lua: properly initialize random number generator

Formerly multiple instances could use the same seed,
which prevented the retry logic in Lua modules (e.g. prefill) from
retrying at different times.

AFAIK security impact is zero aside from potential thundering-herd
problem with many kresd instances.

NEWS

@@ -17,6 +17,7 @@ Bugfixes
 - lua resolve(): correctly include EDNS0 in the virtual packet (!963)
   Custom modules might have been confused by that.
 - do not leak bogus data into SERVFAIL answers (#396)
+- improve random Lua number generator initialization (!979)
 
 Incompatible changes
 --------------------

daemon/engine.c

@@ -500,6 +500,13 @@ static int init_state(struct engine *engine)
 	lua_setglobal(engine->L, "map");
 	lua_pushlightuserdata(engine->L, engine);
 	lua_setglobal(engine->L, "__engine");
+	/* Random number generator */
+	lua_getfield(engine->L, LUA_GLOBALSINDEX, "math");
+	lua_getfield(engine->L, -1, "randomseed");
+	lua_remove(engine->L, -2);
+	lua_Number seed = kr_rand_bytes(sizeof(lua_Number));
+	lua_pushnumber(engine->L, seed);
+	lua_call(engine->L, 1, 0);
 	return kr_ok();
 }
 

modules/prefill/prefill.lua

@@ -142,10 +142,6 @@ function forward_references.fill_cache()
 	restart_timer(rz_cur_interval)
 end
 
-function prefill.init()
-	math.randomseed(os.time())
-end
-
 function prefill.deinit()
 	stop_timer()
 end