Lua userdata store their destructor in their metatable;

src/api/l_math.c

@@ -295,7 +295,7 @@ int luaopen_lovr_math(lua_State* L) {
   luax_registertype(L, RandomGenerator);
 
   for (int i = 0; i < MAX_MATH_TYPES; i++) {
-    _luax_registertype(L, lovrMathTypeNames[i], lovrMathTypes[i]);
+    _luax_registertype(L, lovrMathTypeNames[i], lovrMathTypes[i], NULL);
     luaL_getmetatable(L, lovrMathTypeNames[i]);
 
     // Remove usual __gc handler

src/core/luax.c

@@ -13,7 +13,14 @@ static int luax_meta__tostring(lua_State* L) {
 
 static int luax_meta__gc(lua_State* L) {
   Proxy* p = lua_touserdata(L, 1);
-  lovrGenericRelease(p->object);
+  if (p) {
+    lua_getmetatable(L, 1);
+    lua_getfield(L, -1, "__destructor");
+    lovrDestructor* destructor = (lovrDestructor*) lua_tocfunction(L, -1);
+    if (destructor) {
+      _lovrRelease(destructor, p->object);
+    }
+  }
   return 0;
 }
 
@@ -101,7 +108,7 @@ void luax_registerloader(lua_State* L, lua_CFunction loader, int index) {
   lua_pop(L, 1);
 }
 
-void _luax_registertype(lua_State* L, const char* name, const luaL_Reg* functions) {
+void _luax_registertype(lua_State* L, const char* name, const luaL_Reg* functions, lovrDestructor* destructor) {
 
   // Push metatable
   luaL_newmetatable(L, name);
@@ -115,6 +122,10 @@ void _luax_registertype(lua_State* L, const char* name, const luaL_Reg* function
   lua_pushcfunction(L, luax_meta__gc);
   lua_setfield(L, -2, "__gc");
 
+  // m.__destructor = destructor (used to release reference)
+  lua_pushcfunction(L, (lua_CFunction) destructor);
+  lua_setfield(L, -2, "__destructor");
+
   // m.name = name
   lua_pushstring(L, name);
   lua_setfield(L, -2, "name");
@@ -132,8 +143,8 @@ void _luax_registertype(lua_State* L, const char* name, const luaL_Reg* function
   lua_pop(L, 1);
 }
 
-void _luax_extendtype(lua_State* L, const char* name, const luaL_Reg* baseFunctions, const luaL_Reg* functions) {
-  _luax_registertype(L, name, functions);
+void _luax_extendtype(lua_State* L, const char* name, const luaL_Reg* baseFunctions, const luaL_Reg* functions, lovrDestructor* destructor) {
+  _luax_registertype(L, name, functions, destructor);
   luaL_getmetatable(L, name);
   luaL_register(L, NULL, baseFunctions);
   lua_pop(L, 1);

src/core/luax.h

@@ -17,8 +17,8 @@ typedef struct {
 #endif
 
 #define luax_len(L, i) (int) lua_objlen(L, i)
-#define luax_registertype(L, T) _luax_registertype(L, #T, lovr ## T)
-#define luax_extendtype(L, S, T) _luax_extendtype(L, #T, lovr ## S, lovr ## T)
+#define luax_registertype(L, T) _luax_registertype(L, #T, lovr ## T, lovr ## T ## Destroy)
+#define luax_extendtype(L, S, T) _luax_extendtype(L, #T, lovr ## S, lovr ## T, lovr ## T ## Destroy)
 #define luax_totype(L, i, T) ((T*) _luax_totype(L, i, T_ ## T))
 #define luax_checktype(L, i, T) ((T*) _luax_checktype(L, i, T_ ## T, #T))
 #define luax_checkfloat(L, i) (float) luaL_checknumber(L, i)
@@ -32,8 +32,8 @@ int luax_print(lua_State* L);
 void luax_setmainthread(lua_State* L);
 void luax_atexit(lua_State* L, luax_destructor destructor);
 void luax_registerloader(lua_State* L, lua_CFunction loader, int index);
-void _luax_registertype(lua_State* L, const char* name, const luaL_Reg* functions);
-void _luax_extendtype(lua_State* L, const char* name, const luaL_Reg* baseFunctions, const luaL_Reg* functions);
+void _luax_registertype(lua_State* L, const char* name, const luaL_Reg* functions, lovrDestructor* destructor);
+void _luax_extendtype(lua_State* L, const char* name, const luaL_Reg* baseFunctions, const luaL_Reg* functions, lovrDestructor* destructor);
 void* _luax_totype(lua_State* L, int index, Type type);
 void* _luax_checktype(lua_State* L, int index, Type type, const char* debug);
 void luax_pushobject(lua_State* L, void* object);

src/core/types.h

@@ -86,3 +86,4 @@ void* _lovrAlloc(size_t size, Type type);
 #define lovrRetain(o) if (o && refcount_increment(_ref(o)->count) >= 0xff) lovrThrow("Ref count overflow")
 #define lovrRelease(T, o) if (o && refcount_decrement(_ref(o)->count) == 0) lovr ## T ## Destroy(o), free(_ref(o))
 #define lovrGenericRelease(o) if (o && refcount_decrement(_ref(o)->count) == 0) lovrTypeInfo[_ref(o)->type].destructor(o), free(_ref(o))
+#define _lovrRelease(f, o) if (o && refcount_decrement(_ref(o)->count) == 0) f(o), free(_ref(o))