support READ-ONLY transactions

Author: shogo82148

connection_go18.go

@@ -45,16 +45,18 @@ func (mc *mysqlConn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver
 		// TODO: support isolation levels
 		return nil, errors.New("mysql: isolation levels not supported")
 	}
-	if opts.ReadOnly {
-		// TODO: support read-only transactions
-		return nil, errors.New("mysql: read-only transactions not supported")
-	}
 
 	if err := mc.watchCancel(ctx); err != nil {
 		return nil, err
 	}
 
-	tx, err := mc.Begin()
+	var err error
+	var tx driver.Tx
+	if opts.ReadOnly {
+		tx, err = mc.beginReadOnly()
+	} else {
+		tx, err = mc.Begin()
+	}
 	mc.finish()
 	if err != nil {
 		return nil, err
@@ -69,6 +71,19 @@ func (mc *mysqlConn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver
 	return tx, err
 }
 
+func (mc *mysqlConn) beginReadOnly() (driver.Tx, error) {
+	if mc.closed.IsSet() {
+		errLog.Print(ErrInvalidConn)
+		return nil, driver.ErrBadConn
+	}
+	err := mc.exec("START TRANSACTION READ ONLY")
+	if err != nil {
+		return nil, err
+	}
+
+	return &mysqlTx{mc}, nil
+}
+
 func (mc *mysqlConn) QueryContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Rows, error) {
 	dargs, err := namedValueToValue(args)
 	if err != nil {

driver_go18_test.go

@@ -468,3 +468,41 @@ func TestContextCancelBegin(t *testing.T) {
 		}
 	})
 }
+
+func TestContextBeginReadOnly(t *testing.T) {
+	runTests(t, dsn, func(dbt *DBTest) {
+		dbt.mustExec("CREATE TABLE test (v INTEGER)")
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+
+		tx, err := dbt.db.BeginTx(ctx, &sql.TxOptions{
+			ReadOnly: true,
+		})
+		if _, ok := err.(*MySQLError); ok {
+			dbt.Skip("It seems that your MySQL does not support READ ONLY transactions")
+			return
+		} else if err != nil {
+			dbt.Fatal(err)
+		}
+
+		// INSERT queries fail in a READ ONLY transaction.
+		_, err = tx.ExecContext(ctx, "INSERT INTO test VALUES (1)")
+		if _, ok := err.(*MySQLError); !ok {
+			dbt.Errorf("expected MySQLError, got %v", err)
+		}
+
+		// SELECT queries can be executed.
+		var v int
+		row := tx.QueryRowContext(ctx, "SELECT COUNT(*) FROM test")
+		if err := row.Scan(&v); err != nil {
+			dbt.Fatal(err)
+		}
+		if v != 0 {
+			dbt.Errorf("expected val to be 0, got %d", v)
+		}
+
+		if err := tx.Commit(); err != nil {
+			dbt.Fatal(err)
+		}
+	})
+}