make it possible to parse a varint at the end of a reader

An io.Reader can read into the buffer and return the io.EOF in the same
call. In fact, that's how the quic.Stream is implemented.
In that case, we shouldn't return an error from quicvarint.Read,
otherwise the caller won't be able to parse a varint sent just before a
stream was closed.

quicvarint/io.go

@@ -31,7 +31,10 @@ func NewReader(r io.Reader) Reader {
 
 func (r *byteReader) ReadByte() (byte, error) {
 	var b [1]byte
-	_, err := r.Reader.Read(b[:])
+	n, err := r.Reader.Read(b[:])
+	if n == 1 && err == io.EOF {
+		err = nil
+	}
 	return b[0], err
 }
 

quicvarint/io_test.go

@@ -22,6 +22,21 @@ func (r *nopWriter) Write(_ []byte) (int, error) {
 	return 0, io.ErrShortBuffer
 }
 
+// eofReader is a reader that returns data and the io.EOF at the same time in the last Read call
+type eofReader struct {
+	Data []byte
+	pos  int
+}
+
+func (r *eofReader) Read(b []byte) (int, error) {
+	n := copy(b, r.Data[r.pos:])
+	r.pos += n
+	if r.pos >= len(r.Data) {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
 var _ io.Writer = &nopWriter{}
 
 var _ = Describe("Varint I/O", func() {
@@ -46,6 +61,34 @@ var _ = Describe("Varint I/O", func() {
 			Expect(err).To(Equal(io.ErrUnexpectedEOF))
 			Expect(val).To(Equal(byte(0)))
 		})
+
+		Context("EOF handling", func() {
+			It("eofReader works correctly", func() {
+				r := &eofReader{Data: []byte("foobar")}
+				b := make([]byte, 3)
+				n, err := r.Read(b)
+				Expect(n).To(Equal(3))
+				Expect(err).ToNot(HaveOccurred())
+				Expect(string(b)).To(Equal("foo"))
+				n, err = r.Read(b)
+				Expect(n).To(Equal(3))
+				Expect(err).To(MatchError(io.EOF))
+				Expect(string(b)).To(Equal("bar"))
+				n, err = r.Read(b)
+				Expect(err).To(MatchError(io.EOF))
+				Expect(n).To(BeZero())
+			})
+
+			It("correctly handles io.EOF", func() {
+				buf := &bytes.Buffer{}
+				Write(buf, 1337)
+
+				r := NewReader(&eofReader{Data: buf.Bytes()})
+				n, err := Read(r)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(n).To(BeEquivalentTo(1337))
+			})
+		})
 	})
 
 	Context("Writer", func() {