support READ-ONLY transactions (#618)

connection.go

@@ -92,11 +92,21 @@ func (mc *mysqlConn) markBadConn(err error) error {
 }
 
 func (mc *mysqlConn) Begin() (driver.Tx, error) {
+	return mc.begin(false)
+}
+
+func (mc *mysqlConn) begin(readOnly bool) (driver.Tx, error) {
 	if mc.closed.IsSet() {
 		errLog.Print(ErrInvalidConn)
 		return nil, driver.ErrBadConn
 	}
-	err := mc.exec("START TRANSACTION")
+	var q string
+	if readOnly {
+		q = "START TRANSACTION READ ONLY"
+	} else {
+		q = "START TRANSACTION"
+	}
+	err := mc.exec(q)
 	if err == nil {
 		return &mysqlTx{mc}, err
 	}

connection_go18.go

@@ -14,7 +14,6 @@ import (
 	"context"
 	"database/sql"
 	"database/sql/driver"
-	"errors"
 )
 
 // Ping implements driver.Pinger interface
@@ -41,15 +40,9 @@ func (mc *mysqlConn) Ping(ctx context.Context) error {
 
 // BeginTx implements driver.ConnBeginTx interface
 func (mc *mysqlConn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver.Tx, error) {
-	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
 	}
-
 	defer mc.finish()
 
 	if sql.IsolationLevel(opts.Isolation) != sql.LevelDefault {
@@ -63,7 +56,7 @@ func (mc *mysqlConn) BeginTx(ctx context.Context, opts driver.TxOptions) (driver
 		}
 	}
 
-	return mc.Begin()
+	return mc.begin(opts.ReadOnly)
 }
 
 func (mc *mysqlConn) QueryContext(ctx context.Context, query string, args []driver.NamedValue) (driver.Rows, error) {

driver_go18_test.go

@@ -520,3 +520,41 @@ func TestContextBeginIsolationLevel(t *testing.T) {
 		tx2.Commit()
 	})
 }
+
+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)
+		}
+	})
+}