fix(discord): nil-conn deref in listen crashes process on network failure — snapshot conn under lock, no lockless g.conn.ReadJSON (#102)

The gateway read loop called g.conn.ReadJSON directly with no lock and no
nil-guard, while the reconnect/close path nils g.conn under g.mu. A network
failure landing mid-iteration crashed the process with a SIGSEGV inside
gorilla/websocket's NextReader on a nil *Conn.

listen now snapshots g.conn under g.mu once per iteration and reads via the
local snapshot; a nil snapshot returns cleanly, which reconnectLoop already
treats as a signal to reconnect. This also removes the data race on g.conn
between listen and the reconnect/close path.

Author: alekspetrov

sdk/integrations/discord/transport.go

@@ -186,8 +186,16 @@ func (g *GatewayClient) listen(ctx context.Context) <-chan GatewayEvent {
 			default:
 			}
 
+			g.mu.Lock()
+			c := g.conn
+			g.mu.Unlock()
+			if c == nil {
+				g.log.Warn("discord: listen: connection is nil, ending read loop")
+				return
+			}
+
 			var event GatewayEvent
-			if err := g.conn.ReadJSON(&event); err != nil {
+			if err := c.ReadJSON(&event); err != nil {
 				code := extractCloseCode(err)
 				if code != 0 {
 					if isFatalCloseCode(code) {

sdk/integrations/discord/transport_test.go

@@ -0,0 +1,102 @@
+package discord
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/gorilla/websocket"
+)
+
+func discardLogger() *slog.Logger {
+	return slog.New(slog.NewTextHandler(io.Discard, nil))
+}
+
+// TestListenNilConnNoPanic verifies that listen() does not dereference a nil
+// g.conn: it must return (closing its output channel) instead of panicking.
+func TestListenNilConnNoPanic(t *testing.T) {
+	g := &GatewayClient{
+		stopCh: make(chan struct{}),
+		log:    discardLogger(),
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	out := g.listen(ctx)
+
+	select {
+	case _, ok := <-out:
+		if ok {
+			t.Fatal("expected output channel to close without emitting an event")
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("listen did not return promptly on a nil connection")
+	}
+}
+
+// TestListenConnRace exercises listen() reading from a snapshot of g.conn
+// while a concurrent goroutine nils g.conn out from under it (the
+// reconnect/close path), the way reconnectLoop and Close do in production.
+// Run with -race to confirm there is no data race on g.conn.
+func TestListenConnRace(t *testing.T) {
+	upgrader := websocket.Upgrader{}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		conn, err := upgrader.Upgrade(w, r, nil)
+		if err != nil {
+			return
+		}
+		defer func() { _ = conn.Close() }()
+		for {
+			if _, _, err := conn.ReadMessage(); err != nil {
+				return
+			}
+		}
+	}))
+	defer srv.Close()
+
+	wsURL := "ws" + strings.TrimPrefix(srv.URL, "http")
+
+	g := &GatewayClient{
+		stopCh: make(chan struct{}),
+		log:    discardLogger(),
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	for i := 0; i < 20; i++ {
+		conn, _, err := websocket.DefaultDialer.Dial(wsURL, nil)
+		if err != nil {
+			t.Fatalf("dial: %v", err)
+		}
+		g.mu.Lock()
+		g.conn = conn
+		g.mu.Unlock()
+
+		out := g.listen(ctx)
+
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			g.mu.Lock()
+			c := g.conn
+			g.conn = nil
+			g.mu.Unlock()
+			if c != nil {
+				_ = c.Close()
+			}
+		}()
+
+		for range out {
+		}
+		wg.Wait()
+	}
+}