examples/chat: Fix race condition

Tricky tricky.

internal/examples/chat/chat.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"io"
 	"log"
+	"net"
 	"net/http"
 	"sync"
 	"time"
@@ -69,14 +70,7 @@ func (cs *chatServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 // subscribeHandler accepts the WebSocket connection and then subscribes
 // it to all future messages.
 func (cs *chatServer) subscribeHandler(w http.ResponseWriter, r *http.Request) {
-	c, err := websocket.Accept(w, r, nil)
-	if err != nil {
-		cs.logf("%v", err)
-		return
-	}
-	defer c.CloseNow()
-
-	err = cs.subscribe(r.Context(), c)
+	err := cs.subscribe(r.Context(), w, r)
 	if errors.Is(err, context.Canceled) {
 		return
 	}
@@ -117,18 +111,39 @@ func (cs *chatServer) publishHandler(w http.ResponseWriter, r *http.Request) {
 //
 // It uses CloseRead to keep reading from the connection to process control
 // messages and cancel the context if the connection drops.
-func (cs *chatServer) subscribe(ctx context.Context, c *websocket.Conn) error {
-	ctx = c.CloseRead(ctx)
-
+func (cs *chatServer) subscribe(ctx context.Context, w http.ResponseWriter, r *http.Request) error {
+	var mu sync.Mutex
+	var c *websocket.Conn
+	var closed bool
 	s := &subscriber{
 		msgs: make(chan []byte, cs.subscriberMessageBuffer),
 		closeSlow: func() {
+			mu.Lock()
+			defer mu.Unlock()
+			closed = true
+			if c != nil {
 				c.Close(websocket.StatusPolicyViolation, "connection too slow to keep up with messages")
+			}
 		},
 	}
 	cs.addSubscriber(s)
 	defer cs.deleteSubscriber(s)
 
+	c2, err := websocket.Accept(w, r, nil)
+	if err != nil {
+		return err
+	}
+	mu.Lock()
+	if closed {
+		mu.Unlock()
+		return net.ErrClosed
+	}
+	c = c2
+	mu.Unlock()
+	defer c.CloseNow()
+
+	ctx = c.CloseRead(ctx)
+
 	for {
 		select {
 		case msg := <-s.msgs: