Background:

When we activate a profile in NetworkManager, we create an internal copy of what was activated.

When later the profile gets modified (via `nmcli connection modify`), the changes don't take effect immediately (*). Changes to a profile only take effect after reactivating the profile.

(*) except "connection.zone" and "connection.metered", which take effect immediately, unless the modification happens with `NM_SETTINGS_UPDATE2_FLAG_NO_REAPPLY` flag.

Btw, you can get the content of the copy, wia the `GetAppliedConnection()` D-Bus call.


Now, there is the `Reapply()` D-Bus call,which takes a new profile and applies the changes right away, without requiring a full activation.

That is used by `nmcli device reapply` and `nmcli device modify` commands.


Reapply tries to do two conflicting things.

- apply all the changes as requested
- do not disrupt the connectivity. That means for example, avoid restarting DHCP and if we restart it, then do in a way that does not temporarily loose all IP addresses.

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There are problem with this.

1) If there are no changes to the profile, then often reapply does not actually do anything.

  nmcli device reapply eth0
  ip link set eth0 mtu 2000
  nmcli device reapply eth0

one might expect that the second reapply call fixes the MTU. However, it might not. Here we should lean more towards: make sure that the settings are really as requested, and take all necessary measures to make sure that's the case.


2) when we reapply the IP configuration and need to restart DHCP, then the IP address goes away. I think reapply should always restart DHCP, even if presumably no configuration changed. This is problem 1) above. The problem 2) is that while we restart DHCP, that the IP address gets removed temporarily. We should instead leave the IP address there, until we get a new lease. NMDhcpClient should abstract this. Similar things for RA, link-local, etc.