> I also would have thought that calling udevadm control --reload would
> force it to reload the contexts for the builtins.
>
> >From the original bug report description there is a call to:
> 'systemctl', 'start', '--no-block', 'systemd-udev-trigger.service'
>
> But if one is doing that, to avoid races, one should call udevadm settle
> -t 3 before re-triggering add.
>

Note that -t 3, --timeout=3 only sets the maxium wait time; this means
that if it
processes any event with in those 3 seconds, that we may not have waited
long
enough for udev when it re-reads link files and says it's not 3 seconds
since the last
time.

The 3 second re-read is arbitrary and ideally should be replaced by content
hashing or
use of inotify such that changes to the file (.link) can be triggered
automatically without
hacky things like sleep 3.

> These things are possibly red herrings too.
>
> --
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> report.
> https://bugs.launchpad.net/bugs/1669564
>
> Title:
> udevadm trigger subsystem-match=net doesn't always run rules
>
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