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In the initial design, foobar-wait-online.service would have
Requisite=foobar.service, so that foobar-wait-online.service could be enabled
unconditionally, irrespective of whether foobar.service itself is enabled.
Unfortunately this doesn't work too well:
1. the message about foobar-wait-online.service being skipped because of a
"missing dependency" *looks* like an is problem. This is mostly cosmetic,
but it also quite confusing. We generally don't want any messages of this
type during default boot.
2. it is impossible to start and wait for the network in an
implementation-agnostic way: systemctl start network-online.target, or
Wants/After=network-online.target in a unit don't work because pulling in
network-online.target pulls in foobar-wait-online.service, but it in turn
does not pull in foobar.service. During startup, foobar.service is pulled in
by multi-user.target, but not in a smaller transaction which does not
include multi-user.target.
This change means that *-wait-online.service should be installed through
presets, so that it can be enabled/disabled at will by the administrator.
Our own systemd-networkd-wait-online.service does this already, and
similar change has been requested for NetworkManager-wait-online.service
(https://bugzilla.redhat.com/show_bug.cgi?id=1455704).
This change should by mostly backwards-compatible, unless somebody has some
wait-online.service enabled, without having the corresponding network
implementation enabled, and they are relying on it not being started. I think
that's relatively unlikely because of issue 1. above, and I'm not aware of this
being the default in any distro. And being able to start the network in an
implementation-agnostic way is pretty important, see
https://bugzilla.redhat.com/show_bug.cgi?id=1452866.
There might be implementations around where the network-online logic
might not talk to any network configuration service (and thus not have
to wait for it), hence let's explicitly order network-online.target
after network.target to avoid any ambiguities.
To make sure we don't delay boot on systems where (some) network links are managed by someone else
we don't block if something else has successfully brought up a link.
We will still block until all links we are aware of that are managed by networkd have been
configured, but if no such links exist, and someone else have configured a link sufficiently
that it has a carrier, it may be that the link is ready so we should no longer block.
Note that in all likelyhood the link is not ready (no addresses/routes configured),
so whatever network managment daemon configured it should provide a similar wait-online
service to block network-online.target until it is ready.
The aim is to block as long as we know networking is not fully configured, but no longer. This
will allow systemd-networkd-wait-online.service to be enabled on any system, even if we don't
know whether networkd is the main/only network manager.
Even in the case networking is fully configured by networkd, the default behavior may not be
sufficient: if two links need to be configured, but the first is fully configured before the
second one appears we will assume the network is up. To work around that, we allow specifying
specific devices to wait for before considering the network up.
This unit is enabled by default, just like systemd-networkd, but will only be pulled in if
anyone pulls in network-online.target.
