Re: Error "initial slot snapshot too large" in create replication slot

At Tue, 13 Sep 2022 16:10:59 +0900 (JST), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in
> At Tue, 13 Sep 2022 12:08:18 +0530, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote in
> > On Tue, Sep 13, 2022 at 11:52 AM Kyotaro Horiguchi
> > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > > That function is called after the SnapBuild reaches
> > > SNAPBUILD_CONSISTENT state ,or SnapBuildInitialSnapshot() rejects
> > > other than that state. That is, IIUC the top-sub relationship of all
> > > the currently running transactions is fully known to reorder buffer.
> > > We need a comment about that.
> >
> > I don't think this assumption is true, any xid started after switching
> > to the SNAPBUILD_FULL_SNAPSHOT and before switching to the
> > SNAPBUILD_CONSISTENT, might still be in progress so we can not
> > identify whether they are subxact or not from reorder buffer.
>
> Yeah, I misunderstood that the relationship is recorded earlier
> (how?). Thus it is not reliable in the first place.
>
> I agree that the best way is oversized xip.
>
>
> By the way, I feel that "is >= than" is redundant or plain wrong..

By the way GetSnapshotData() does this:

> snapshot->subxip = (TransactionId *)
> malloc(GetMaxSnapshotSubxidCount() * sizeof(TransactionId));
...
> if (!snapshot->takenDuringRecovery)
...
> else
> {
> subcount = KnownAssignedXidsGetAndSetXmin(snapshot->subxip, &xmin,
> xmax);

It is possible that the subxip is overrun. We need to expand the array
somehow. Or assign the array of the size (GetMaxSnapshotXidCount() +
GetMaxSnapshotSubxidCount()) for takenDuringRecovery snapshots.

(I feel deja vu..)

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center