ApplySchema --batch-size fails with --allow-zero-in-date strategy flag

[shlomi-noach]

#13693 introduced a --batch-size flag, to be used with CREATE statements only and with direct ddl strategy.

When the ddl strategy includes --allow-zero-in-date, ApplySchema --batch-size <greater_than_1> fails; this happens because TabletExecutor attempts to inject query modifiers (specifically /*vt+ allowZeroInDate=true */) into the SQL query. This is the relevant code:

vitess/go/vt/schemamanager/tablet_executor.go

Lines 460 to 473 in 974579d

	if exec.ddlStrategySetting != nil && exec.ddlStrategySetting.IsAllowZeroInDateFlag() {
	// --allow-zero-in-date Applies to DDLs
	stmt, err := sqlparser.Parse(string(sql))
	if err != nil {
	errChan <- ShardWithError{Shard: tablet.Shard, Err: err.Error()}
	return
	}
	if ddlStmt, ok := stmt.(sqlparser.DDLStatement); ok {
	// Add comments directive to allow zero in date
	const directive = `/*vt+ allowZeroInDate=true */`
	ddlStmt.SetComments(ddlStmt.GetParsedComments().Prepend(directive))
	sql = sqlparser.String(ddlStmt)
	}
	}

Problem is that with --batch-size the SQL can contain multiple queries.

The bug was not caught because it only happens when --allow-zero-in-date is added, which wasn't tested in #13693's tests.