database/sql: prefer to return Rows.lasterr rather then a static error

Fixes #25829

Change-Id: I400fdaf0ef3a23bc0d61c4873ffa298e0cf0fc6a
Reviewed-on: https://go-review.googlesource.com/c/145204Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -2605,6 +2605,15 @@ type Rows struct {
 	lastcols []driver.Value
 }
 
+// lasterrOrErrLocked returns either lasterr or the provided err.
+// rs.closemu must be read-locked.
+func (rs *Rows) lasterrOrErrLocked(err error) error {
+	if rs.lasterr != nil && rs.lasterr != io.EOF {
+		return rs.lasterr
+	}
+	return err
+}
+
 func (rs *Rows) initContextClose(ctx, txctx context.Context) {
 	if ctx.Done() == nil && (txctx == nil || txctx.Done() == nil) {
 		return
@@ -2728,22 +2737,22 @@ func (rs *Rows) NextResultSet() bool {
 func (rs *Rows) Err() error {
 	rs.closemu.RLock()
 	defer rs.closemu.RUnlock()
-	if rs.lasterr == io.EOF {
-		return nil
-	}
-	return rs.lasterr
+	return rs.lasterrOrErrLocked(nil)
 }
 
+var errRowsClosed = errors.New("sql: Rows are closed")
+var errNoRows = errors.New("sql: no Rows available")
+
 // Columns returns the column names.
 // Columns returns an error if the rows are closed.
 func (rs *Rows) Columns() ([]string, error) {
 	rs.closemu.RLock()
 	defer rs.closemu.RUnlock()
 	if rs.closed {
-		return nil, errors.New("sql: Rows are closed")
+		return nil, rs.lasterrOrErrLocked(errRowsClosed)
 	}
 	if rs.rowsi == nil {
-		return nil, errors.New("sql: no Rows available")
+		return nil, rs.lasterrOrErrLocked(errNoRows)
 	}
 	rs.dc.Lock()
 	defer rs.dc.Unlock()
@@ -2757,10 +2766,10 @@ func (rs *Rows) ColumnTypes() ([]*ColumnType, error) {
 	rs.closemu.RLock()
 	defer rs.closemu.RUnlock()
 	if rs.closed {
-		return nil, errors.New("sql: Rows are closed")
+		return nil, rs.lasterrOrErrLocked(errRowsClosed)
 	}
 	if rs.rowsi == nil {
-		return nil, errors.New("sql: no Rows available")
+		return nil, rs.lasterrOrErrLocked(errNoRows)
 	}
 	rs.dc.Lock()
 	defer rs.dc.Unlock()
@@ -2916,8 +2925,9 @@ func (rs *Rows) Scan(dest ...interface{}) error {
 		return rs.lasterr
 	}
 	if rs.closed {
+		err := rs.lasterrOrErrLocked(errRowsClosed)
 		rs.closemu.RUnlock()
-		return errors.New("sql: Rows are closed")
+		return err
 	}
 	rs.closemu.RUnlock()