Re: snapshot too old issues, first around wraparound and then more.

On Fri, Apr 3, 2020 at 6:52 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Thu, Apr 2, 2020 at 5:17 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > Since this is a feature that can result in wrong query results (and
> > quite possibly crashes / data corruption), I don't think we can just
> > leave this unfixed. But given the amount of code / infrastructure
> > changes required to get this into a working feature, I don't see how we
> > can unleash those changes onto the stable branches.
>

As per my initial understanding, the changes required are (a) There
seem to be multiple places where TestForOldSnapshot is missing, (b)
TestForOldSnapshot itself need to be reviewed carefully to see if it
has problems, (c) Some of the members of OldSnapshotControlData like
head_timestamp and xid_by_minute are not maintained accurately, (d)
handling of wraparound for xids in the in-memory data-structure for
this feature is required, (e) test infrastructure is not good enough
to catch bugs or improve this feature.

Now, this sounds like a quite of work but OTOH, if we see most of the
critical changes required will be in only a few functions like
TransactionIdLimitedForOldSnapshots(),
MaintainOldSnapshotTimeMapping(), TestForOldSnapshot(). I don't deny
the possibility that we might need much more work or we need to come
up with quite a different design to address all these problems but
unless Kevin or someone else doesn't come up with a solution to
address all of these problems, we can't be sure of that.

> I don't think that the feature can be allowed to remain in anything
> like its current form. The current design is fundamentally unsound.
>

Agreed, but OTOH, not giving time to Kevin or others who might be
interested to support this work is also not fair. I think once
somebody comes up with patches for problems we can decide whether this
feature can be salvaged in back-branches or we need to disable it in a
hard-way. Now, if Kevin himself is not interested in fixing or nobody
shows up to help, then surely we can take the decision sooner but
giving time for a couple of weeks (or even till we are near for PG13
release) in this case doesn't seem like a bad idea.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com