[mythtv-users] delaying backend startup until the network is up

[Ian Campbell]

On Wed, 2018-12-19 at 14:33 +1300, Stephen Worthington wrote:
> On Tue, 18 Dec 2018 15:46:40 +0000, you wrote:
> 
> > On Mon, 2018-12-10 at 17:03 -0500, James Abernathy wrote:
> > > on my production system I use without issue the systemd override
> > > provided by. 
> > > 
> > > sudo systemctl edit mythtv-backend.service
> > > 
> > > [Unit]
> > > After=NetworkManager-wait-online.service
> > 
> > I notice that a lot of the examples (general ones, not myth
> > specific)
> > on the web seem to also have a Wants= line with the same target. Is
> > there some subtle distinction between having one vs having both
> > lines?
> > (I don't run systemd on my myth box and don't really grok this
> > systemd
> > thing).
> 
> The distinction is not at all subtle.  If you have an "After=" line,
> then the unit with that line will not be started until the specified
> unit has completed startup.  But if you do not have a "Wants=" (or
> "Requires=" or something similar), and there is no other cause for the
> Wants= unit to be started, it will not be started, and then your
> After= unit will also not be started.  After= sets up ordering between
> units.  Wants=, requires= and the like cause startup of other units.

IOW the use of `After=` only, withouit some other `Want=` or similar
dependency is likely insufficient to ensure the thing is run at all?

> > I also see that the service referenced in [0] is named `network-
> > online.service` (but that it says to enable `NetworkManager-wait-
> > online.service` to satisfy it). Maybe that doesn't matter and
> > you've
> > just harmlessly "dereferenced" one link in a dependency chain.
> 
> The network-online.target (not .service) is a systemd "special"
> target.  It happens when minimal networking is available.  That
> usually means that only the localhost interface is available - to
> meet
> the requirements for network-online.target, only one interface needs
> to be available, and localhost is normally the one that comes up
> first
> as it is pure software and does not need to wait for any hardware.

This contradicts  
https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/h which
says re `network-online.target` (under "How do I make sure that my
service starts after the network is really online?"):

> This will ensure that all configured network devices are up and have
> an IP address assigned before the service is started. 
> network-online.target will time out after 90s. Enabling this might 
> considerably delay your boot even if the timeout is not reached.

Are you saying the "all configured network devices" statement here is
incorrect (or somehow misleading?), contrary to what you said below
(trimmed) it also says "an IP address assigned" not merely "up".

Nevertheless, James, have you tried following the suggestions on that
systemd documentation page, specifically using:

   After=network-online.target
   Wants=network-online.target

_and_ enabling the `NetworkManager-wait-online.service` (to satisfy the
network-online.target)? I think that would be:

    # systemctl enable NetworkManager-wait-online.service

According to the page the command to check is:

    # systemctl is-enabled NetworkManager-wait-online.service

I think that is definitately worth trying before getting into other
more complex workarounds suggested here. If it doesn't work I'd also
encourage you to report a bug with your distro, at the very least the
docs need fixing to not so unambiguously recommend this as the approach
to use if it isn't actually intended to work that way.

> Also, it is fairly common for server PCs not to run NetworkManager,

Note that the OP who I was addressing does appear to be using
NetworkManager.

Ian.