Re: [HACKERS] PATCH: two slab-like memory allocators

On 2017-03-06 12:40:18 -0500, Robert Haas wrote:
> On Wed, Mar 1, 2017 at 5:55 PM, Andres Freund <andres@anarazel.de> wrote:
> > The issue was that on 32bit platforms the Datum returned by some
> > functions (int2int4_sum in this case) isn't actually a separately
> > allocated Datum, but rather just something embedded in a larger
> > struct.  That, combined with the following code:
> >         if (!peraggstate->resulttypeByVal && !*isnull &&
> >                 !MemoryContextContains(CurrentMemoryContext,
> >                                                            DatumGetPointer(*result)))
> > seems somewhat problematic to me.  MemoryContextContains() can give
> > false positives when used on memory that's not a distinctly allocated
> > chunk, and if so, we violate memory lifetime rules.  It's quite
> > unlikely, given the required bit patterns, but nonetheless it's making
> > me somewhat uncomfortable.
> >
> > Do others think this isn't an issue and we can just live with it?
> 
> I think it's 100% broken to call MemoryContextContains() on something
> that's not guaranteed to be a palloc'd chunk.

I agree, but to me it seems the only fix would be to just yank out the
whole optimization?

- Andres