Re: WAL segments removed from primary despite the fact that logical replication slot needs it.

On Wed, Nov 16, 2022 at 12:08 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Nov 16, 2022 at 11:44 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > > > > Found one in the time frame you mentioned:
> > > > > 2022-11-10 21:03:24.612
UTC,"upgrayedd","canvas",21748,"10.1.238.101:35640",636d671b.54f4,39,"idle",2022-11-1021:03:23
UTC,7/0,0,DEBUG,00000,"failedto increase restart lsn: proposed 1039D/8B5773D8, after 1039D/9170B010, current candidate
1039D/83825958,current after 1039D/8B5773D8, flushed up to 1039D/91F41B50",,,,,,,,,"focal14" 
> > > > >
> > > >
> > > > Thanks!
> > > >
> > > > LSN 1039D/8B5773D8 seems to be related to this issue. If we advance
> > > > slot's restart_lsn to this LSN, we remove WAL files older than
> > > > 000000000001039D0000008A.
> > > >
> > > > In LogicalIncreaseRestartDecodingForSlot(), since
> > > > "current_lsn(1039D/9170B010) <
> > > > slot->data.confirmed_flush(1039D/91F41B50)", we executed the following
> > > > part and called LogicalConfirmReceivedLocation():
> > > >
> > > >     else if (current_lsn <= slot->data.confirmed_flush)
> > > >     {
> > > >         slot->candidate_restart_valid = current_lsn;
> > > >         slot->candidate_restart_lsn = restart_lsn;
> > > >
> > > >         /* our candidate can directly be used */
> > > >         updated_lsn = true;
> > > >     }
> > > >
> > >
> > > If this would have been executed in
> > > LogicalIncreaseRestartDecodingForSlot(), then the values displayed in
> > > the above DEBUG messages "current candidate 1039D/83825958, current
> > > after 1039D/8B5773D8" should be the same as proposed and after
> > > "proposed 1039D/8B5773D8, after 1039D/9170B010". Am, I missing
> > > something?
> >
> > Oh, you're right.
> >
> > Given restart_lsn was 1039D/8B5773D8, slot->data.restart_lsn was equal
> > to or greater than 1039D/8B5773D8 at that time but
> > slot->candidate_restart_lsn was 1039D/83825958, right? Which is weird.
> >
>
> Yes, that is weird but it had been a bit obvious if the same LOG would
> have printed slot->data.restart_lsn. This means that somehow slot's
> 'candidate_restart_lsn' somehow went behind its 'restart_lsn'. I can't
> figure out yet how that can happen but if that happens then the slot's
> restart_lsn can retreat in LogicalConfirmReceivedLocation() because we
> don't check if slot's candidate_restart_lsn is lesser than its
> restart_lsn before assigning the same in line
> MyReplicationSlot->data.restart_lsn =
> MyReplicationSlot->candidate_restart_lsn;. I think that should be
> checked. Note that we call LogicalConfirmReceivedLocation() can be
> called from ProcessStandbyReplyMessage(), so once the wrong
> candidate_restart_lsn is set, it can be assigned to restart_lsn from
> other code paths as well.
>
> I am not able to think how 'candidate_restart_lsn' can be set to an
> LSN value prior to 'restart_lsn'.
>

In the below part of the code, we use the value of
'last_serialized_snapshot' for restart_lsn.
else if (txn == NULL &&
builder->reorder->current_restart_decoding_lsn != InvalidXLogRecPtr &&
builder->last_serialized_snapshot != InvalidXLogRecPtr)
LogicalIncreaseRestartDecodingForSlot(lsn,
  builder->last_serialized_snapshot);

Now, say, after restart, we start reading from slot's restart_lsn
which is 1039D/8B5773D8. At this LSN, we restored a snapshot that has
the last_seriealized_snapshot set to 1039D/83825958. If that happens,
then in LogicalIncreaseRestartDecodingForSlot, we can set these values
to slot's candidate_*_lsn variables. Say, if this happens, next time
whenever LogicalConfirmReceivedLocation() is called the value of
slot's restart_lsn will be moved back. Once it is moved back, yet
another restart will lead to this problem. Does this theory makes
sense?


--
With Regards,
Amit Kapila.