snapshot: sagernet/gvisor v0.0.0-20260727.0-sing-box-mod.1 + SPEC 048 guard

Обновление снапшота с v0.0.0-20250811.0 на пин, которого требует
sing-box после мержа 235 коммитов (upstream d620bbbf2 "Update gvisor to
20260727.0"). Прежний снапшот был взят 2026-08-04 ровно с той версии,
на которой тогда стоял апстрим; разрыв возник 2026-08-05 вместе с его
бампом.

За год апстрим-gvisor изменил ~14 000 строк в 292 файлах. Значимое для
нас — сетевой стек: tcp/connect.go (PMTU-discovery + исправление
начального RTT/RTO: раньше задержка ACK внутри стека завышала стартовый
таймаут на несколько RTT), tcp/snd.go, tcp/rcv.go, stack/conntrack.go,
stack/packet_buffer.go. Всего 30 файлов в TCP и 37 в stack.

Баг SPEC 048 апстрим НЕ исправил — проверено по коду новой версии:
handleConnecting по-прежнему проверяет состояние endpoint'а, но не ep.h,
а performHandshake так же зануляет h и отпускает мьютекс до Close().
Поэтому guard перенесён (12 строк) вместе со своим тестом (45 строк).

Red/green проверен на новой базе: без guard'а тест падает с той же
nil-паникой, что в полевом крашдампе; с ним зелёный.

pkg/context/context.go

@@ -58,25 +58,32 @@ type Blocker interface {
 	// is interrupted.
 	Block(C <-chan struct{}) error
 
+	// BlockWithTimeout blocks until an event is received from C, the timeout
+	// has elapsed (only if haveTimeout is true), or some external interrupt.
+	//
+	// It returns:
+	//   - The remaining timeout, which is guaranteed to be 0 if the timeout
+	//     expired, and is unspecified if haveTimeout is false.
+	//   - An error which if the timeout expired or if interrupted.
+	BlockWithTimeout(C chan struct{}, haveTimeout bool, timeout time.Duration) (time.Duration, error)
+
 	// BlockWithTimeoutOn blocks until either the conditions of Block are
 	// satisfied, or the timeout is hit. Note that deadlines are not supported
 	// since the notion of "with respect to what clock" is not resolved.
 	//
-	// The return value is per BlockOn.
+	// It returns:
+	//   - The remaining timeout, which is guaranteed to be 0 if the timeout
+	//     expired.
+	//   - Boolean as per BlockOn return value.
 	BlockWithTimeoutOn(waiter.Waitable, waiter.EventMask, time.Duration) (time.Duration, bool)
 
 	// UninterruptibleSleepStart indicates the beginning of an uninterruptible
-	// sleep state (equivalent to Linux's TASK_UNINTERRUPTIBLE). If deactivate
-	// is true and the Context represents a Task, the Task's AddressSpace is
-	// deactivated.
-	UninterruptibleSleepStart(deactivate bool)
+	// sleep state (equivalent to Linux's TASK_UNINTERRUPTIBLE).
+	UninterruptibleSleepStart()
 
 	// UninterruptibleSleepFinish indicates the end of an uninterruptible sleep
-	// state that was begun by a previous call to UninterruptibleSleepStart. If
-	// activate is true and the Context represents a Task, the Task's
-	// AddressSpace is activated. Normally activate is the same value as the
-	// deactivate parameter passed to UninterruptibleSleepStart.
-	UninterruptibleSleepFinish(activate bool)
+	// state that was begun by a previous call to UninterruptibleSleepStart.
+	UninterruptibleSleepFinish()
 }
 
 // NoTask is an implementation of Blocker that does not block.
@@ -94,7 +101,7 @@ func (nt *NoTask) Interrupt() {
 
 // Interrupted implements Blocker.Interrupted.
 func (nt *NoTask) Interrupted() bool {
-	return nt.cancel != nil && len(nt.cancel) > 0
+	return len(nt.cancel) > 0
 }
 
 // Killed implements Blocker.Killed.
@@ -131,34 +138,47 @@ func (nt *NoTask) BlockOn(w waiter.Waitable, mask waiter.EventMask) bool {
 	}
 }
 
-// BlockWithTimeoutOn implements Blocker.BlockWithTimeoutOn.
-func (nt *NoTask) BlockWithTimeoutOn(w waiter.Waitable, mask waiter.EventMask, duration time.Duration) (time.Duration, bool) {
+// BlockWithTimeout implements Blocker.BlockWithTimeout.
+func (nt *NoTask) BlockWithTimeout(C chan struct{}, haveTimeout bool, timeout time.Duration) (time.Duration, error) {
+	if !haveTimeout {
+		return timeout, nt.Block(C)
+	}
+
 	if nt.cancel == nil {
 		nt.cancel = make(chan struct{}, 1)
 	}
-	e, ch := waiter.NewChannelEntry(mask)
-	w.EventRegister(&e)
-	defer w.EventUnregister(&e)
 	start := time.Now() // In system time.
-	t := time.AfterFunc(duration, func() { ch <- struct{}{} })
+	remainingTimeout := func() time.Duration {
+		rt := timeout - time.Since(start)
+		if rt < 0 {
+			rt = 0
+		}
+		return rt
+	}
 	select {
 	case <-nt.cancel:
-		return time.Since(start), false // Interrupted.
-	case _, ok := <-ch:
-		if ok && t.Stop() {
-			// Timer never fired.
-			return time.Since(start), ok
-		}
-		// Timer fired, remain is zero.
-		return time.Duration(0), ok
+		return remainingTimeout(), errors.New("interrupted system call") // Interrupted.
+	case <-C:
+		return remainingTimeout(), nil
+	case <-time.After(timeout):
+		return 0, errors.New("timeout expired")
 	}
 }
 
+// BlockWithTimeoutOn implements Blocker.BlockWithTimeoutOn.
+func (nt *NoTask) BlockWithTimeoutOn(w waiter.Waitable, mask waiter.EventMask, timeout time.Duration) (time.Duration, bool) {
+	e, ch := waiter.NewChannelEntry(mask)
+	w.EventRegister(&e)
+	defer w.EventUnregister(&e)
+	left, err := nt.BlockWithTimeout(ch, true, timeout)
+	return left, err == nil
+}
+
 // UninterruptibleSleepStart implmenents Blocker.UninterruptedSleepStart.
-func (*NoTask) UninterruptibleSleepStart(bool) {}
+func (*NoTask) UninterruptibleSleepStart() {}
 
 // UninterruptibleSleepFinish implmenents Blocker.UninterruptibleSleepFinish.
-func (*NoTask) UninterruptibleSleepFinish(bool) {}
+func (*NoTask) UninterruptibleSleepFinish() {}
 
 // Context represents a thread of execution (hereafter "goroutine" to reflect
 // Go idiosyncrasy). It carries state associated with the goroutine across API
@@ -236,3 +256,28 @@ func (ctx *withValue) Value(key any) any {
 	}
 	return ctx.Context.Value(key)
 }
+
+// WithValues returns a copy of parent in which the values associated with keys
+// are the corresponding values in the map.
+func WithValues(parent Context, values map[any]any) Context {
+	if len(values) == 0 {
+		return parent
+	}
+	return &withValues{
+		Context: parent,
+		values:  values,
+	}
+}
+
+type withValues struct {
+	Context
+	values map[any]any
+}
+
+// Value implements Context.Value.
+func (ctx *withValues) Value(key any) any {
+	if val, ok := ctx.values[key]; ok {
+		return val
+	}
+	return ctx.Context.Value(key)
+}