nd6: centralize link-local packet send decision

Previously, ethip6 and lowpan6 each had their own copy of code that
used internal nd6 data structures to decide whether to send a packet
on the local link right away, or queue it while nd6 performed local
address resolution.  This patch moves that code into nd6, thereby
eliminating all remaining cases of external access to internal nd6
data structures, as well as the need to expose two specific nd6
functions.

As a side effect, the patch effectively fixes two bugs in the lowpan6
code that were already fixed in the ethip6 code.

src/core/ipv6/nd6.c

@@ -98,6 +98,8 @@ static s8_t nd6_get_router(const ip6_addr_t *router_addr, struct netif *netif);
 static s8_t nd6_new_router(const ip6_addr_t *router_addr, struct netif *netif);
 static s8_t nd6_get_onlink_prefix(ip6_addr_t *prefix, struct netif *netif);
 static s8_t nd6_new_onlink_prefix(ip6_addr_t *prefix, struct netif *netif);
+static s8_t nd6_get_next_hop_entry(const ip6_addr_t *ip6addr, struct netif *netif);
+static err_t nd6_queue_packet(s8_t neighbor_index, struct pbuf *q);
 
 #define ND6_SEND_FLAG_MULTICAST_DEST 0x01
 #define ND6_SEND_FLAG_ALLNODES_DEST 0x02
@@ -1562,7 +1564,7 @@ nd6_new_onlink_prefix(ip6_addr_t *prefix, struct netif *netif)
  *         suitable next hop was found, ERR_MEM if no cache entry
  *         could be created
  */
-s8_t
+static s8_t
 nd6_get_next_hop_entry(const ip6_addr_t *ip6addr, struct netif *netif)
 {
 #ifdef LWIP_HOOK_ND6_GET_GW
@@ -1684,7 +1686,7 @@ nd6_get_next_hop_entry(const ip6_addr_t *ip6addr, struct netif *netif)
  * @param q packet to be queued
  * @return ERR_OK if succeeded, ERR_MEM if out of memory
  */
-err_t
+static err_t
 nd6_queue_packet(s8_t neighbor_index, struct pbuf *q)
 {
   err_t result = ERR_MEM;
@@ -1861,6 +1863,60 @@ nd6_send_q(s8_t i)
 #endif /* LWIP_ND6_QUEUEING */
 }
 
+/**
+ * A packet is to be transmitted to a specific IPv6 destination on a specific
+ * interface. Check if we can find the hardware address of the next hop to use
+ * for the packet. If so, give the hardware address to the caller, which should
+ * use it to send the packet right away. Otherwise, enqueue the packet for
+ * later transmission while looking up the hardware address, if possible.
+ *
+ * As such, this function returns one of three different possible results:
+ *
+ * - ERR_OK with a non-NULL 'hwaddrp': the caller should send the packet now.
+ * - ERR_OK with a NULL 'hwaddrp': the packet has been enqueued for later.
+ * - not ERR_OK: something went wrong; forward the error upward in the stack.
+ *
+ * @param netif The lwIP network interface on which the IP packet will be sent.
+ * @param q The pbuf(s) containing the IP packet to be sent.
+ * @param ip6addr The destination IPv6 address of the packet.
+ * @param hwaddrp On success, filled with a pointer to a HW address or NULL.
+ * @return
+ * - ERR_OK on success, ERR_RTE if no route was found for the packet,
+ * or ERR_MEM if low memory conditions prohibit sending the packet at all.
+ */
+err_t
+nd6_packet_send_check(struct netif *netif, struct pbuf *q, const ip6_addr_t *ip6addr, const u8_t **hwaddrp)
+{
+  s8_t i;
+
+  /* Get next hop record. */
+  i = nd6_get_next_hop_entry(ip6addr, netif);
+  if (i < 0) {
+    /* failed to get a next hop neighbor record. */
+    return i;
+  }
+
+  /* Now that we have a destination record, send or queue the packet. */
+  if (neighbor_cache[i].state == ND6_STALE) {
+    /* Switch to delay state. */
+    neighbor_cache[i].state = ND6_DELAY;
+    neighbor_cache[i].counter.delay_time = LWIP_ND6_DELAY_FIRST_PROBE_TIME / ND6_TMR_INTERVAL;
+  }
+  /* @todo should we send or queue if PROBE? send for now, to let unicast NS pass. */
+  if ((neighbor_cache[i].state == ND6_REACHABLE) ||
+      (neighbor_cache[i].state == ND6_DELAY) ||
+      (neighbor_cache[i].state == ND6_PROBE)) {
+
+    /* Tell the caller to send out the packet now. */
+    *hwaddrp = neighbor_cache[i].lladdr;
+    return ERR_OK;
+  }
+
+  /* We should queue packet on this interface. */
+  *hwaddrp = NULL;
+  return nd6_queue_packet(i, q);
+}
+
 
 /**
  * Get the Path MTU for a destination.