Re: fixing old_snapshot_threshold's time->xid mapping

On Sat, Apr 18, 2020 at 11:47 AM Thomas Munro <thomas.munro@gmail.com> wrote:
>
> On Fri, Apr 17, 2020 at 2:12 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> > What about a contrib function that lets you clobber
> > oldSnapshotControl->current_timestamp?  It looks like all times in
> > this system come ultimately from GetSnapshotCurrentTimestamp(), which
> > uses that variable to make sure that time never goes backwards.
>
> Here's a draft TAP test that uses that technique successfully, as a
> POC.  It should probably be extended to cover more cases, but I
> thought I'd check what people thought of the concept first before
> going further.  I didn't see a way to do overlapping transactions with
> PostgresNode.pm, so I invented one (please excuse the bad perl); am I
> missing something?  Maybe it'd be better to do 002 with an isolation
> test instead, but I suppose 001 can't be in an isolation test, since
> it needs to connect to multiple databases, and it seemed better to do
> them both the same way.  It's also not entirely clear to me that
> isolation tests can expect a database to be fresh and then mess with
> dangerous internal state, whereas TAP tests set up and tear down a
> cluster each time.
>
> I think I found another bug in MaintainOldSnapshotTimeMapping(): if
> you make time jump by more than old_snapshot_threshold in one go, then
> the map gets cleared and then no early pruning or snapshot-too-old
> errors happen.  That's why in 002_too_old.pl it currently advances
> time by 10 minutes twice, instead of 20 minutes once.  To be
> continued.

IMHO that doesn't seems to be a problem.  Because even if we jump more
than old_snapshot_threshold in one go we don't clean complete map
right.  The latest snapshot timestamp will become the headtimestamp.
So in TransactionIdLimitedForOldSnapshots if (current_ts -
old_snapshot_threshold) is still >= head_timestap then we can still do
early pruning.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com