Re: Garbage collection of userdata inside userdata

On Wednesday, 15 July 2026 00:27:17 EEST 'Stefano Cossu' via lua-l wrote:
> Hello,
> 
> I have a Lua C module defining two different userdata types, one is
> called a Dataset and the other a TermSet. Both can be used and
> garbage-collected independently. A Dataset object has a TermSet as a
> "gr" attribute. I am setting this term set as a uservalue for the dataset.
> 
> The problem is that if I access the term set from the dataset, it gets
> garbage collected when it falls out of scope, and then, when the dataset
> is garbage collected too, it calls a free function that attempts to free
> the term set that was already freed, resulting in a double-free error.
> 
> This is the function used for creating the object:
> 
> // Called by dataset factories
> int dataset_to_udata (lua_State *L, VOLK_Store *store)
> {
>      VOLK_Dataset **dsp = lua_newuserdatauv (L, sizeof (*dsp), 2);
>      luaL_getmetatable (L, "volksdata.Dataset");
>      lua_setmetatable (L, -2);
>      VOLK_Dataset *ds = VOLK_dataset_new (store);
>      LUA_NLCHECK (ds, "Error creating dataset.");
>      *dsp = ds;
> 
>      // Set store uservalue.
>      lua_pushvalue(L, 1);
>      lua_setiuservalue (L, -2, 1);
> 
>      // Set gr uservalue.
>      VOLK_TermSet **ts_p = lua_newuserdata (L, sizeof (*ts_p));
>      LUA_NLCHECK (ts_p, "Error allocating memory for term set.");
>      luaL_getmetatable (L, "volksdata.TermSet");
>      lua_setmetatable (L, -2);
>      lua_setiuservalue (L, -2, 2);
> 
>      return 1;
> }
> 
> And the access function:
> 
> static int
> get_graphs (lua_State *L)
> {
>      (void) check_dataset (L, 1);
>      lua_getiuservalue (L, 1, 2);
> 
>      return 1;
> }
> 
> I thought that setting the uservalue would prevent the term set from
> being garbage collected when it falls out of scope, but I guess that it
> still gets garbage collected when the dataset is destroyed.
> 
> Which approach shall I use? I guess I'd want to prevent the independent
> term set from ever being garbage collected. I can't use a light userdata
> pointer because the access function must return a full usable Lua
> object. I also shouldn't duplicate the term set because it's expensive
> and the two copies could fall out of sync, which is undesirable.
> 
> Thanks for any suggestions.
> s

If your objects are GC-managed, conceptually, you should *never* free them 
manually. Instead, what I'd do is have the term set in one uservalue slot in 
the dataset userdata. This way, dataset will keep its term set alive until it 
itself falls out of scope *by design*.

Furthermore, do all deallocation stuff only in the `__gc` metamethod of the 
uservalues, and *never* deallocate one GC'd object's data from aonther's 
`__gc` metamethod.

P.S.: Since lua 5.3-ish (afaik), userdata can have multiple uservalue slots, 
use that to your advantage.



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