network: introduce link_reconfigure()

Will be used in later commits.

src/network/networkd-link.c

@@ -2855,6 +2855,78 @@ static int link_configure_duid(Link *link) {
         return 0;
 }
 
+int link_reconfigure(Link *link) {
+        Network *network;
+        int r;
+
+        if (IN_SET(link->state, LINK_STATE_PENDING, LINK_STATE_LINGER))
+                return 0;
+
+        r = network_get(link->manager, link->sd_device, link->ifname,
+                        &link->mac, &network);
+        if (r == -ENOENT) {
+                link_enter_unmanaged(link);
+                return 0;
+        } else if (r == 0 && network->unmanaged) {
+                link_enter_unmanaged(link);
+                return 0;
+        } else if (r < 0)
+                return r;
+
+        if (link->network == network)
+                return 0;
+
+        log_link_info(link, "Re-configuring with %s", network->filename);
+
+        /* Dropping old .network file */
+        r = link_stop_clients(link, false);
+        if (r < 0) {
+                link_enter_failed(link);
+                return r;
+        }
+
+        if (link_dhcp4_server_enabled(link))
+                (void) sd_dhcp_server_stop(link->dhcp_server);
+
+        r = link_drop_config(link);
+        if (r < 0)
+                return r;
+
+        if (!IN_SET(link->state, LINK_STATE_UNMANAGED, LINK_STATE_PENDING)) {
+                log_link_debug(link, "State is %s, dropping config", link_state_to_string(link->state));
+                r = link_drop_foreign_config(link);
+                if (r < 0)
+                        return r;
+        }
+
+        link_free_carrier_maps(link);
+        link_free_engines(link);
+        link->network = network_unref(link->network);
+
+        /* Then, apply new .network file */
+        r = network_apply(network, link);
+        if (r < 0)
+                return r;
+
+        r = link_new_carrier_maps(link);
+        if (r < 0)
+                return r;
+
+        link_set_state(link, LINK_STATE_INITIALIZED);
+
+        /* link_configure_duid() returns 0 if it requests product UUID. In that case,
+         * link_configure() is called later asynchronously. */
+        r = link_configure_duid(link);
+        if (r <= 0)
+                return r;
+
+        r = link_configure(link);
+        if (r < 0)
+                return r;
+
+        return 0;
+}
+
 static int link_initialized_and_synced(Link *link) {
         Network *network;
         int r;

src/network/networkd-link.h

@@ -204,6 +204,8 @@ uint32_t link_get_ipv6_accept_ra_route_table(Link *link);
 int link_request_set_routes(Link *link);
 int link_request_set_nexthop(Link *link);
 
+int link_reconfigure(Link *link);
+
 #define ADDRESS_FMT_VAL(address)                   \
         be32toh((address).s_addr) >> 24,           \
         (be32toh((address).s_addr) >> 16) & 0xFFu, \