fix: suppress net.ErrClosed on concurrent Close

Close and CloseNow return net.ErrClosed when the read loop or
write path wins the casClosing race. The defer that suppresses
net.ErrClosed is only registered on the winner path, so the
user-facing call surfaces a wrapped "use of closed network
connection" even though the close succeeded.

Move the defer before the casClosing check so both paths
suppress net.ErrClosed, matching the godoc contract that
additional calls to Close are no-ops.

Author: mafredri

close.go

@@ -98,20 +98,16 @@ func CloseStatus(err error) StatusCode {
 // complete.
 func (c *Conn) Close(code StatusCode, reason string) (err error) {
 	defer errd.Wrap(&err, "failed to close WebSocket")
-
-	if c.casClosing() {
-		err = c.waitGoroutines()
-		if err != nil {
-			return err
-		}
-		return net.ErrClosed
-	}
 	defer func() {
 		if errors.Is(err, net.ErrClosed) {
 			err = nil
 		}
 	}()
 
+	if c.casClosing() {
+		return c.waitGoroutines()
+	}
+
 	err = c.closeHandshake(code, reason)
 
 	err2 := c.close()
@@ -131,20 +127,16 @@ func (c *Conn) Close(code StatusCode, reason string) (err error) {
 // Use when you do not want the overhead of the close handshake.
 func (c *Conn) CloseNow() (err error) {
 	defer errd.Wrap(&err, "failed to immediately close WebSocket")
-
-	if c.casClosing() {
-		err = c.waitGoroutines()
-		if err != nil {
-			return err
-		}
-		return net.ErrClosed
-	}
 	defer func() {
 		if errors.Is(err, net.ErrClosed) {
 			err = nil
 		}
 	}()
 
+	if c.casClosing() {
+		return c.waitGoroutines()
+	}
+
 	err = c.close()
 
 	err2 := c.waitGoroutines()

conn_test.go

@@ -397,8 +397,8 @@ func TestConn(t *testing.T) {
 		assert.Success(t, err2)
 		err1 = c1.CloseNow()
 		err2 = c2.CloseNow()
-		assert.ErrorIs(t, websocket.ErrClosed, err1)
-		assert.ErrorIs(t, websocket.ErrClosed, err2)
+		assert.Success(t, err1)
+		assert.Success(t, err2)
 	})
 
 	t.Run("MidReadClose", func(t *testing.T) {