RE: Synchronizing slots from primary to standby

On Friday, December 1, 2023 12:51 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:

Hi,

>
> On Fri, Dec 1, 2023 at 9:40 AM Zhijie Hou (Fujitsu)
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > On Wednesday, November 29, 2023 5:12 PM Zhijie Hou (Fujitsu)
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > I was reviewing slotsync worker design and here
> > are few comments on 0002 patch:
>
> Thanks for reviewing the patch.
>
> >
> >
> > 3. In synchronize_one_slot, do we need to skip the slot sync and drop if the
> > local slot is a physical one ?
> >
>
> IMO, if a local slot exists which is a physical one, it will be a user
> created slot and in that case worker will error out on finding
> existing slot with same name. And the case where local slot is
> physical one but not user-created is not possible on standby (assuming
> we have correct check on primary disallowing setting 'failover'
> property for physical slot). Do you have some other scenario in mind,
> which I am missing here?

I was thinking about the race condition when it has confirmed that the slot is
not a user created one and enter "sync_state == SYNCSLOT_STATE_READY" branch,
but at this moment, if someone uses "DROP_REPLICATION_SLOT" to drop this slot and
recreate another one(e.g. a physical one), then the slotsync worker will
overwrite the fields of this physical slot. Although this affects user created
logical slots in similar cases as well.

And the same is true for slotsync_drop_initiated_slots() and
drop_obsolete_slots(), as we don't lock the slots in the list, if user tri to
drop and re-create old slot concurrently, then we could drop user created slot
here.

Best Regards,
Hou zj