Potential fix for the LSN issue (#600)

KeepAlive messages are now sent only after we confirm no records were read

flow/connectors/postgres/cdc.go

@@ -359,13 +359,13 @@ func (p *PostgresCDCSource) consumeStream(
 
 			if xld.WALStart > clientXLogPos {
 				clientXLogPos = xld.WALStart
-				records.UpdateLatestCheckpoint(int64(clientXLogPos))
 			}
 
 			if len(localRecords) == 0 {
 				// given that we have no records it is safe to update the flush wal position
 				// to the clientXLogPos. clientXLogPos can be moved forward due to PKM messages.
 				consumedXLogPos = clientXLogPos
+				records.UpdateLatestCheckpoint(int64(clientXLogPos))
 			}
 		}
 	}
@@ -392,7 +392,7 @@ func (p *PostgresCDCSource) processMessage(batch *model.CDCRecordStream, xld pgl
 		// for a commit message, update the last checkpoint id for the record batch.
 		log.Debugf("CommitMessage => CommitLSN: %v, TransactionEndLSN: %v",
 			msg.CommitLSN, msg.TransactionEndLSN)
-		batch.UpdateLatestCheckpoint(int64(xld.WALStart))
+		batch.UpdateLatestCheckpoint(int64(msg.CommitLSN))
 		p.commitLock = false
 	case *pglogrepl.RelationMessage:
 		// treat all relation messages as correponding to parent if partitioned.