encoding/csv: add Error method to Writer

Fixed issue 3931

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6923049

src/pkg/encoding/csv/writer.go

@@ -92,10 +92,17 @@ func (w *Writer) Write(record []string) (err error) {
 }
 
 // Flush writes any buffered data to the underlying io.Writer.
+// To check if an error occured during the Flush, call Error.
 func (w *Writer) Flush() {
 	w.w.Flush()
 }
 
+// Error reports any error that has occurred during a previous Write or Flush.
+func (w *Writer) Error() error {
+	_, err := w.w.Write(nil)
+	return err
+}
+
 // WriteAll writes multiple CSV records to w using Write and then calls Flush.
 func (w *Writer) WriteAll(records [][]string) (err error) {
 	for _, record := range records {

src/pkg/encoding/csv/writer_test.go

@@ -6,6 +6,7 @@ package csv
 
 import (
 	"bytes"
+	"errors"
 	"testing"
 )
 
@@ -42,3 +43,30 @@ func TestWrite(t *testing.T) {
 		}
 	}
 }
+
+type errorWriter struct{}
+
+func (e errorWriter) Write(b []byte) (int, error) {
+	return 0, errors.New("Test")
+}
+
+func TestError(t *testing.T) {
+	b := &bytes.Buffer{}
+	f := NewWriter(b)
+	f.Write([]string{"abc"})
+	f.Flush()
+	err := f.Error()
+
+	if err != nil {
+		t.Errorf("Unexpected error: %s\n", err)
+	}
+
+	f = NewWriter(errorWriter{})
+	f.Write([]string{"abc"})
+	f.Flush()
+	err = f.Error()
+
+	if err == nil {
+		t.Error("Error should not be nil")
+	}
+}