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

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.

-- 
Robert Haas
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