updog check-update return code

[tjkirch]

updog.service runs updog check-update which returns 1 if there are no new updates. That's a healthy state for the system, so the service shouldn't be marked as failed (and the system as degraded) when there are no updates.

We need a way for the systemd service not to fail when there are no updates. We should either:

• Always make it return 0 when there are no updates
• Make it return 0 unless a --fail-if-no-updates flag is given
• Make it return 0 if a --allow-no-updates flag is given

[tjkirch]

Or, @sam-aws suggests that we could just remove the service for now, since it's just informational.

[sam-aws]

Always make it return 0 when there are no updates

NAK, this was the whole drama with dogswatch

Make it return 0 unless a --fail-if-no-updates flag is given
Make it return 0 if a --allow-no-updates flag is given

This feels like adding too much complexity to the operation, but I can see some utility in at least checking if the TUF repo is available. Then again the --all option would do that too.

The updog service was added way early on as more of a placeholder than anything else; I'd lean towards removing it and replacing it with something that advertises boot/update success (eg. something that calls #674). That could even be covered by the existing mark-successful-boot.service.

[iliana]

+1 to removing updog.service