Re: Improve pg_sync_replication_slots() to wait for primary to advance - Mailing list pgsql-hackers

On Mon, Sep 15, 2025 at 6:17 PM Ajin Cherian <itsajin@gmail.com> wrote:
>
> On Wed, Sep 10, 2025 at 2:45 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > On Tue, Sep 9, 2025 at 5:37 PM Ajin Cherian <itsajin@gmail.com> wrote:
> > >
> > > Attached v11 patch addressing the above comments.
> > >
> >
> > Please find a few comments:
> >
> > 1)
> >
> > + Retry is done after 2
> > + * sec wait. Exits early if promotion is triggered or certain critical
> >
> > We can say: Retry is done after SLOTSYNC_API_NAPTIME_MS wait.
> >
>
> Changed.
>
> > 2)
> > + /*
> > + * Fetch remote slot info for the given slot_names. If slot_names is NIL,
> > + * fetch all failover-enabled slots. Note that we reuse slot_names from
> > + * the previous iteration; re-fetching all failover slots each time could
> > + * cause an endless loop.
> > + */
> >
> > a)
> > the previous iteration --> the first iteration.
> >
> > b) Also we can mention the reason why we take names from first
> > iteration instead of going for pending ones alone, something like:
> >
> > Instead of reprocessing only the pending slots in each iteration, it's
> > better to process all the slots received in the first iteration.
> > This ensures that by the time we're done, all slots reflect the latest values.
> >
> > 3)
> > + remote_slots = fetch_remote_slots(wrconn, slot_names);
> > +
> > +
> > + /* Attempt to synchronize slots */
> > + synchronize_slots(wrconn, remote_slots, &slot_persistence_pending);
> >
> > One extra blank line can be removed
> >
>
> Fixed.
>
> > 4)
> >
> > + /* Clean up slot_names if allocated in TopMemoryContext */
> > + if (slot_names)
> > + list_free_deep(slot_names);
> >
> > Can we please move it before 'ReplicationSlotCleanup'.
> >
>
> Fixed.
>
> > 5)
> > In case of error in subsequent iteration, slot_names allocated from
> > TopMemoryContext will be left unfreed?
> >
>
> I've changed the logic so that even on error, slot_names are freed.

I see that you have freed 'slot_names' in config-changed and promotion
case but the ERROR can come from other flows as well. The idea was to
somehow to free it (if possible) in slotsync_failure_callback() by
passing it as an argument, like we do for 'wrconn'.

> > 6)
> > + ListCell   *lc;
> > + bool first_slot = true;
> >
> > Shall we move these two to concerned if-block:
> > if (slot_names != NIL)
> >
>
> Changed.
>
> > 7)
> >  * The slot_persistence_pending flag is used by the pg_sync_replication_slots
> >  * API to track if any slots could not be persisted and need to be retried.
> >
> > a) Instead of mentioning only about slot_persistence_pending argument
> > in concerned function's header, we shall define all the arguments.
> >
> > b) We can remove the 'flag' term from the comments as it is a
> > function-argument now.
> >
>
> Changed.
>
> > 8)
> > I think we should add briefly in the header of the file about the new
> > behaviour of API.
> >
>
> Added.
>
> Attaching patch v12 addressing these comments.
>

Thank You for the patch. Please find a few comments:

1)
+ bool slot_persistence_pending = false;

We can move this declaration outside of the loop. And I think we don't
need to initialize as we are resetting it to false before each
iteration.

2)

+ /* Switch to long-lived TopMemoryContext to store slot names */
+ oldcontext = MemoryContextSwitchTo(TopMemoryContext);
+
+ /* Extract slot names from the remote slots */
+ slot_names = extract_slot_names(remote_slots);
+
+ MemoryContextSwitchTo(oldcontext);

I think it will be better if we move 'MemoryContextSwitchTo' calls
inside extract_slot_names() itself. The entire logic related to
'slot_names' will then be consolidated in one place

3)
+ * The slot_persistence_pending flag is used by the pg_sync_replication_slots
+ * API to track if any slots could not be persisted and need to be retried.

Can we change it to below. We can have it started in a new line after
a blank line (see how remote_slot_precedes, found_consistent_snapshot
are defined)

*slot_persistence_pending is set to true if any of the slots fail to
persist. It is utilized by the pg_sync_replication_slots() API.

Please change it in both synchronize_one_slot() and
update_and_persist_local_synced_slot()

4)
a)
+ Update the
+ * slot_persistence_pending flag, so the API can retry.
  */

b)
 /* update the flag, so that the API can retry */

It will be good if we can remove 'flag' usage from both occurrences in
update_and_persist_local_synced_slot().

5)
Similar to ProcessSlotSyncInterrupts() for worker, shall we have one
such function for API which can have all 3 things:

{
/*
 * If we've been promoted, then no point
 * continuing.
 */
if (SlotSyncCtx->stopSignaled)
{
   ereport(ERROR,
  (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
  errmsg("exiting from slot synchronization as"
  " promotion is triggered")));
}

CHECK_FOR_INTERRUPTS();

if (ConfigReloadPending)
    slotsync_api_reread_config();
}

And similar to the worker case, we can have it checked in the
beginning of the loop. Thoughts?

thanks
Shveta