When Interface::handle_config_3 (third step of applying a new configuration to
interfaces) tried to detach the interface from the current IP config because
the old and new IP config differed, it did so using the new domain. The former
steps of the reconfiguration already installed the new domain reference at the
interface. Therefore, also the DHCP server of the new domain was used. This,
however caused uncaught exceptions because detaching from an IP config
includes dissolving all DHCP allocations. This dissolving of DHCP allocations
now operated on a DHCP server (the one of the new domain) that wasn't related
to the allocations and, in the worst case, caused an uncaught exception
because the IPs were out of its range.
That said, this commit ensures that detaching an interface from an IP config
is always done on the domain from which the IP config originated. Normally,
this is the domain the interface is attached to. But in the case of
Interface::handle_config_3, it is another - the former domain the interface
was attached to.
The commit also adapts the nic_router_dhcp_* tests in a way that they
reconfigure the router in a way that would trigger the uncaught exception
without the fix.
Fixes#4200
be644098d7