database/sql: return context errors from Rows.Scan

The previous implementation would return "sql: Rows are closed" for any
context errors, which can be confusing for context timeouts or
cancelations.

Fixes #24431

Change-Id: I884904ec43204c43f4e94e2335b2802aab77a888
Reviewed-on: https://go-review.googlesource.com/104276Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql.go

@@ -2870,6 +2870,11 @@ func rowsColumnInfoSetupConnLocked(rowsi driver.Rows) []*ColumnType {
 // string inputs parseable by strconv.ParseBool.
 func (rs *Rows) Scan(dest ...interface{}) error {
 	rs.closemu.RLock()
+
+	if rs.lasterr != nil && rs.lasterr != io.EOF {
+		rs.closemu.RUnlock()
+		return rs.lasterr
+	}
 	if rs.closed {
 		rs.closemu.RUnlock()
 		return errors.New("sql: Rows are closed")

src/database/sql/sql_test.go

@@ -325,8 +325,8 @@ func TestQueryContext(t *testing.T) {
 			}
 			t.Fatalf("Scan: %v", err)
 		}
-		if index == 2 && err == nil {
-			t.Fatal("expected an error on last scan")
+		if index == 2 && err != context.Canceled {
+			t.Fatalf("Scan: %v; want context.Canceled", err)
 		}
 		got = append(got, r)
 		index++