mdns: add config option to use netif ext callback

e.g. if the calling stack should not invoke the mdns functions due
to high stack usage, disable the option and trigger it yourself.

Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>

src/apps/mdns/mdns.c

@@ -102,7 +102,9 @@ static const ip_addr_t v6group = DNS_MQUERY_IPV6_GROUP_INIT;
 
 static u8_t mdns_netif_client_id;
 static struct udp_pcb *mdns_pcb;
+#if MDNS_RESP_USENETIF_EXTCALLBACK
 NETIF_DECLARE_EXT_CALLBACK(netif_callback)
+#endif
 static mdns_name_result_cb_t mdns_name_result_cb;
 
 #define NETIF_TO_HOST(netif) (struct mdns_host*)(netif_get_client_data(netif, mdns_netif_client_id))
@@ -1912,7 +1914,7 @@ dealloc:
   pbuf_free(p);
 }
 
-#if LWIP_NETIF_EXT_STATUS_CALLBACK
+#if LWIP_NETIF_EXT_STATUS_CALLBACK && MDNS_RESP_USENETIF_EXTCALLBACK
 static void
 mdns_netif_ext_status_callback(struct netif *netif, netif_nsc_reason_t reason, const netif_ext_callback_args_t *args)
 {
@@ -1940,7 +1942,7 @@ mdns_netif_ext_status_callback(struct netif *netif, netif_nsc_reason_t reason, c
     mdns_resp_announce(netif);
   }
 }
-#endif
+#endif /* LWIP_NETIF_EXT_STATUS_CALLBACK && MDNS_RESP_USENETIF_EXTCALLBACK */
 
 static err_t
 mdns_send_probe(struct netif* netif, const ip_addr_t *destination)
@@ -2396,8 +2398,10 @@ mdns_resp_init(void)
 
   mdns_netif_client_id = netif_alloc_client_data_id();
 
+#if MDNS_RESP_USENETIF_EXTCALLBACK
   /* register for netif events when started on first netif */
   netif_add_ext_callback(&netif_callback, mdns_netif_ext_status_callback);
+#endif
 }
 
 #endif /* LWIP_MDNS_RESPONDER */

src/include/lwip/apps/mdns_opts.h

@@ -59,6 +59,13 @@
 #define MDNS_MAX_SERVICES               1
 #endif
 
+/** MDNS_RESP_USENETIF_EXTCALLBACK==1: register an ext_callback on the netif
+ * to automatically restart probing/announcing on status or address change.
+ */
+#ifndef MDNS_RESP_USENETIF_EXTCALLBACK
+#define MDNS_RESP_USENETIF_EXTCALLBACK  LWIP_NETIF_EXT_STATUS_CALLBACK
+#endif
+
 /**
  * MDNS_DEBUG: Enable debugging for multicast DNS.
  */