zone: add often used zone_soa() into API

src/knot/events/handlers/refresh.c

@@ -781,18 +781,6 @@ uint32_t bootstrap_next(uint32_t interval)
 	return interval;
 }
 
-/*! \brief Get SOA from zone. */
-static const knot_rdataset_t *zone_soa(zone_t *zone)
-{
-	assert(zone);
-
-	if (zone->contents == NULL) {
-		return NULL;
-	}
-
-	return node_rdataset(zone->contents->apex, KNOT_RRTYPE_SOA);
-}
-
 /*! \brief Schedule expire event, unless it is already scheduled. */
 static void start_expire_timer(conf_t *conf, zone_t *zone, const knot_rdataset_t *soa)
 {

src/knot/zone/zone.c

@@ -226,6 +226,15 @@ void zone_clear_preferred_master(zone_t *zone)
 	pthread_mutex_unlock(&zone->preferred_lock);
 }
 
+const knot_rdataset_t *zone_soa(const zone_t *zone)
+{
+	if (!zone || zone_contents_is_empty(zone->contents)) {
+		return NULL;
+	}
+
+	return node_rdataset(zone->contents->apex, KNOT_RRTYPE_SOA);
+}
+
 /*!
  * \brief Get preferred zone master while checking its existence.
  */

src/knot/zone/zone.h

@@ -140,6 +140,9 @@ void zone_set_preferred_master(zone_t *zone, const struct sockaddr_storage *addr
 /*! \brief Clears the current preferred master address. */
 void zone_clear_preferred_master(zone_t *zone);
 
+/*! \brief Get zone SOA RR. */
+const knot_rdataset_t *zone_soa(const zone_t *zone);
+
 typedef int (*zone_master_cb)(conf_t *conf, zone_t *zone, const conf_remote_t *remote,
                               void *data);