dnsproxy: Delay cache removal

The idea here is to delay cache removal few seconds if there
are no cache users any more (refcount goes to 0). This is useful
for IPv6 RDNSS where new DNS servers are created right after old
one is removed. In this case we do not want to loose the cache
that still has perfectly valid data.

1 files changed, 22 insertions, 4 deletions

diff --git a/src/dnsproxy.c b/src/dnsproxy.c
index 4033f2ab..5006739f 100644
--- a/src/dnsproxy.c
+++ b/src/dnsproxy.c
@@ -1627,6 +1627,18 @@ static void cache_element_destroy(gpointer value)
 		cache_size = 0;
 }
 
+static gboolean try_remove_cache(gpointer user_data)
+{
+	if (__sync_fetch_and_sub(&cache_refcount, 1) == 1) {
+		DBG("No cache users, removing it.");
+
+		g_hash_table_destroy(cache);
+		cache = NULL;
+	}
+
+	return FALSE;
+}
+
 static void destroy_server(struct server_data *server)
 {
 	GList *list;
@@ -1656,10 +1668,16 @@ static void destroy_server(struct server_data *server)
 	}
 	g_free(server->interface);
 
-	if (__sync_fetch_and_sub(&cache_refcount, 1) == 1) {
-		g_hash_table_destroy(cache);
-		cache = NULL;
-	}
+	/*
+	 * We do not remove cache right away but delay it few seconds.
+	 * The idea is that when IPv6 DNS server is added via RDNSS, it has a
+	 * lifetime. When the lifetime expires we decrease the refcount so it
+	 * is possible that the cache is then removed. Because a new DNS server
+	 * is usually created almost immediately we would then loose the cache
+	 * without any good reason. The small delay allows the new RDNSS to
+	 * create a new DNS server instance and the refcount does not go to 0.
+	 */
+	g_timeout_add_seconds(3, try_remove_cache, NULL);
 
 	g_free(server);
 }