Packaging policy discussion: After=network-online.target

[Christopher James Halse Rogers]

Hello everyone,

There's an nfs-utils SRU¹ hanging around waiting for a policy decision 
on use of the After=network-online.target systemd unit dependency. I'm 
not an expert here, but it looks like part of my SRU rotation today is 
starting the discussion on this so we can resolve it one way or another!

I am not an expert in this area, but as I understand it, the tradeoff 
here is:
1. Without a dependency on After=network-online.target there is no 
guarantee that the network interface(s) will be usable at the time the 
nfs-utils unit triggers, and nfs-utils will fail if the relevant ntwork 
interface is not usable, or
2. With a dependency on After=network-online.target nfs-utils will 
reliably start, but if there are any interfaces which are configured 
but do not come up this will result in the boot hanging until the 
timeout is hit.

In mitigation of (2), there are apparently a number of default packages 
which already have a dependency on After=network-online.target, so boot 
hanging if interfaces are down is the status quo?

The obvious thing to do here would be to follow Debian, but as far as I 
can tell there is not currently a Debian policy about this - the best I 
can find is an ancient draft of a best-practises-guide² suggesting 
that pacakages SHOULD handle networking dynamically, but if they do not 
MUST have a dependency on After=network-online.target

As far I understand it, handling networking dynamically requires 
upstream code changes (although maybe fairly simple code changes?).

It seems unlikely that, whatever we decide, we'll immediately do a full 
sweep of the archive and fix everything, so it looks like our choice is 
between:

1. The long-term goal is to have no After=network-online.target 
dependencies in default boot (stretch goal: in main). Whenever we run 
into a package-fails-if-network-is-not-yet-up bug, we patch the code 
and submit upstream. Over time we audit existing users of 
After=network-online.target and patch them for dynamic networking, as 
time permits.

2. We don't expect to be able to reach no After=network-online.target 
dependencies in the default boot, so it's not a priority to avoid them. 
Whenever we run into a package-fails-if-network-is-not-yet-up bug, we 
add an After=network-online.target dependency.

Option (1) seems to be the technically superior option (and is 
recommended by systemd upstream³), but appears to require more work. I 
have limited insight into how much work that would be; someone from 
Foundations or Server probably needs to weigh in on that.

Option (2) seems to be formalisation of the status-quo, so would seem 
to be less work.

Let the discussion begin!

¹: https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/1918141
²: 
https://github.com/ajtowns/debian-init-policy/blob/master/systemd-best-practices.pad
³: https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/