source-postgres: Detect tuple/relation mismatch

This should never happen but we observed it once. I still have no
idea how it could possibly happen so we need more information in
case it happens again someday.

source-postgres/replication.go

@@ -716,6 +716,30 @@ func (s *replicationStream) decodeTuple(
 		return nil, nil
 	}
 
+	// Check if tuple columns match relation columns. They should never differ, so if
+	// they do something is very wrong and we need to log a bunch of debug info.
+	if len(tuple.Columns) > len(rel.Columns) {
+		var relColumnNames []string
+		var relColumnTypes []uint32
+		for _, col := range rel.Columns {
+			relColumnNames = append(relColumnNames, col.Name)
+			relColumnTypes = append(relColumnTypes, col.DataType)
+		}
+		var tupleColumnTypes []uint8
+		var tupleColumnData []string
+		for _, col := range tuple.Columns {
+			tupleColumnTypes = append(tupleColumnTypes, col.DataType)
+			tupleColumnData = append(tupleColumnData, string(col.Data))
+		}
+		logrus.WithFields(logrus.Fields{
+			"relNames":    relColumnNames,
+			"relTypeOIDs": relColumnTypes,
+			"tupleTypes":  tupleColumnTypes,
+			"tupleData":   tupleColumnData,
+		}).Warn("tuple has more columns than relation")
+		return nil, fmt.Errorf("tuple has %d columns but relation only has %d columns", len(tuple.Columns), len(rel.Columns))
+	}
+
 	var fields = make(map[string]interface{})
 	for idx, col := range tuple.Columns {
 		if keyOnly && (rel.Columns[idx].Flags&1) == 0 {