[neighbour] Use discovery protocol field to identify incomplete neighbours

Use the discovery protocol pointer field (rather than the running
state of the discovery timer) to determine whether or not neighbour
discovery is ongoing, as a precursor to allowing the timer to be
(ab)used for adding deliberate latency to transmitted packets.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/neighbour.c

@@ -186,6 +186,9 @@ static void neighbour_discovered ( struct neighbour *neighbour,
 	/* Stop retransmission timer */
 	stop_timer ( &neighbour->timer );
 
+	/* Mark discovery as complete */
+	neighbour->discovery = NULL;
+
 	/* Transmit any packets in queue.  Take out a temporary
 	 * reference on the entry to prevent it from going out of
 	 * scope during the call to net_tx().
@@ -300,6 +303,7 @@ int neighbour_tx ( struct io_buffer *iobuf, struct net_device *netdev,
 		   struct neighbour_discovery *discovery,
 		   const void *net_source ) {
 	struct neighbour *neighbour;
+	int rc;
 
 	/* Find or create neighbour cache entry */
 	neighbour = neighbour_find ( netdev, net_protocol, net_dest );
@@ -310,19 +314,24 @@ int neighbour_tx ( struct io_buffer *iobuf, struct net_device *netdev,
 		neighbour_discover ( neighbour, discovery, net_source );
 	}
 
-	/* If a link-layer address is available then transmit
-	 * immediately, otherwise queue for later transmission.
+	/* If discovery is still in progress then queue for later
+	 * transmission.
 	 */
-	if ( neighbour_has_ll_dest ( neighbour ) ) {
-		return net_tx ( iobuf, netdev, net_protocol, neighbour->ll_dest,
-				netdev->ll_addr );
-	} else {
+	if ( neighbour->discovery ) {
 		DBGC2 ( neighbour, "NEIGHBOUR %s %s %s deferring packet\n",
 			netdev->name, net_protocol->name,
 			net_protocol->ntoa ( net_dest ) );
 		list_add_tail ( &iobuf->list, &neighbour->tx_queue );
 		return 0;
 	}
+
+	/* Otherwise, transmit immediately */
+	if ( ( rc = net_tx ( iobuf, netdev, net_protocol, neighbour->ll_dest,
+			     netdev->ll_addr ) ) != 0 ) {
+		return rc;
+	}
+
+	return 0;
 }
 
 /**