oz_(un)protect() API unsafe

[Denys Duchier]

Raphael Collet <raph at info.ucl.ac.be> writes:

> Denys, this is unclear to me, too.  Could you please explain the problem, and
> the proposed fix?

I am thinking in terms of a little itch I am scratching involving pthreads.  It
will usually be the case that tasks delegated to pthreads will need to hold on
to some Oz data structure, if only to have a way to return a result or indicate
termination etc...  They will therefore need to OZ_protect it on initiation and
OZ_unprotect it on termination.  If it turns out that 2 tasks need to hold on to
the same Oz data structure, the first one to terminate will OZ_unprotect the
data and that data may well be garbage collected before the second task
terminates.

The fix is simply to add a counter to each protected data: the counter is
incremented by oz_protect and decremented by oz_unprotect.  You would only really
unlink the data from the protected list if the counter reaches 0.

Cheers,

--Denys