Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

On Wed, Oct 4, 2017 at 10:46 PM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> Wong, Yi Wen wrote:
>> My interpretation of README.HOT is the check is just to ensure the chain is continuous; in which case the condition should be:
>>
>> > if (TransactionIdIsValid(priorXmax) &&
>> > !TransactionIdEquals(priorXmax, HeapTupleHeaderGetRawXmin(htup)))
>> > break;
>>
>> So the difference is GetRawXmin vs GetXmin, because otherwise we get the FreezeId instead of the Xmin when the transaction happened
>
> I independently arrived at the same conclusion. Since I was trying with
> 9.3, the patch differs -- in the old version we must explicitely test
> for the FrozenTransactionId value, instead of using GetRawXmin.
> Attached is the patch I'm using, and my own oneliner test (pretty much
> the same I posted earlier) seems to survive dozens of iterations without
> showing any problem in REINDEX.

Confirmed, the problem goes away with this patch on 9.3.

> This patch is incomplete, since I think there are other places that need
> to be patched in the same way (EvalPlanQualFetch? heap_get_latest_tid?).
> Of course, for 9.4 and onwards we need to patch like you described.

I have just done a lookup of the source code, and here is an
exhaustive list of things in need of surgery:
- heap_hot_search_buffer
- heap_get_latest_tid
- heap_lock_updated_tuple_rec
- heap_prune_chain
- heap_get_root_tuples
- rewrite_heap_tuple
- EvalPlanQualFetch (twice)

> This bit in EvalPlanQualFetch caught my attention ... why is it saying
> xmin never changes? It does change with freezing.
>
> /*
> * If xmin isn't what we're expecting, the slot must have been
> * recycled and reused for an unrelated tuple. This implies that
> * the latest version of the row was deleted, so we need do
> * nothing. (Should be safe to examine xmin without getting
> * buffer's content lock, since xmin never changes in an existing
> * tuple.)
> */
> if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
> priorXmax))

Agreed. That's not good.
--
Michael