timesyncd may try to sync when system is not really online

[Alvin Šipraga <ALSI@xxxxxxxxxxxxxxx>]

Hello,

systemd-timesyncd listens for changes in network state and will start a 
timesync when it determines that the system is online. It does this via 
network_is_online(), which just queries the global carrier and address 
state advertised by systemd-networkd.

systemd-networkd determines the global carrier and address state by 
simply iterating through all links and taking the maximum of the link 
carrier and address states.

However, a routable link does not necessarily mean that the system is 
online. One example is with wireless access point interfaces: an AP is 
set up with hostapd and networkd configures a static IP address on that 
interface, making it routable. By the above, this causes timesyncd to 
attempt a timesync that is doomed to fail.

We are having trouble with this logic because we start hitting 
timesyncd's internal rate limiter. After a non-AP interface gets an IP 
and we are truly "online", timesyncd will try to start a timesync, but 
may defer it for up to 30 seconds due to rate limiting.

Ideally, timesyncd should be able to distinguish between the two cases 
such that it can ignore the state of the AP interface and avoid spurious 
timesync attempts.

I can think of a few solutions to this problem but I wanted to consult 
the mailing list to see if anybody has some better ideas. Here are some 
proposals, in order of descending preference:


1. Add a new [Link] option NotForOnline= (or similar) to 
systemd.networkd(5) to indicate that this link being routable does not 
imply that the system is online. This could also implicitly set 
RequiredForOnline=no if desired. Then:

   i) Parse the above option in network_is_online() to exclude certain
      links; or
  ii) parse the above option only in timesyncd. Note that the only user
      of network_is_online() is timesyncd anyway.

2. Add a new [Link] option IgnoreLinkState= (or similar) to 
systemd.network(5) to indicate to systemd-networkd that this link's 
carrier and address state should not be factored into the calculation of 
global carrier and address state.

3. Same as (1) above, but rather than adding a new option, just parse 
RequiredForOnline=.

4. Something like the above, but set the ignored or allowed interfaces 
in timesyncd.conf.

5. Make the rate limiter user-configurable so that a user can disable or 
tune it accordingly.


Options (1) and (2) seem to be the only decent solutions IMO. But I'm a 
bit cautious of (2) because I don't know what other stuff might be 
depending on systemd's global state.

Happy to hear other ideas on this. Could be that we are also doing 
something wrong and there is already a knob to twist. I can provide 
further details if necessary.

I'll prepare a patch once I've waited some time for any feedback here.

Kind regards,
Alvin
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