Re: Fix misaligned access of ItemPointerData on ARM

On 2015-05-21 15:34:00 -0400, Tom Lane wrote:
> Piotr Stefaniak <postgres(at)piotr-stefaniak(dot)me> writes:
> > But due to how ExecRowMark struct is laid out in memory, the packed
> > struct ItemPointerData begins at an uneven offset, leading to misaligned
> > access whenever BlockIdData is set by ItemPointerSetInvalid() (and
> > likely in some other places, too).
>
> But BlockIdData is laid out and accessed as two 16-bit fields, so there
> should be no problem. On what platform exactly do you see a failure?

It's probably aligned on a byte boundary:

typedef struct ExecRowMark
{
Relation relation; /* opened and suitably locked relation */
Index rti; /* its range table index */
Index prti; /* parent range table index, if child */
Index rowmarkId; /* unique identifier for resjunk columns */
RowMarkType markType; /* see enum in nodes/plannodes.h */
bool noWait; /* NOWAIT option */
ItemPointerData curCtid; /* ctid of currently locked tuple, if any */
} ExecRowMark;

due to the packedness curCtid will quite possibly be stored without any
padding after after noWait.