Re: Potential G2-item cycles under serializable isolation

Hi,

On 2020-06-08 18:26:01 -0700, Peter Geoghegan wrote:
> On Sun, Jun 7, 2020 at 7:54 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > The issue seems to be that heapam's mechanism for adding a "conflict
> > out" can get some details subtly wrong. This can happen in the event
> > of a concurrently updated tuple where the updating xact aborts. So for
> > each pair of transactions that committed in each G2-Item, there seemed
> > to be a third updating transaction that aborted, messing things up. I
> > think that the "conflict out" stuff ought to behave as if an updated
> > tuple was never updated when it's not visible to our xact anyway. We
> > should use the XID that created the original tuple for "conflict out"
> > purposes -- not the (possibly aborted) updater's XID (i.e. not the
> > HeapTupleHeaderGetUpdateXid() XID).
>
> The functionality in question (the code from the
> HeapCheckForSerializableConflictOut() case statement) was originally
> discussed here, with the details finalized less than a week before SSI
> was committed in 2011:
>
> https://www.postgresql.org/message-id/flat/1296499247.11513.777.camel%40jdavis#9e407424df5f8794360b6e84de60200a
>
> It hasn't really changed since that time.
>
> Kevin, Jeff: What do you think of the fix I have proposed? Attached is
> a revised version, with better comments and a proper commit message.
> Somebody should review this patch before I proceed with commit.

This should be testable with isolationtester, right?

Greetings,

Andres Freund