Ignore invalidated slots while computing oldest catalog Xmin

Once a logical slot has acquired a catalog_xmin, it doesn't let go of
it, even when invalidated by exceeding the max_slot_wal_keep_size, which
means that dead catalog tuples are not removed by vacuum anymore since
the point is invalidated, until the slot is dropped.  This could be
catastrophic if catalog churn is high.

Change the computation of Xmin to ignore invalidated slots,
to prevent dead rows from accumulating.

Backpatch to 13, where slot invalidation appeared.

Author: Sirisha Chamarthi <sirichamarthi22@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/CAKrAKeUEDeqquN9vwzNeG-CN8wuVsfRYbeOUV9qKO_RHok=j+g@mail.gmail.com

Author: alvherre

src/backend/replication/slot.c

@@ -846,15 +846,22 @@ ReplicationSlotsComputeRequiredXmin(bool already_locked)
 		ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
 		TransactionId effective_xmin;
 		TransactionId effective_catalog_xmin;
+		bool		invalidated;
 
 		if (!s->in_use)
 			continue;
 
 		SpinLockAcquire(&s->mutex);
 		effective_xmin = s->effective_xmin;
 		effective_catalog_xmin = s->effective_catalog_xmin;
+		invalidated = (!XLogRecPtrIsInvalid(s->data.invalidated_at) &&
+					   XLogRecPtrIsInvalid(s->data.restart_lsn));
 		SpinLockRelease(&s->mutex);
 
+		/* invalidated slots need not apply */
+		if (invalidated)
+			continue;
+
 		/* check the data xmin */
 		if (TransactionIdIsValid(effective_xmin) &&
 			(!TransactionIdIsValid(agg_xmin) ||

src/backend/storage/ipc/procarray.c

@@ -3900,6 +3900,9 @@ ProcArraySetReplicationSlotXmin(TransactionId xmin, TransactionId catalog_xmin,
 
 	if (!already_locked)
 		LWLockRelease(ProcArrayLock);
+
+	elog(DEBUG1, "xmin required by slots: data %u, catalog %u",
+		 xmin, catalog_xmin);
 }
 
 /*