moves the writeLock to writeFrame to avoid deadlock

Author: FrauElster

close.go

@@ -169,12 +169,6 @@ func (c *Conn) closeHandshake(code StatusCode, reason string) error {
 }
 
 func (c *Conn) writeClose(code StatusCode, reason string) error {
-	c.closeMu.Lock()
-	defer c.closeMu.Unlock()
-	if c.closeWritten {
-		return nil
-	}
-
 	ce := CloseError{
 		Code:   code,
 		Reason: reason,
@@ -199,7 +193,6 @@ func (c *Conn) writeClose(code StatusCode, reason string) error {
 	if err != nil && !errors.Is(err, net.ErrClosed) {
 		return err
 	}
-	c.closeWritten = true
 	return nil
 }
 

conn.go

@@ -73,10 +73,10 @@ type Conn struct {
 	closeReadCtx  context.Context
 	closeReadDone chan struct{}
 
-	closed       chan struct{}
-	closeMu      sync.Mutex
-	closing      bool
-	closeWritten bool
+	closed    chan struct{}
+	closeMu   sync.Mutex
+	closing   bool
+	closeSent bool
 
 	pingCounter   atomic.Int64
 	activePingsMu sync.Mutex

write.go

@@ -20,6 +20,9 @@ import (
 	"github.com/coder/websocket/internal/util"
 )
 
+// ErrAlreadyClosed is returned when a close frame has already been sent and another is attempted.
+var ErrAlreadyClosed = errors.New("close frame already sent")
+
 // Writer returns a writer bounded by the context that will write
 // a WebSocket message of type dataType to the connection.
 //
@@ -268,6 +271,10 @@ func (c *Conn) writeFrame(ctx context.Context, fin bool, flate bool, opcode opco
 		}
 	}()
 
+	if opcode == opClose && c.closeSent {
+		return 0, ErrAlreadyClosed
+	}
+
 	c.writeHeader.fin = fin
 	c.writeHeader.opcode = opcode
 	c.writeHeader.payloadLength = int64(len(p))
@@ -303,6 +310,10 @@ func (c *Conn) writeFrame(ctx context.Context, fin bool, flate bool, opcode opco
 		}
 	}
 
+	if opcode == opClose {
+		c.closeSent = true
+	}
+
 	select {
 	case <-c.closed:
 		if opcode == opClose {