bufio: save error from underlying ReadFrom

Updates #35194

Change-Id: Ib854bc6250ddeb606d6ff6240179e23b98e4ac62
Reviewed-on: https://go-review.googlesource.com/c/go/+/203999
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bufio/bufio.go

@@ -708,7 +708,9 @@ func (b *Writer) WriteString(s string) (int, error) {
 func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
 	if b.Buffered() == 0 {
 		if w, ok := b.wr.(io.ReaderFrom); ok {
-			return w.ReadFrom(r)
+			n, err = w.ReadFrom(r)
+			b.err = err
+			return n, err
 		}
 	}
 	var m int

src/bufio/bufio_test.go

@@ -1535,6 +1535,26 @@ func TestPartialReadEOF(t *testing.T) {
 	}
 }
 
+type writerWithReadFromError struct{}
+
+func (w writerWithReadFromError) ReadFrom(r io.Reader) (int64, error) {
+	return 0, errors.New("writerWithReadFromError error")
+}
+
+func (w writerWithReadFromError) Write(b []byte) (n int, err error) {
+	return 10, nil
+}
+
+func TestWriterReadFromMustSetUnderlyingError(t *testing.T) {
+	var wr = NewWriter(writerWithReadFromError{})
+	if _, err := wr.ReadFrom(strings.NewReader("test2")); err == nil {
+		t.Fatal("expected ReadFrom returns error, got nil")
+	}
+	if _, err := wr.Write([]byte("123")); err == nil {
+		t.Fatal("expected Write returns error, got nil")
+	}
+}
+
 func BenchmarkReaderCopyOptimal(b *testing.B) {
 	// Optimal case is where the underlying reader implements io.WriterTo
 	srcBuf := bytes.NewBuffer(make([]byte, 8192))