mysql(ticdc): handle the case when @@sql_mode is null (pingcap#7992) (p…

…ingcap#8000)

close pingcap#7989

cdc/sink/mysql/mysql.go

@@ -387,13 +387,17 @@ func needSwitchDB(ddl *model.DDLEvent) bool {
 	return true
 }
 
-func querySQLMode(ctx context.Context, db *sql.DB) (sqlMode string, err error) {
+func querySQLMode(ctx context.Context, db *sql.DB) (string, error) {
 	row := db.QueryRowContext(ctx, "SELECT @@SESSION.sql_mode;")
-	err = row.Scan(&sqlMode)
+	var sqlMode sql.NullString
+	err := row.Scan(&sqlMode)
 	if err != nil {
 		err = cerror.WrapError(cerror.ErrMySQLQueryError, err)
 	}
-	return
+	if !sqlMode.Valid {
+		sqlMode.String = ""
+	}
+	return sqlMode.String, err
 }
 
 // check whether the target charset is supported

cdc/sink/mysql/mysql_test.go

@@ -947,6 +947,10 @@ func TestReduceReplace(t *testing.T) {
 }
 
 func mockTestDB(adjustSQLMode bool) (*sql.DB, error) {
+	return mockTestDBWithSQLMode(adjustSQLMode, "ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE")
+}
+
+func mockTestDBWithSQLMode(adjustSQLMode bool, sqlMode interface{}) (*sql.DB, error) {
 	// mock for test db, which is used querying TiDB session variable
 	db, mock, err := sqlmock.New()
 	if err != nil {
@@ -955,7 +959,7 @@ func mockTestDB(adjustSQLMode bool) (*sql.DB, error) {
 	if adjustSQLMode {
 		mock.ExpectQuery("SELECT @@SESSION.sql_mode;").
 			WillReturnRows(sqlmock.NewRows([]string{"@@SESSION.sql_mode"}).
-				AddRow("ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE"))
+				AddRow(sqlMode))
 	}
 
 	columns := []string{"Variable_name", "Value"}
@@ -998,7 +1002,7 @@ func TestAdjustSQLMode(t *testing.T) {
 		}()
 		if dbIndex == 0 {
 			// test db
-			db, err := mockTestDB(true)
+			db, err := mockTestDBWithSQLMode(true, nil)
 			require.Nil(t, err)
 			return db, nil
 		}

pkg/sink/mysql/db_helper.go

@@ -175,13 +175,17 @@ func generateDSNByConfig(
 	return dsnCfg.FormatDSN(), nil
 }
 
-func querySQLMode(ctx context.Context, db *sql.DB) (sqlMode string, err error) {
+func querySQLMode(ctx context.Context, db *sql.DB) (string, error) {
 	row := db.QueryRowContext(ctx, "SELECT @@SESSION.sql_mode;")
-	err = row.Scan(&sqlMode)
+	var sqlMode sql.NullString
+	err := row.Scan(&sqlMode)
 	if err != nil {
 		err = cerror.WrapError(cerror.ErrMySQLQueryError, err)
 	}
-	return
+	if !sqlMode.Valid {
+		sqlMode.String = ""
+	}
+	return sqlMode.String, err
 }
 
 // check whether the target charset is supported