bufio: minor fixes

* note end-of-line and EOF behavior for ReadLine
* diagnose broken Readers

Fixes #3825.
Fixes #4276.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/6907060

src/pkg/bufio/bufio.go

@@ -64,6 +64,8 @@ func NewReader(rd io.Reader) *Reader {
 	return NewReaderSize(rd, defaultBufSize)
 }
 
+var errNegativeRead = errors.New("bufio: reader returned negative count from Read")
+
 // fill reads a new chunk into the buffer.
 func (b *Reader) fill() {
 	// Slide existing data to beginning.
@@ -75,6 +77,9 @@ func (b *Reader) fill() {
 
 	// Read new data.
 	n, e := b.rd.Read(b.buf[b.w:])
+	if n < 0 {
+		panic(errNegativeRead)
+	}
 	b.w += n
 	if e != nil {
 		b.err = e
@@ -282,6 +287,9 @@ func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
 // of the line. The returned buffer is only valid until the next call to
 // ReadLine. ReadLine either returns a non-nil line or it returns an error,
 // never both.
+//
+// The text returned from ReadLine does not include the line end ("\r\n" or "\n").
+// No indication or error is given if the input ends without a final line end.
 func (b *Reader) ReadLine() (line []byte, isPrefix bool, err error) {
 	line, err = b.ReadSlice('\n')
 	if err == ErrBufferFull {

src/pkg/bufio/bufio_test.go

@@ -939,6 +939,29 @@ func (w *writeCountingDiscard) Write(p []byte) (int, error) {
 	return len(p), nil
 }
 
+type negativeReader int
+
+func (r *negativeReader) Read([]byte) (int, error) { return -1, nil }
+
+func TestNegativeRead(t *testing.T) {
+	// should panic with a description pointing at the reader, not at itself.
+	// (should NOT panic with slice index error, for example.)
+	b := NewReader(new(negativeReader))
+	defer func() {
+		switch err := recover().(type) {
+		case nil:
+			t.Fatal("read did not panic")
+		case error:
+			if !strings.Contains(err.Error(), "reader returned negative count from Read") {
+				t.Fatal("wrong panic: %v", err)
+			}
+		default:
+			t.Fatalf("unexpected panic value: %T(%v)", err, err)
+		}
+	}()
+	b.Read(make([]byte, 100))
+}
+
 // An onlyReader only implements io.Reader, no matter what other methods the underlying implementation may have.
 type onlyReader struct {
 	r io.Reader