Merge pull request #4532 from ATKasem/fix/listener-fd-leak-error-paths

🐛 fix: close the listener on Listen error paths to avoid FD leaks

Author: ReneWerner87

listen.go

@@ -267,6 +267,16 @@ func (app *App) Listen(addr string, config ...ListenConfig) error {
 		return fmt.Errorf("failed to listen: %w", err)
 	}
 
+	// Close the listener on any path that doesn't reach Serve (which otherwise
+	// takes ownership of it) — an early error return or a panicking hook — so
+	// the bound socket isn't leaked.
+	served := false
+	defer func() {
+		if !served {
+			_ = ln.Close() //nolint:errcheck // best-effort cleanup on the error path
+		}
+	}()
+
 	// prepare the server for the start
 	app.startupProcess()
 
@@ -285,6 +295,7 @@ func (app *App) Listen(addr string, config ...ListenConfig) error {
 		}
 	}
 
+	served = true
 	return app.server.Serve(ln)
 }
 
@@ -377,6 +388,7 @@ func (*App) createListener(addr string, tlsConfig *tls.Config, cfg *ListenConfig
 
 	if cfg.ListenerNetwork == NetworkUnix {
 		if err = os.Chmod(addr, cfg.UnixSocketFileMode); err != nil {
+			_ = listener.Close() //nolint:errcheck // best-effort cleanup on the error path
 			return nil, fmt.Errorf("cannot chmod %#o for %q: %w", cfg.UnixSocketFileMode, addr, err)
 		}
 	}

listen_test.go

@@ -38,6 +38,30 @@ func Test_Listen(t *testing.T) {
 	require.NoError(t, app.Listen(":0", ListenConfig{DisableStartupMessage: true}))
 }
 
+// go test -run Test_Listen_ClosesListenerOnBeforeServeError
+func Test_Listen_ClosesListenerOnBeforeServeError(t *testing.T) {
+	// Grab a free port, then release it so we can bind it via Listen.
+	probe, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	addr := probe.Addr().String()
+	require.NoError(t, probe.Close())
+
+	app := New()
+	err = app.Listen(addr, ListenConfig{
+		DisableStartupMessage: true,
+		BeforeServeFunc: func(_ *App) error {
+			return errors.New("stop before serving")
+		},
+	})
+	require.Error(t, err)
+
+	// The listener must have been closed on the error path, so the port is
+	// immediately bindable again (a leaked listener would keep it bound).
+	ln, err := net.Listen("tcp", addr)
+	require.NoError(t, err, "listener leaked: port still bound after BeforeServeFunc error")
+	require.NoError(t, ln.Close())
+}
+
 // go test -run Test_Listen_Graceful_Shutdown
 func Test_Listen_Graceful_Shutdown(t *testing.T) {
 	t.Run("Basic Graceful Shutdown", func(t *testing.T) {