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

On 2017-02-28 20:29:36 -0800, Andres Freund wrote:
> On 2017-02-28 20:18:35 -0800, Andres Freund wrote:
> > - Andres, hoping the buildfarm turns greener
> 
> Oh well, that didn't work. Investigating.

The fix for that was fairly trivial, and the buildfarm has cooled down.

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?

Regards,

Andres