Re: Potential G2-item cycles under serializable isolation

On 6/11/20 6:51 PM, Peter Geoghegan wrote:
> On Wed, Jun 10, 2020 at 8:14 PM Kyle Kingsbury <aphyr@jepsen.io> wrote:
>> I wanted to let you know that I've put together a draft of a report on these
>> findings, and if you've got any comments you'd like to offer, I'd be happy to
>> hear them, either on-list or privately.
>> http://jepsen.io/analyses/postgresql-12.3?draft-token=Kets1Quayfs
> You should mention that a fix was committed this morning:
>
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=5940ffb221316ab73e6fdc780dfe9a07d4221ebb

Ah, fantastic!

> You may want to draw attention to the isolation test, since it's a
> perfect distillation of the bug. I'm certain that it cannot be
> simplified any further.

I mentioned it in the discussion, but now I can offer a direct link. Great work. :)

> Note that the test involves three transactions/sessions -- not two.
> Elle always complained about a pair of transactions that had a cycle,
> which were similar to the "foo" and "bar" sessions from the test. But
> Elle never said anything about a third transaction (that played the
> role of the "trouble" transaction from the test). Of course, this
> makes perfect sense -- the third/"trouble" transaction *should* be
> totally irrelevant. Note that the UPDATE has to come from a third
> session/transaction for things to go awry. And note that the order of
> each statement has to match the order from the isolation test. For
> example, no problems occur if you flip the order of "trouble_update"
> and "bar_select" -- because that would mean that the physical tuple
> does not get physically updated before being examined by "bar_select".

Ah, thank you. I didn't quite understand this from the second patch commit message.

> You should also specifically mention that the next upcoming minor
> release will be on August 13th, 2020:
>
> https://www.postgresql.org/developer/roadmap/
>
> Users that are concerned about this bug will be able to get a new
> minor release with the fix by that date, at the latest.

That's great news. Will do! :)

--Kyle