system stack: self-heal the TCP forwarder accept loop (sing-box-lx SPEC 040)

Upstream acceptLoop treats any Accept error as terminal and silently
returns, leaving the stack alive but every new TCP SYN NAT-rewritten onto
a dead port (instant RST) until a full restart. When the listener fd is
closed out from under the stack (a stray close on a reused fd number from
another runtime in the same process), all new TCP dies forever while
UDP/QUIC/DNS keep working.

- System.Close() now marks a deliberate shutdown first; acceptLoop still
  exits quietly on it.
- Any other Accept error is logged (the errno names the killer path),
  the listener is recreated on the same address, the forwarder port is
  republished atomically, and the loop keeps serving.
- If the rebind fails, the loop logs an error and gives up - no worse
  than upstream.
- acceptRecoveries counter is kept as telemetry.

tcpPort/tcpPort6 become atomic (written by the heal path, read from the
tunLoop dispatch/NAT path); listener replacement is serialized against
Close() with a mutex.

stack_system_selfheal_test.go

@@ -0,0 +1,117 @@
+package tun
+
+// lx/040 (SPECS/TASKS/040-SINGTUN_ACCEPTLOOP_SELFHEAL): acceptLoop self-heal.
+//
+// Red/green против апстрима 2d9b8aed5fe2: там acceptLoop(listener) при любой
+// ошибке Accept молча выходит навсегда — восстановления нет, порт не меняется,
+// новый connect вечно бьётся в мёртвый сокет. Для red-прогона на чистом
+// апстрим-чекауте достаточно адаптировать хелперы ниже (currentTCPPort →
+// s.tcpPort, spawnAcceptLoop → go s.acceptLoop(ln)): тест упадёт по таймауту
+// ожидания восстановления.
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/netip"
+	"testing"
+	"time"
+
+	"github.com/sagernet/sing/common/logger"
+)
+
+func newSelfHealTestSystem(t *testing.T) *System {
+	t.Helper()
+	s := &System{
+		ctx:          context.Background(),
+		logger:       logger.NOP(),
+		inet4Address: netip.MustParseAddr("127.0.0.1"),
+		udpTimeout:   time.Minute,
+	}
+	s.tcpNat = NewNat(s.ctx, s.udpTimeout)
+	ln, err := s.listenTCP(false)
+	if err != nil {
+		t.Fatalf("listenTCP: %v", err)
+	}
+	s.tcpListener = ln
+	s.tcpPort.Store(uint32(ln.Addr().(*net.TCPAddr).Port))
+	spawnAcceptLoop(s, ln)
+	return s
+}
+
+func currentTCPPort(s *System) uint32 {
+	return s.tcpPort.Load()
+}
+
+func spawnAcceptLoop(s *System, ln net.Listener) {
+	go s.acceptLoop(ln, false)
+}
+
+func dialForwarder(t *testing.T, port uint32) error {
+	t.Helper()
+	conn, err := net.DialTimeout("tcp4", fmt.Sprintf("127.0.0.1:%d", port), time.Second)
+	if err == nil {
+		_ = conn.Close()
+	}
+	return err
+}
+
+// Убийство listener'а мимо System.Close (эмуляция чужого close по
+// переиспользованному fd-номеру) должно приводить к пересозданию listener'а
+// и продолжению приёма TCP, а не к вечной смерти петли.
+func TestSystemAcceptLoopSelfHeal(t *testing.T) {
+	s := newSelfHealTestSystem(t)
+	oldPort := currentTCPPort(s)
+
+	if err := dialForwarder(t, oldPort); err != nil {
+		t.Fatalf("healthy listener refused connect: %v", err)
+	}
+
+	// Убить listener из-под стека: closing НЕ выставлен.
+	_ = s.tcpListener.Close()
+
+	deadline := time.Now().Add(5 * time.Second)
+	healed := false
+	for time.Now().Before(deadline) {
+		if s.acceptRecoveries.Load() > 0 {
+			healed = true
+			break
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	if !healed {
+		t.Fatalf("acceptLoop did not recover within 5s (upstream behavior: silent permanent death)")
+	}
+
+	newPort := currentTCPPort(s)
+	if newPort == oldPort {
+		t.Fatalf("recovered port equals dead port %d — relisten did not publish a new port", oldPort)
+	}
+	if err := dialForwarder(t, newPort); err != nil {
+		t.Fatalf("connect to recreated listener (port %d) failed: %v", newPort, err)
+	}
+	if got := s.acceptRecoveries.Load(); got != 1 {
+		t.Fatalf("acceptRecoveries = %d, want 1", got)
+	}
+
+	s.closing.Store(true)
+	_ = s.tcpListener.Close()
+}
+
+// Штатное закрытие (closing выставлен, как это делает System.Close) обязано
+// оставаться тихим: без пересозданий и без роста счётчика.
+func TestSystemAcceptLoopQuietOnClose(t *testing.T) {
+	s := newSelfHealTestSystem(t)
+	oldPort := currentTCPPort(s)
+
+	s.closing.Store(true)
+	_ = s.tcpListener.Close()
+
+	time.Sleep(300 * time.Millisecond)
+	if got := s.acceptRecoveries.Load(); got != 0 {
+		t.Fatalf("deliberate close triggered %d recoveries, want 0", got)
+	}
+	if port := currentTCPPort(s); port != oldPort {
+		t.Fatalf("deliberate close changed port %d → %d", oldPort, port)
+	}
+}