fix: suppress net.ErrClosed on concurrent Close

Close and CloseNow returned a wrapped net.ErrClosed when the
internal read loop or write path won the casClosing race before
the user-facing call. The defer that suppresses net.ErrClosed
was only registered on the CAS winner path.

Separate user-facing close tracking from internal close
coordination. A userClosed atomic tracks whether the user has
called Close/CloseNow before. Combined with isClosed, this
gives three distinct behaviors:

- First user call, close in progress or peer closed: nil.
- Concurrent user calls while close is in progress: nil,
  returns instantly without blocking.
- User call after close has fully completed: net.ErrClosed.

Author: mafredri

close.go

@@ -99,11 +99,7 @@ func CloseStatus(err error) StatusCode {
 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
-		}
+	if c.userClosed.Swap(true) && c.isClosed() {
 		return net.ErrClosed
 	}
 	defer func() {
@@ -112,6 +108,10 @@ func (c *Conn) Close(code StatusCode, reason string) (err error) {
 		}
 	}()
 
+	if c.casClosing() {
+		return c.waitGoroutines()
+	}
+
 	err = c.closeHandshake(code, reason)
 
 	err2 := c.close()
@@ -132,11 +132,7 @@ func (c *Conn) Close(code StatusCode, reason string) (err error) {
 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
-		}
+	if c.userClosed.Swap(true) && c.isClosed() {
 		return net.ErrClosed
 	}
 	defer func() {
@@ -145,6 +141,10 @@ func (c *Conn) CloseNow() (err error) {
 		}
 	}()
 
+	if c.casClosing() {
+		return c.waitGoroutines()
+	}
+
 	err = c.close()
 
 	err2 := c.waitGoroutines()

conn.go

@@ -77,9 +77,10 @@ type Conn struct {
 	closeReadCtx  context.Context
 	closeReadDone chan struct{}
 
-	closing atomic.Bool
-	closeMu sync.Mutex // Protects following.
-	closed  chan struct{}
+	userClosed atomic.Bool // Set by Close/CloseNow on first user call.
+	closing    atomic.Bool
+	closeMu    sync.Mutex // Protects following.
+	closed     chan struct{}
 
 	pingCounter    atomic.Int64
 	activePingsMu  sync.Mutex