[Patch] Fix another unbound alloca (BZ 13761)

[Roland McGrath]

> It seems to me that freeing dataset is just wrong if it's allocated via 
> mempool_alloc and things in mempool_alloc are GC'd.

Correct.

> ISTM the safe thing to do would be something like:
> 
>       if (he == NULL)
>          dataset = (struct dataset *) mempool_alloc (db, total + n, 1);
>       else if (! __libc_use_alloca (alloca_used + total + n))
>          dataset = (struct dataset *) malloc (...);

Agreed.  Note that in the existing code ALLOCA_USED is a Boolean, and it is
used to mean "cannot be placed in the cache".  So the bookkeeping would
change somewhat to support the malloc case (which is alloca_used=true in
the current sense of that variable, but also has to be freed).

> While it might be safe/appropriate to use mempools in the case where the 
> needed space is large and transient, it's hard to prove.

The purpose of the mempool stuff is to use the shared memory space that's
available to clients as a cache without interacting with nscd.  So it's
never appropriate for something that is not going to be looked up in that
cache later.


Thanks,
Roland