Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

On Mon, Feb 5, 2018 at 1:45 PM, Peter Geoghegan <pg@bowt.ie> wrote:
>> So, I guess another option might be to call VALGRIND_MAKE_MEM_DEFINED
>> on the buffer.  "We know what we're doing, trust us!"
>>
>> In some ways, that seems better than inserting a suppression, because
>> it only affects the memory in the buffer.
>
> I think that that would also work, and would be simpler, but also
> slightly inferior to using the proposed suppression. If there is
> garbage in logtape.c buffers, we still generally don't want to do
> anything important on the basis of those values. We make one exception
> with the suppression, which is a pretty typical kind of exception to
> make -- don't worry if we write() poisoned bytes, since those are
> bound to be alignment related.
>
> OTOH, as I've said we are generally bound to write some kind of
> logtape.c garbage, which will almost certainly not be of the
> uninitialized memory variety. So, while I feel that the suppression is
> better, the advantage is likely microscopic.

Attached patch does it to the tail of the buffer, as Tom suggested on
the -committers thread.

Note that there is one other place in logtape.c that can write a
partial block like this: LogicalTapeRewindForRead(). I haven't
bothered to do anything there, since it cannot possibly be affected by
this issue for the same reason that serial sorts cannot be -- it's
code that is only used by a tuplesort that really needs to spill to
disk, and merge multiple runs (or for tapes that have already been
frozen, that are expected to never reallocate logtape.c buffers).

-- 
Peter Geoghegan