nic_router: fix pure virtual call in Interface

The Interface constructor previously tried to attach to a domain.  This
might include sending a DHCP request to get the domain a valid IP config.
But in order to achieve this, the constructor used a pure virtual method
of Interface which crashes due to the unfinished vtable. To fix this bug,
the attach attempt was moved to a new Interface::init method.

Issue #2730

repos/os/src/server/nic_router/component.cc

@@ -153,11 +153,14 @@ Session_component *Net::Root::_create_session(char const *args)
 			throw Insufficient_ram_quota();
 		}
 		Session_label const label(label_from_args(args));
-		return new (md_alloc())
+		Session_component &component = *new (md_alloc())
 			Session_component(*md_alloc(), _timer, ram_quota - session_size,
 			                  _buf_ram, tx_buf_size, rx_buf_size, _region_map,
 			                  _mac_alloc.alloc(), _ep, _router_mac, label,
 			                  _interfaces, _config());
+
+		component.init();
+		return &component;
 	}
 	catch (Mac_allocator::Alloc_failed) {
 		error("failed to allocate MAC address");

repos/os/src/server/nic_router/interface.cc

@@ -1131,6 +1131,11 @@ Interface::Interface(Genode::Entrypoint     &ep,
 	_interfaces(interfaces)
 {
 	_interfaces.insert(this);
+}
+
+
+void Interface::init()
+{
 	try { _attach_to_domain(_policy.determine_domain_name(), true); }
 	catch (Domain_tree::No_match) { }
 }

repos/os/src/server/nic_router/interface.h

@@ -322,6 +322,8 @@ class Net::Interface : private Interface_list::Element
 
 		void link_state_sigh(Genode::Signal_context_capability sigh);
 
+		void init();
+
 
 		/***************
 		 ** Accessors **

repos/os/src/server/nic_router/main.cc

@@ -81,6 +81,7 @@ Configuration &Net::Main::_init_config()
 
 Net::Main::Main(Env &env) : _env(env)
 {
+	_uplink.init();
 	_config_rom.sigh(_config_handler);
 	env.parent().announce(env.ep().manage(_root));
 }