bufio: fix potential endless loop in ReadByte

Fixes #7745.

LGTM=bradfitz, r
R=r, bradfitz
CC=golang-codereviews
https://golang.org/cl/86220044

src/pkg/bufio/bufio.go

@@ -88,15 +88,22 @@ func (b *Reader) fill() {
 		b.r = 0
 	}
 
-	// Read new data.
-	n, err := b.rd.Read(b.buf[b.w:])
-	if n < 0 {
-		panic(errNegativeRead)
-	}
-	b.w += n
-	if err != nil {
-		b.err = err
+	// Read new data: try a limited number of times.
+	for i := maxConsecutiveEmptyReads; i > 0; i-- {
+		n, err := b.rd.Read(b.buf[b.w:])
+		if n < 0 {
+			panic(errNegativeRead)
+		}
+		b.w += n
+		if err != nil {
+			b.err = err
+			return
+		}
+		if n > 0 {
+			return
+		}
 	}
+	b.err = io.ErrNoProgress
 }
 
 func (b *Reader) readErr() error {
@@ -151,6 +158,9 @@ func (b *Reader) Read(p []byte) (n int, err error) {
 			// Large read, empty buffer.
 			// Read directly into p to avoid copy.
 			n, b.err = b.rd.Read(p)
+			if n < 0 {
+				panic(errNegativeRead)
+			}
 			if n > 0 {
 				b.lastByte = int(p[n-1])
 				b.lastRuneSize = -1

src/pkg/bufio/bufio_test.go

@@ -14,6 +14,7 @@ import (
 	"strings"
 	"testing"
 	"testing/iotest"
+	"time"
 	"unicode/utf8"
 )
 
@@ -174,6 +175,34 @@ func TestReader(t *testing.T) {
 	}
 }
 
+type zeroReader struct{}
+
+func (zeroReader) Read(p []byte) (int, error) {
+	return 0, nil
+}
+
+func TestZeroReader(t *testing.T) {
+	var z zeroReader
+	r := NewReader(z)
+
+	c := make(chan error)
+	go func() {
+		_, err := r.ReadByte()
+		c <- err
+	}()
+
+	select {
+	case err := <-c:
+		if err == nil {
+			t.Error("error expected")
+		} else if err != io.ErrNoProgress {
+			t.Error("unexpected error:", err)
+		}
+	case <-time.After(time.Second):
+		t.Error("test timed out (endless loop in ReadByte?)")
+	}
+}
+
 // A StringReader delivers its data one string segment at a time via Read.
 type StringReader struct {
 	data []string