Remove hideTempErr to allow downstream users to check for errors like net.ErrClosed (#894)

Since this change
https://github.com/gorilla/websocket/pull/840/files#diff-4f427d2b022907c552328e63f137561f6de92396d7a6e8f6c2ea1bcf0db52654L190-R197
we can no longer determinate if the errors coming from ReadMessage() are
net.ErrClosed for example
Hardcoding the error message is not great option because it may vary
from OS to OS and system locale

Author: UnAfraid

conn.go

@@ -192,13 +192,6 @@ func newMaskKey() [4]byte {
 	return k
 }
 
-func hideTempErr(err error) error {
-	if e, ok := err.(net.Error); ok {
-		err = &netError{msg: e.Error(), timeout: e.Timeout()}
-	}
-	return err
-}
-
 func isControl(frameType int) bool {
 	return frameType == CloseMessage || frameType == PingMessage || frameType == PongMessage
 }
@@ -364,7 +357,6 @@ func (c *Conn) RemoteAddr() net.Addr {
 // Write methods
 
 func (c *Conn) writeFatal(err error) error {
-	err = hideTempErr(err)
 	c.writeErrMu.Lock()
 	if c.writeErr == nil {
 		c.writeErr = err
@@ -1033,7 +1025,7 @@ func (c *Conn) NextReader() (messageType int, r io.Reader, err error) {
 	for c.readErr == nil {
 		frameType, err := c.advanceFrame()
 		if err != nil {
-			c.readErr = hideTempErr(err)
+			c.readErr = err
 			break
 		}
 
@@ -1073,7 +1065,7 @@ func (r *messageReader) Read(b []byte) (int, error) {
 				b = b[:c.readRemaining]
 			}
 			n, err := c.br.Read(b)
-			c.readErr = hideTempErr(err)
+			c.readErr = err
 			if c.isServer {
 				c.readMaskPos = maskBytes(c.readMaskKey, c.readMaskPos, b[:n])
 			}
@@ -1096,7 +1088,7 @@ func (r *messageReader) Read(b []byte) (int, error) {
 		frameType, err := c.advanceFrame()
 		switch {
 		case err != nil:
-			c.readErr = hideTempErr(err)
+			c.readErr = err
 		case frameType == TextMessage || frameType == BinaryMessage:
 			c.readErr = errors.New("websocket: internal error, unexpected text or binary in Reader")
 		}

conn_test.go

@@ -814,3 +814,25 @@ func TestFormatMessageType(t *testing.T) {
 		t.Error("failed to format message type")
 	}
 }
+
+type fakeNetClosedReader struct {
+}
+
+func (r fakeNetClosedReader) Read([]byte) (int, error) {
+	return 0, net.ErrClosed
+}
+
+func TestConnectionClosed(t *testing.T) {
+	var b1, b2 bytes.Buffer
+
+	client := newTestConn(fakeNetClosedReader{}, &b1, false)
+	server := newTestConn(fakeNetClosedReader{}, &b2, true)
+
+	if _, _, err := server.NextReader(); !errors.Is(err, net.ErrClosed) {
+		t.Fatalf("server expects a net.ErrClosed error, %v returned", err)
+	}
+
+	if _, _, err := client.NextReader(); !errors.Is(err, net.ErrClosed) {
+		t.Fatalf("client expects a net.ErrClosed error, %v returned", err)
+	}
+}