Expression filter: UPDATE filter is ignored if all of update-old-value filters are empty

[kennytm]

What did you do?

1. Prepare a table

   -- upstream MySQL
   CREATE TABLE test.t (a BIGINT PRIMARY KEY, r VARCHAR(10), s INT);
   INSERT INTO test.t VALUES (1, 'a', 2);
   
   -- downstream TiDB
   CREATE TABLE test.t (a BIGINT PRIMARY KEY, r VARCHAR(10), s INT);
   INSERT INTO test.t VALUES (1, 'a', 2);

2. Create a task which only the update-new-value filter is specified

   expression-filter:
       e02:
           schema: test
           table: t
           update-new-value-expr: "r = 'a'"
   mysql-instances:
     - source-id: mysql-replica-01
       expression-filters: ['e02']
   ...

3. In the upstream MySQL, execute an UPDATE statement

   -- Upstream MySQL
   UPDATE test.t SET s = s + 1 WHERE a = 1;

What did you expect to see?

The UPDATE DML should be skipped, because the changed row has r = 'a'.

What did you see instead?

The DML is actually synchronized, i.e. SELECTing the row gives s = 3.

Versions of the cluster

v6.3.0

current status of DM cluster (execute query-status <task-name> in dmctl)

No response

[kennytm]

The issue is caused by oldValueFilters == nil inside the genAndFilterUpdateDMLs() function. The loop is thus completely skipped over and a RowChange is always generated.

tiflow/dm/syncer/dml.go

Lines 197 to 199 in bc176c3

	for j := range oldValueFilters {
	// AND logic
	oldExpr, newExpr := oldValueFilters[j], newValueFilters[j]

The reason why oldValueFilters == nil I believe is the wrong short-circuit check used in GetUpdateExprs(). Although the doc suggests it will return two lists of the same length, if g.hasUpdateOldFilter[tableID] is missing, g.updateOldExprs[tableID] will remain an unpopulated zero-length list (instead of becoming ["1", "1", "1", …]).

tiflow/dm/syncer/expr_filter_group.go

Lines 108 to 120 in bc176c3

	// GetUpdateExprs returns two lists of expression filters for given table, to filter UPDATE events by old values and new
	// values respectively. The two lists should have same length, and the corresponding expressions is AND logic.
	// This function will lazy calculate expressions if not initialized.
	func (g *ExprFilterGroup) GetUpdateExprs(table *filter.Table, ti *model.TableInfo) ([]expression.Expression, []expression.Expression, error) {
	tableID := utils.GenTableID(table)
	retOld, ok1 := g.updateOldExprs[tableID]
	retNew, ok2 := g.updateNewExprs[tableID]
	if ok1 || ok2 {
	return retOld, retNew, nil
	}
	if _, ok := g.hasUpdateOldFilter[tableID]; ok {

(And I suspect filling in only update-old-value-expr will instead cause an index-out-of-bound panic in dml.go:199, but I haven't tried.)