Re: Synchronizing slots from primary to standby

On Tue, Jan 9, 2024 at 5:44 PM Zhijie Hou (Fujitsu)
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> V58-0002
>

+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.

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.

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?

--
With Regards,
Amit Kapila.