ike-sa: Optionally try to migrate to the best path on routing priority changes

When multihomed, a setup might prefer to dynamically stay on the cheapest
available path by using MOBIKE migrations. If the cheapest path goes away and
comes back, we currently stay on the more expensive path to reduce noise and
prevent potential migration issues. This is usually just fine for links not
generating real cost.

If we have more expensive links in the setup, it can be desirable to always
migrate to the cheapest link available. By setting charon.prefer_best_path,
charon tries to migrate to the path using the highest priority link, allowing
an external application to update routes to indirectly control MOBIKE behavior.
This option has no effect if MOBIKE is unavailable.

src/libcharon/sa/ike_sa.c

@@ -2442,6 +2442,25 @@ static bool is_current_path_valid(private_ike_sa_t *this)
 {
 	bool valid = FALSE;
 	host_t *src;
+
+	if (supports_extension(this, EXT_MOBIKE) &&
+		lib->settings->get_bool(lib->settings,
+								"%s.prefer_best_path", FALSE, lib->ns))
+	{
+		/* check if the current path is the best path; migrate otherwise */
+		src = charon->kernel->get_source_addr(charon->kernel, this->other_host,
+											  NULL);
+		if (src)
+		{
+			valid = src->ip_equals(src, this->my_host);
+			src->destroy(src);
+		}
+		if (!valid)
+		{
+			DBG1(DBG_IKE, "old path is not preferred anymore");
+		}
+		return valid;
+	}
 	src = charon->kernel->get_source_addr(charon->kernel, this->other_host,
 										  this->my_host);
 	if (src)
@@ -2452,6 +2471,10 @@ static bool is_current_path_valid(private_ike_sa_t *this)
 		}
 		src->destroy(src);
 	}
+	if (!valid)
+	{
+		DBG1(DBG_IKE, "old path is not available anymore, try to find another");
+	}
 	return valid;
 }
 
@@ -2478,7 +2501,6 @@ static bool is_any_path_valid(private_ike_sa_t *this)
 			break;
 	}
 
-	DBG1(DBG_IKE, "old path is not available anymore, try to find another");
 	enumerator = create_peer_address_enumerator(this);
 	while (enumerator->enumerate(enumerator, &addr))
 	{