Re: [Caml-list] Presumed bug in OCaml's garbage collector or in its Weak module...

[Josh Berdine <josh@berdine.net>]

On Fri, Jun 17 2016, Martin R. Neuhäußer wrote:

> Thanks for the good point! And yes, the weak sets use Weak.get_copy internally.

I have also run into this when writing C bindings for a library with ref-counted memory management, where the OCaml finalizers decremented the C counts.  Since the functions in the structure passed to Weak.Make can be passed shallow copies of possibly-dead values, these functions need to be prepared for the underlying C values to have already been finalized.  That's tough to deal with.  (I ended up just letting Z3 hash-cons it's own terms, and kept pointers into Z3 out of my own hash-consed terms.)

> However, the exact semantics of Weak.get_copy is still a bit unclear to me. It creates a „shallow copy“ of a value that might independently be garbage collected. If that shallow copy can survive the finalization of its source, that might cause the problem. Are there any restrictions enforced on values obtained by Weak.get_copy? Or could this even be misused to create arbitrary copies of custom blocks that propagate anywhere?

If the functions passed to Weak.Make store the shallow copy into some global state, it seems that they could live on without constraint.

Cheers, Josh