Re: Synchronizing slots from primary to standby

Hi,

On 11/13/23 2:57 PM, Zhijie Hou (Fujitsu) wrote:
> On Friday, November 10, 2023 4:16 PM Drouvot, Bertrand <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>> Yeah good point, agree to just error out in all the case then (if we discard the
>> sync_ reserved wording proposal, which seems to be the case as probably not
>> worth the extra work).
>
> Thanks for the discussion!
>
> Here is the V33 patch set which includes the following changes:

Thanks for working on it!

>
> 1) Drop slots with state 'i' in promotion flow after we shut down WalReceiver.

@@ -3557,10 +3558,15 @@ WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
* this only after failure, so when you promote, we still
* finish replaying as much as we can from archive and
* pg_wal before failover.
+ *
+ * Drop the slots for which sync is initiated but not yet
+ * completed i.e. they are still waiting for the primary
+ * server to catch up.
*/
if (StandbyMode && CheckForStandbyTrigger())
{
XLogShutdownWalRcv();
+ slotsync_drop_initiated_slots();
return XLREAD_FAIL;
}

I had a closer look and it seems this is not located at the right place.

Indeed, it's added here:

switch (currentSource)
{
case XLOG_FROM_ARCHIVE:
case XLOG_FROM_PG_WAL:

While in our case we are in

case XLOG_FROM_STREAM:

So I think we should move slotsync_drop_initiated_slots() in the
XLOG_FROM_STREAM case. Maybe before shutting down the sync slot worker?
(the TODO item number 2 you mentioned up-thread)

BTW in order to prevent any corner case, would'nt also be better to

replace:

+ /*
+ * Do not allow consumption of a "synchronized" slot until the standby
+ * gets promoted.
+ */
+ if (RecoveryInProgress() && (slot->data.sync_state != SYNCSLOT_STATE_NONE))

with something like:

if ((RecoveryInProgress() && (slot->data.sync_state != SYNCSLOT_STATE_NONE)) || slot->data.sync_state == SYNCSLOT_STATE_INITIATED)

to ensure slots in 'i' case can never be used?

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com