Apply the resolvers instead of only writing them to the profile

Setting DNS rewrote the connection profile and stopped there, so the
running device kept resolving through whatever it had before. Nothing
noticed while something else always activated the device afterwards: the
profile was written first and NetworkManager picked the resolvers up
when it got round to the interface on its own.

Turning a DHCP client off now applies the profile when the device is not
activated, which moved that activation ahead of the DNS write. A caller
that sets an address, disables DHCP, and then sets resolvers -- the
order a static configuration is naturally applied in -- ends with the
device activated on a profile that had no DNS in it yet, and the write
that follows never reaches the system. The profile carries the
resolvers, resolv.conf carries none, and ipv4.ignore-auto-dns has by
then ruled out the lease's resolvers as well: an interface holding an
address, a route, and no way to resolve a name.

Reapply the device after the change. Unlike turning a client off, this
risks nothing for a caller connected over the interface -- reapply
changes the device in place and does not tear the link down -- so it is
not conditioned on the device being unactivated, which is exactly the
case that needs it.

networkManager.go

@@ -869,5 +869,18 @@ func (nm *networkManager) SetIfaceDNS(ctx context.Context, iface string, servers
 		}
 	}
 
+	// Put the resolvers into effect. Writing them to the profile alone leaves
+	// the running device on whatever it resolved through before, and the
+	// ignore-auto-dns above means that is now nothing at all where the old
+	// resolvers came from a lease -- a device holding an address, a route, and
+	// no way to resolve a name. Unlike turning a DHCP client off, this carries
+	// no risk for a caller connected over the interface: reapply changes the
+	// device in place and does not tear the link down.
+	if len(targets) != 0 {
+		if rerr := reapplyDevice(ctx, iface); rerr != nil {
+			logger.Printf("error applying the connection profile to %s: %v", iface, rerr)
+		}
+	}
+
 	return errors.Join(errs...)
 }

tests/networkManager/results/1/test

@@ -4,3 +4,4 @@ connection modify test ipv4.routes 10.253.2.0/24 203.0.113.22 100
 connection modify test ipv6.routes abcd:ef12:3455:10::/64 abcd:ef12:3456:10::1 100
 connection modify test_eth0 ipv4.dns 8.8.8.8,1.1.1.1 ipv4.dns-search example.com ipv4.ignore-auto-dns yes
 connection modify test_eth0 ipv6.dns 2001:4860:4860::8888 ipv6.dns-search example.com ipv6.ignore-auto-dns yes
+device reapply test_eth0

tests/networkManager/results/2/test

@@ -4,6 +4,7 @@ connection modify test ipv4.routes 10.253.2.0/24 203.0.113.22 100
 connection modify test ipv6.routes abcd:ef12:3455:10::/64 abcd:ef12:3456:10::1 100
 connection modify test_eth0 ipv4.dns 8.8.8.8,1.1.1.1 ipv4.dns-search example.com ipv4.ignore-auto-dns yes
 connection modify test_eth0 ipv6.dns 2001:4860:4860::8888 ipv6.dns-search example.com ipv6.ignore-auto-dns yes
+device reapply test_eth0
 connection modify test_eth0 ipv4.method auto ipv4.addresses 1.2.10.4/24 ipv4.gateway 1.2.10.254
 connection modify test_eth0 ipv6.method auto ipv6.addresses ipv6.gateway
 connection modify main ipv4.routes