Improve errors.

- Use new closeError type for reporting close frames to the application.
- Use closeError with code 1006 when the peer closes connection without
  sending a close frame. The error io.ErrUnexpectedEOF was used
  previously. This change helps developers distinguish abnormal closure
  and an unexpected EOF in the JSON parser.

conn.go

@@ -70,18 +70,30 @@ var (
 	ErrReadLimit = errors.New("websocket: read limit exceeded")
 )
 
-type websocketError struct {
+// netError satisfies the net Error interface.
+type netError struct {
 	msg       string
 	temporary bool
 	timeout   bool
 }
 
-func (e *websocketError) Error() string   { return e.msg }
-func (e *websocketError) Temporary() bool { return e.temporary }
-func (e *websocketError) Timeout() bool   { return e.timeout }
+func (e *netError) Error() string   { return e.msg }
+func (e *netError) Temporary() bool { return e.temporary }
+func (e *netError) Timeout() bool   { return e.timeout }
+
+// closeError represents close frame.
+type closeError struct {
+	code int
+	text string
+}
+
+func (e *closeError) Error() string {
+	return "websocket: close " + strconv.Itoa(e.code) + " " + e.text
+}
 
 var (
-	errWriteTimeout        = &websocketError{msg: "websocket: write timeout", timeout: true}
+	errWriteTimeout        = &netError{msg: "websocket: write timeout", timeout: true}
+	errUnexpectedEOF       = &closeError{code: CloseAbnormalClosure, text: io.ErrUnexpectedEOF.Error()}
 	errBadWriteOpCode      = errors.New("websocket: bad write message type")
 	errWriteClosed         = errors.New("websocket: write closed")
 	errInvalidControlFrame = errors.New("websocket: invalid control frame")
@@ -527,7 +539,7 @@ func (c *Conn) readFull(p []byte) (err error) {
 	if n == len(p) {
 		err = nil
 	} else if err == io.EOF {
-		err = io.ErrUnexpectedEOF
+		err = errUnexpectedEOF
 	}
 	return
 }
@@ -649,17 +661,17 @@ func (c *Conn) advanceFrame() (int, error) {
 		}
 	case CloseMessage:
 		c.WriteControl(CloseMessage, []byte{}, time.Now().Add(writeWait))
-		if len(payload) < 2 {
-			return noFrame, io.EOF
+		closeCode := CloseNoStatusReceived
+		closeText := ""
+		if len(payload) >= 2 {
+			closeCode = int(binary.BigEndian.Uint16(payload))
+			closeText = string(payload[2:])
 		}
-		closeCode := binary.BigEndian.Uint16(payload)
 		switch closeCode {
 		case CloseNormalClosure, CloseGoingAway:
 			return noFrame, io.EOF
 		default:
-			return noFrame, errors.New("websocket: close " +
-				strconv.Itoa(int(closeCode)) + " " +
-				string(payload[2:]))
+			return noFrame, &closeError{code: closeCode, text: closeText}
 		}
 	}
 
@@ -739,7 +751,7 @@ func (r messageReader) Read(b []byte) (int, error) {
 
 	err := r.c.readErr
 	if err == io.EOF && r.seq == r.c.readSeq {
-		err = io.ErrUnexpectedEOF
+		err = errUnexpectedEOF
 	}
 	return 0, err
 }

conn_test.go

@@ -152,7 +152,7 @@ func TestCloseBeforeFinalFrame(t *testing.T) {
 
 	w, _ := wc.NextWriter(BinaryMessage)
 	w.Write(make([]byte, bufSize+bufSize/2))
-	wc.WriteControl(CloseMessage, []byte{}, time.Now().Add(10*time.Second))
+	wc.WriteControl(CloseMessage, FormatCloseMessage(CloseNormalClosure, ""), time.Now().Add(10*time.Second))
 	w.Close()
 
 	op, r, err := rc.NextReader()
@@ -160,8 +160,8 @@ func TestCloseBeforeFinalFrame(t *testing.T) {
 		t.Fatalf("NextReader() returned %d, %v", op, err)
 	}
 	_, err = io.Copy(ioutil.Discard, r)
-	if err != io.ErrUnexpectedEOF {
-		t.Fatalf("io.Copy() returned %v, want %v", err, io.ErrUnexpectedEOF)
+	if err != errUnexpectedEOF {
+		t.Fatalf("io.Copy() returned %v, want %v", err, errUnexpectedEOF)
 	}
 	_, _, err = rc.NextReader()
 	if err != io.EOF {
@@ -184,12 +184,12 @@ func TestEOFBeforeFinalFrame(t *testing.T) {
 		t.Fatalf("NextReader() returned %d, %v", op, err)
 	}
 	_, err = io.Copy(ioutil.Discard, r)
-	if err != io.ErrUnexpectedEOF {
-		t.Fatalf("io.Copy() returned %v, want %v", err, io.ErrUnexpectedEOF)
+	if err != errUnexpectedEOF {
+		t.Fatalf("io.Copy() returned %v, want %v", err, errUnexpectedEOF)
 	}
 	_, _, err = rc.NextReader()
-	if err != io.ErrUnexpectedEOF {
-		t.Fatalf("NextReader() returned %v, want %v", err, io.ErrUnexpectedEOF)
+	if err != errUnexpectedEOF {
+		t.Fatalf("NextReader() returned %v, want %v", err, errUnexpectedEOF)
 	}
 }