clusterer: issue sync request under lock

(cherry picked from commit bf178b0)

modules/clusterer/api.h

@@ -179,6 +179,7 @@ typedef int (*register_capability_f)(str *cap, cl_packet_cb_f packet_cb,
  * Request to synchronize data for a given capability from another node.
  */
 typedef int (*request_sync_f)(str * capability, int cluster_id);
+
 /*
  * Returns a BIN packet in which to include a distinct "chunk" of data
  * (e.g. info about a single usrloc contact) to sync.

modules/clusterer/sync.c

@@ -96,6 +96,8 @@ int cl_request_sync(str *capability, int cluster_id)
 	LM_DBG("requesting %.*s sync in cluster %d\n",
 	       capability->len, capability->s, cluster_id);
 
+	lock_start_read(cl_list_lock);
+
 	cluster = get_cluster_by_id(cluster_id);
 	if (!cluster) {
 		LM_ERR("Unknown cluster [%d]\n", cluster_id);
@@ -151,6 +153,8 @@ int cl_request_sync(str *capability, int cluster_id)
 		}
 	}
 
+	lock_stop_read(cl_list_lock);
+
 	return 0;
 }