fix: Idempotency wrapper for callers

Example for a caller assuming idempotency: uhcp (-> `gnrc_netif_ipv6_add_prefix` -> `NETOPT_IPV6_IID`)

sys/net/gnrc/netif/gnrc_netif.c

@@ -280,7 +280,7 @@ int gnrc_netif_get_from_netdev(gnrc_netif_t *netif, gnrc_netapi_opt_t *opt)
                     assert(opt->data_len == sizeof(netopt_ipv6_rfc7217_iid_data));
                     netopt_ipv6_rfc7217_iid_data *data =
                             (netopt_ipv6_rfc7217_iid_data *) opt->data;
-                    res = ipv6_get_rfc7217_iid(
+                    res = ipv6_get_rfc7217_iid_idempotent(
                             data->iid, netif, data->pfx, data->dad_ctr);
                     break;
 #endif

sys/net/gnrc/network_layer/ipv6/nib/_nib-slaac.c

@@ -216,6 +216,30 @@ int ipv6_get_rfc7217_iid(eui64_t *iid, gnrc_netif_t *netif, const ipv6_addr_t *p
 
     return 0;
 }
+
+inline int ipv6_get_rfc7217_iid_idempotent(eui64_t *iid, gnrc_netif_t *netif, const ipv6_addr_t *pfx,
+                                    uint8_t *dad_ctr) {
+    int idx;
+    if ((idx = gnrc_netif_ipv6_addr_pfx_idx(netif, pfx, SLAAC_PREFIX_LENGTH)) >= 0) {
+        /* if the prefix is already known,
+         * do not cause generation of a potentially different
+         * stable privacy address (keyword DAD_Counter).
+         * refer to https://datatracker.ietf.org/doc/html/rfc4862#section-5.5.3 d)
+         * */
+        DEBUG("nib: Not calling IDGEN, prefix already known.\n");
+
+        //write out params
+        //- dad_ctr
+        *dad_ctr = gnrc_netif_ipv6_addr_gen_retries(netif, idx);
+        //- iid
+        ipv6_addr_t *addr = &netif->ipv6.addrs[idx];
+        memcpy(iid, &addr->u64[1], sizeof(*iid));
+
+        return 1;
+    }
+
+    return ipv6_get_rfc7217_iid(iid, netif, pfx,dad_ctr);
+}
 #endif
 
 #if IS_ACTIVE(CONFIG_GNRC_IPV6_NIB_SLAAC)

sys/net/gnrc/network_layer/ipv6/nib/_nib-slaac.h

@@ -149,6 +149,13 @@ bool _stable_privacy_should_retry_idgen(uint8_t *dad_ctr, const char *reason);
  */
 int ipv6_get_rfc7217_iid(eui64_t *iid, gnrc_netif_t *netif, const ipv6_addr_t *pfx,
                          uint8_t *dad_ctr);
+
+/**
+ * @brief @ref ipv6_get_rfc7217_iid for those callers which assume idempotency.
+ * @return 1 if ignored for idempotency, else @ref ipv6_get_rfc7217_iid
+ */
+int ipv6_get_rfc7217_iid_idempotent(eui64_t *iid, gnrc_netif_t *netif, const ipv6_addr_t *pfx,
+                                    uint8_t *dad_ctr);
 #endif
 
 #if IS_ACTIVE(CONFIG_GNRC_IPV6_NIB_SLAAC) || defined(DOXYGEN)