encoding/base64: don't ignore underlying souce read error in decode

Fixes #3577

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6137054

src/pkg/encoding/base64/base64.go

@@ -318,7 +318,7 @@ func (d *decoder) Read(p []byte) (n int, err error) {
 	}
 	nn, d.err = io.ReadAtLeast(d.r, d.buf[d.nbuf:nn], 4-d.nbuf)
 	d.nbuf += nn
-	if d.nbuf < 4 {
+	if d.err != nil || d.nbuf < 4 {
 		return 0, d.err
 	}
 

src/pkg/encoding/base64/base64_test.go

@@ -6,9 +6,11 @@ package base64
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"io/ioutil"
 	"testing"
+	"time"
 )
 
 type testpair struct {
@@ -226,3 +228,50 @@ func TestNewLineCharacters(t *testing.T) {
 		}
 	}
 }
+
+type nextRead struct {
+	n   int   // bytes to return
+	err error // error to return
+}
+
+// faultInjectReader returns data from source, rate-limited
+// and with the errors as written to nextc.
+type faultInjectReader struct {
+	source string
+	nextc  <-chan nextRead
+}
+
+func (r *faultInjectReader) Read(p []byte) (int, error) {
+	nr := <-r.nextc
+	if len(p) > nr.n {
+		p = p[:nr.n]
+	}
+	n := copy(p, r.source)
+	r.source = r.source[n:]
+	return n, nr.err
+}
+
+// tests that we don't ignore errors from our underlying reader
+func TestDecoderIssue3577(t *testing.T) {
+	next := make(chan nextRead, 10)
+	wantErr := errors.New("my error")
+	next <- nextRead{5, nil}
+	next <- nextRead{10, wantErr}
+	d := NewDecoder(StdEncoding, &faultInjectReader{
+		source: "VHdhcyBicmlsbGlnLCBhbmQgdGhlIHNsaXRoeSB0b3Zlcw==", // twas brillig...
+		nextc:  next,
+	})
+	errc := make(chan error)
+	go func() {
+		_, err := ioutil.ReadAll(d)
+		errc <- err
+	}()
+	select {
+	case err := <-errc:
+		if err != wantErr {
+			t.Errorf("got error %v; want %v", err, wantErr)
+		}
+	case <-time.After(5 * time.Second):
+		t.Errorf("timeout; Decoder blocked without returning an error")
+	}
+}