Re: Improve pg_sync_replication_slots() to wait for primary to advance

Hi Ajin,

I have reviewed v20 and got a few comments:

> On Oct 30, 2025, at 18:18, Ajin Cherian <itsajin@gmail.com> wrote:
> 
> <v20-0001-Improve-initial-slot-synchronization-in-pg_sync_.patch>

1 - slotsync.c
```
+		if (slot_names)
+			list_free_deep(slot_names);
 
 		/* Cleanup the synced temporary slots */
 		ReplicationSlotCleanup(true);
@@ -1762,5 +2026,5 @@ SyncReplicationSlots(WalReceiverConn *wrconn)
 		/* We are done with sync, so reset sync flag */
 		reset_syncing_flag();
 	}
-	PG_END_ENSURE_ERROR_CLEANUP(slotsync_failure_callback, PointerGetDatum(wrconn));
+	PG_END_ENSURE_ERROR_CLEANUP(slotsync_failure_callback, PointerGetDatum(&fparams));
```

I am afraid there is a risk of double memory free. Slot_names has been assigned to fparams.slot_names within the  for loop, and it’s freed after the loop. If something gets wrong and slotsync_failure_callback() is called, the function will free fparams.slot_names again.

2 - slotsync.c
```
+			/*
+			 * 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 first iteration; re-fetching all failover slots each time could
+			 * cause an endless loop. 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.
+			 */
+			remote_slots = fetch_remote_slots(wrconn, slot_names);
+
+			/* Attempt to synchronize slots */
+			some_slot_updated = synchronize_slots(wrconn, remote_slots,
+												  &slot_persistence_pending);
+
+			/*
+			 * If slot_persistence_pending is true, extract slot names
+			 * for future iterations (only needed if we haven't done it yet)
+			 */
+			if (slot_names == NIL && slot_persistence_pending)
+			{
+				slot_names = extract_slot_names(remote_slots);
+
+				/* Update the failure structure so that it can be freed on error */
+				fparams.slot_names = slot_names;
+			}
```

I am thinking if that could be a problem. As you now extract_slot_names() only in the first iteration, if a slot is dropped, and a new slot comes with the same name, will the new slot be incorrectly synced?

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/