Re: logical decoding and replication of sequences, take 2

On Thu, Dec 14, 2023 at 2:45 PM Ashutosh Bapat
<ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> wrote:
>
> On Thu, Dec 14, 2023 at 12:37 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > I think you forgot to attach the patch.
>
> Sorry. Here it is.
>
> On Thu, Dec 14, 2023 at 2:36 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > >
> > It looks like the solution works. But this is the only place where we
> > process a change before SNAPSHOT reaches FULL. But this is also the
> > only record which affects a decision to queue/not a following change.
> > So it should be ok. The sequence_hash'es as separate for each
> > transaction and they are cleaned when processing COMMIT record.
> > >
> >
> > But it is possible that even commit or abort also happens before the
> > snapshot reaches full state in which case the hash table will have
> > stale or invalid (for aborts) entries. That will probably be cleaned
> > at a later point by running_xact records.
>
> Why would cleaning wait till running_xact records? Won't txn entry
> itself be removed when processing commit/abort record? At the same the
> sequence hash will be cleaned as well.
>
> > Now, I think in theory, it
> > is possible that the same RelFileLocator can again be allocated before
> > we clean up the existing entry which can probably confuse the system.
>
> How? The transaction allocating the first time would be cleaned before
> it happens the second time. So shouldn't matter.
>

It can only be cleaned if we process it but xact_decode won't allow us
to process it and I don't think it would be a good idea to add another
hack for sequences here. See below code:

xact_decode(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
{
SnapBuild *builder = ctx->snapshot_builder;
ReorderBuffer *reorder = ctx->reorder;
XLogReaderState *r = buf->record;
uint8 info = XLogRecGetInfo(r) & XLOG_XACT_OPMASK;

/*
* If the snapshot isn't yet fully built, we cannot decode anything, so
* bail out.
*/
if (SnapBuildCurrentState(builder) < SNAPBUILD_FULL_SNAPSHOT)
return;

--
With Regards,
Amit Kapila.