Re: Weak tables and userdata finalization

[Mark Hamburg <mark@...>]

On Sep 2, 2011, at 8:44 AM, Mark Hamburg wrote:

> I believe I just worked through a bug in my project caused by weak caches together with userdata finalization, but I wanted to confirm that my analysis was correct and not just the result of sleep deprivation.
> 
> * Lua 5.1.4
> * Two userdatas each with __gc metamethods. Let's call them UD1 and UD2.
> * We also have a table T.
> * T references UD2 -- e.g., T.x == UD2
> * T is accessible in a weak cache C (getmetatable( C ).__mode == 'kv') and C[ 'key' ] == T
> * UD1 has an environment that also references T -- e.g., getfenv( UD1 ).y == T
> 
> So, here is what I think is happening to me that I want to confirm is possible.
> 
> * The GC finds that neither UD1 nor UD2 (nor T) are accessible via strong links, so it queues UD1 and UD2 for finalization.
> * As part of queueing UD1 for finalization, it marks its environment so that that remains available for the __gc metamethod and this in turn marks T.
> * T then survives in the cache.
> * We fetch T out of the cache.
> * This doesn't stop UD2 from being finalized which then makes us unhappy when we go to use use UD2 via T.x.
> 
> Except that writing this down, it doesn't jive with my recollection of what the GC does. In particular, I would have expected the reference to T in C to have been cleared even if T is kept alive by the finalization logic.
> 
> The symptom, however, is clearly that we go to use UD2 and it has already been finalized. UD1, on the other hand, was just a theory.
> 
> Any other insights into how we could end up running through weak caches and reaching an object that has been or will be finalized? I know about the dangers with keys on weak tables since those are identified as potentially being needed by the __gc metamethod, but I am not iterating weak tables anywhere in a way that would discover such finalized keys.

Maybe I just confirmed my original theory and demonstrated that my recollection was wrong:

  udsize = luaC_separateudata(L, 0);  /* separate userdata to be finalized */
  marktmu(g);  /* mark `preserved' userdata */
  udsize += propagateall(g);  /* remark, to propagate `preserveness' */
  cleartable(g->weak);  /* remove collected objects from weak tables */

Basic lesson: Weak tables plus finalization are a dangerous mix?

Mark