Hi, On Thu, Jan 11, 2024 at 04:22:56PM +0530, Amit Kapila wrote: > On Tue, Jan 9, 2024 at 6:39 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > +static bool > > +synchronize_one_slot(WalReceiverConn *wrconn, RemoteSlot *remote_slot) > > { > > ... > > + /* Slot ready for sync, so sync it. */ > > + else > > + { > > + /* > > + * Sanity check: With hot_standby_feedback enabled and > > + * invalidations handled appropriately as above, this should never > > + * happen. > > + */ > > + if (remote_slot->restart_lsn < slot->data.restart_lsn) > > + elog(ERROR, > > + "cannot synchronize local slot \"%s\" LSN(%X/%X)" > > + " to remote slot's LSN(%X/%X) as synchronization" > > + " would move it backwards", remote_slot->name, > > + LSN_FORMAT_ARGS(slot->data.restart_lsn), > > + LSN_FORMAT_ARGS(remote_slot->restart_lsn)); > > ... > > } > > > > I was thinking about the above code in the patch and as far as I can > > think this can only occur if the same name slot is re-created with > > prior restart_lsn after the existing slot is dropped. Normally, the > > newly created slot (with the same name) will have higher restart_lsn > > but one can mimic it by copying some older slot by using > > pg_copy_logical_replication_slot(). > > > > I don't think as mentioned in comments even if hot_standby_feedback is > > temporarily set to off, the above shouldn't happen. It can only lead > > to invalidated slots on standby.
I also think so. > > > > To close the above race, I could think of the following ways: > > 1. Drop and re-create the slot. > > 2. Emit LOG/WARNING in this case and once remote_slot's LSN moves > > ahead of local_slot's LSN then we can update it; but as mentioned in > > your previous comment, we need to update all other fields as well. If > > we follow this then we probably need to have a check for catalog_xmin > > as well. IIUC, this would be a sync slot (so not usable until promotion) that could not be used anyway (invalidated), so I'll vote for drop / re-create then. > > Now, related to this the other case which needs some handling is what > > if the remote_slot's restart_lsn is greater than local_slot's > > restart_lsn but it is a re-created slot with the same name. In that > > case, I think the other properties like 'two_phase', 'plugin' could be > > different. So, is simply copying those sufficient or do we need to do > > something else as well? > > > I'm not sure to follow here. If the remote slot is re-created then it would be also dropped / re-created locally, or am I missing something? Regards, -- Bertrand Drouvot PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com