KRT: Fix route learn scan when route changed

When a kernel route changed, function krt_learn_scan() noticed that and
replaced the route in internal kernel FIB, but after that, function
krt_learn_prune() failed to propagate the new route to the nest, because
it confused the new route with the (removed) old best route and decided
that the best route did not changed.

Wow, the original code (and the bug) is almost 17 years old.

nest/route.h

@@ -223,8 +223,8 @@ typedef struct rte {
     struct {				/* Routes generated by krt sync (both temporary and inherited ones) */
       s8 src;				/* Alleged route source (see krt.h) */
       u8 proto;				/* Kernel source protocol ID */
-      u8 type;				/* Kernel route type */
       u8 seen;				/* Seen during last scan */
+      u8 best;				/* Best route in network, propagated to core */
       u32 metric;			/* Kernel metric */
     } krt;
   } u;

sysdep/bsd/krt-sock.c

@@ -493,9 +493,8 @@ krt_read_route(struct ks_msg *msg, struct krt_proto *p, int scan)
   e->net = net;
   e->u.krt.src = src;
   e->u.krt.proto = src2;
-
+  e->u.krt.seen = 0;
-  /* These are probably too Linux-specific */
+  e->u.krt.best = 0;
-  e->u.krt.type = 0;
   e->u.krt.metric = 0;
 
   if (scan)

sysdep/linux/netlink.c

@@ -1102,7 +1102,8 @@ nl_parse_route(struct nlmsghdr *h, int scan)
   e->net = net;
   e->u.krt.src = src;
   e->u.krt.proto = i->rtm_protocol;
-  e->u.krt.type = i->rtm_type;
+  e->u.krt.seen = 0;
+  e->u.krt.best = 0;
   e->u.krt.metric = 0;
 
   if (a[RTA_PRIORITY])

sysdep/unix/krt.c

@@ -417,46 +417,58 @@ again:
       net *n = (net *) f;
       rte *e, **ee, *best, **pbest, *old_best;
 
-      old_best = n->routes;
+      /*
+       * Note that old_best may be NULL even if there was an old best route in
+       * the previous step, because it might be replaced in krt_learn_scan().
+       * But in that case there is a new valid best route.
+       */
+
+      old_best = NULL;
       best = NULL;
       pbest = NULL;
       ee = &n->routes;
       while (e = *ee)
 	{
+	  if (e->u.krt.best)
+	    old_best = e;
+
 	  if (!e->u.krt.seen)
 	    {
 	      *ee = e->next;
 	      rte_free(e);
 	      continue;
 	    }
+
 	  if (!best || best->u.krt.metric > e->u.krt.metric)
 	    {
 	      best = e;
 	      pbest = ee;
 	    }
+
 	  e->u.krt.seen = 0;
+	  e->u.krt.best = 0;
 	  ee = &e->next;
 	}
       if (!n->routes)
 	{
 	  DBG("%I/%d: deleting\n", n->n.prefix, n->n.pxlen);
 	  if (old_best)
-	    {
+	    krt_learn_announce_delete(p, n);
-	      krt_learn_announce_delete(p, n);
+
-	      n->n.flags &= ~KRF_INSTALLED;
-	    }
 	  FIB_ITERATE_PUT(&fit, f);
 	  fib_delete(fib, f);
 	  goto again;
 	}
+
+      best->u.krt.best = 1;
       *pbest = best->next;
       best->next = n->routes;
       n->routes = best;
-      if (best != old_best || !(n->n.flags & KRF_INSTALLED) || p->reload)
+
+      if ((best != old_best) || p->reload)
 	{
 	  DBG("%I/%d: announcing (metric=%d)\n", n->n.prefix, n->n.pxlen, best->u.krt.metric);
 	  krt_learn_announce_update(p, best);
-	  n->n.flags |= KRF_INSTALLED;
 	}
       else
 	DBG("%I/%d: uptodate (metric=%d)\n", n->n.prefix, n->n.pxlen, best->u.krt.metric);
@@ -515,31 +527,31 @@ krt_learn_async(struct krt_proto *p, rte *e, int new)
   best = n->routes;
   bestp = &n->routes;
   for(gg=&n->routes; g=*gg; gg=&g->next)
+  {
     if (best->u.krt.metric > g->u.krt.metric)
       {
 	best = g;
 	bestp = gg;
       }
+
+    g->u.krt.best = 0;
+  }
+
   if (best)
     {
+      best->u.krt.best = 1;
       *bestp = best->next;
       best->next = n->routes;
       n->routes = best;
     }
+
   if (best != old_best)
     {
       DBG("krt_learn_async: distributing change\n");
       if (best)
-	{
+	krt_learn_announce_update(p, best);
-	  krt_learn_announce_update(p, best);
-	  n->n.flags |= KRF_INSTALLED;
-	}
       else
-	{
+	krt_learn_announce_delete(p, n);
-	  n->routes = NULL;
-	  krt_learn_announce_delete(p, n);
-	  n->n.flags &= ~KRF_INSTALLED;
-	}
     }
 }
 
@@ -564,7 +576,7 @@ krt_dump(struct proto *P)
 static void
 krt_dump_attrs(rte *e)
 {
-  debug(" [m=%d,p=%d,t=%d]", e->u.krt.metric, e->u.krt.proto, e->u.krt.type);
+  debug(" [m=%d,p=%d]", e->u.krt.metric, e->u.krt.proto);
 }
 
 #endif