luzer: initialize FDP metatable only once

The FDP metatable is recreating each time when we initialize the FDP
object. It increases the GC consumption and makes the code less
JIT-friendly since traces are exited by the guard on the FDP methods
objects that checked the exact pointer of the `GCobj` which is new for
any new FDP provider.

This patch fixes that by initializing the metatable only once on the
luzer library loading.

Author: Buristan

CHANGELOG.md

@@ -28,3 +28,4 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Stack overflow due to recursive traceback calls.
 - Fix a crash due to incorrect `argv` building (#13).
 - Fix parsing command-line flags (#23).
+- Multiple initialization of the FDP metatable.

luzer/fuzzed_data_provider.cc

@@ -261,16 +261,13 @@ const luaL_Reg methods[] =
 	{ NULL, NULL }
 };
 
-int
-luaL_fuzzed_data_provider(lua_State *L)
+/*
+ * Create the metatable once on the luzer loading to be more GC and JIT
+ * friendly. `luaL_fuzzed_data_provider()` is called in the loop inside
+ * `LLVMFuzzerRunDriver()`.
+ */
+void fdp_metatable_init(lua_State *L)
 {
-	int index = lua_gettop(L);
-	if (index != 1)
-		luaL_error(L, "Usage: luzer.FuzzedDataProvider(string)");
-
-	const char *data = luaL_checkstring(L, 1);
-	size_t size = strlen(data);
-
 	luaL_newmetatable(L, FDP_LUA_UDATA_NAME);
 	lua_pushvalue(L, -1);
 	lua_setfield(L, -2, "__index");
@@ -279,6 +276,18 @@ luaL_fuzzed_data_provider(lua_State *L)
 #else
 	luaL_setfuncs(L, methods, 0);
 #endif
+	lua_pop(L, 1); /* Remove the metatable from the stack. */
+}
+
+int
+luaL_fuzzed_data_provider(lua_State *L)
+{
+	int index = lua_gettop(L);
+	if (index != 1)
+		luaL_error(L, "Usage: luzer.FuzzedDataProvider(string)");
+
+	const char *data = luaL_checkstring(L, 1);
+	size_t size = strlen(data);
 
 	lua_userdata_t *lfdp;
 	lfdp = (lua_userdata_t*)lua_newuserdata(L, sizeof(*lfdp));

luzer/fuzzed_data_provider.h

@@ -4,6 +4,7 @@
 #ifdef __cplusplus
 extern "C" {
 #endif
+	void fdp_metatable_init(lua_State *L);
 	int luaL_fuzzed_data_provider(lua_State *L);
 #ifdef __cplusplus
 } /* extern "C" */

luzer/luzer.c

@@ -462,5 +462,7 @@ int luaopen_luzer_impl(lua_State *L)
 	lua_pushstring(L, LUA_RELEASE);
 	lua_rawset(L, -3);
 
+	fdp_metatable_init(L);
+
 	return 1;
 }