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.

sandboxexec/sandbox/oci.go

@@ -0,0 +1,165 @@
+// Copyright 2026 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sandbox
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	specs "github.com/opencontainers/runtime-spec/specs-go"
+)
+
+// BundleConfig holds configuration for creating an OCI bundle.
+type BundleConfig struct {
+	ID               string
+	RuntimeDir       string
+	EnableNetworking bool
+	Mounts           []Mount
+	Env              []string
+	Annotations      map[string]string
+	WorkingDir       string
+	Hostname         string
+}
+
+// NewBundle creates a temporary OCI bundle on the fly with the given configuration.
+func NewBundle(cfg BundleConfig) (string, error) {
+	// Create a bundle directory for the sandbox.
+	bundleDir := filepath.Join(cfg.RuntimeDir, cfg.ID)
+	rootfsDir := filepath.Join(bundleDir, "rootfs")
+
+	if err := os.MkdirAll(rootfsDir, 0o755); err != nil {
+		return "", fmt.Errorf("failed to create bundle directories: %w", err)
+	}
+
+	// Define the OCI Specification programmatically.
+	namespaces := []specs.LinuxNamespace{
+		{Type: specs.PIDNamespace},
+		{Type: specs.MountNamespace},
+		{Type: specs.UTSNamespace},
+		{Type: specs.IPCNamespace},
+	}
+
+	if os.Geteuid() != 0 {
+		namespaces = append(namespaces, specs.LinuxNamespace{Type: specs.UserNamespace})
+	}
+	if cfg.EnableNetworking {
+		namespaces = append(namespaces, specs.LinuxNamespace{Type: specs.NetworkNamespace})
+	}
+
+	spec := &specs.Spec{
+		Version:     "1.0.0",
+		Annotations: cfg.Annotations,
+		Root: &specs.Root{
+			Path:     "rootfs",
+			Readonly: false,
+		},
+		Process: &specs.Process{
+			Terminal: false,
+			User:     specs.User{UID: 0, GID: 0},
+			// Keeps the sandbox alive in the background.
+			Args: []string{"sleep", "infinity"},
+			Cwd:  cfg.WorkingDir,
+		},
+		Mounts: []specs.Mount{
+			// Mandatory Linux API Filesystems
+			{Destination: "/proc", Type: "proc", Source: "proc"},
+			{Destination: "/dev", Type: "tmpfs", Source: "tmpfs"},
+		},
+		// enable basic namespaces for gVisor.
+		Linux: &specs.Linux{
+			Namespaces: namespaces,
+		},
+		Hostname: cfg.Hostname,
+	}
+
+	baseEnv := []string{"PATH=/bin:/usr/bin:/usr/local/bin"}
+	spec.Process.Env = append(baseEnv, cfg.Env...)
+
+	if os.Geteuid() != 0 {
+		spec.Linux.UIDMappings = []specs.LinuxIDMapping{
+			{ContainerID: 0, HostID: uint32(os.Geteuid()), Size: 1},
+		}
+		spec.Linux.GIDMappings = []specs.LinuxIDMapping{
+			{ContainerID: 0, HostID: uint32(os.Getegid()), Size: 1},
+		}
+	}
+
+	// Map host binaries & libraries as readonly. The binaries will be
+	// executed in gVisor sandbox, not on the host.
+	for _, p := range []string{"/bin", "/usr", "/lib", "/lib64", "/etc/alternatives"} {
+		if _, err := os.Stat(p); err == nil {
+			opts := []string{"rbind", "ro", "nosuid", "nodev"}
+			if p == "/etc/alternatives" {
+				opts = []string{"rbind", "ro"}
+			}
+			spec.Mounts = append(spec.Mounts, specs.Mount{
+				Destination: p,
+				Type:        "bind",
+				Source:      p,
+				Options:     opts,
+			})
+		}
+	}
+
+	// Add custom mounts. Custom mounts overriding default host mounts create duplicate OCI
+	// entries. The later entry overrides the earlier one, as expected by OCI specs.
+	for _, m := range cfg.Mounts {
+		switch m.Type {
+		case MountTypeBind:
+			opts := []string{"rbind"}
+			if m.ReadOnly {
+				opts = append(opts, "ro")
+			} else {
+				opts = append(opts, "rw")
+			}
+			spec.Mounts = append(spec.Mounts, specs.Mount{
+				Destination: filepath.Clean(m.Destination),
+				Source:      filepath.Clean(m.Source),
+				Type:        "bind",
+				Options:     opts,
+			})
+		case MountTypeTmpfs:
+			spec.Mounts = append(spec.Mounts, specs.Mount{
+				Destination: filepath.Clean(m.Destination),
+				Source:      "tmpfs",
+				Type:        "tmpfs",
+			})
+		case MountTypeProc:
+			spec.Mounts = append(spec.Mounts, specs.Mount{
+				Destination: filepath.Clean(m.Destination),
+				Source:      "proc",
+				Type:        "proc",
+			})
+		}
+	}
+
+	// Write the spec to config.json
+	configPath := filepath.Join(bundleDir, "config.json")
+	configFile, err := os.Create(configPath)
+	if err != nil {
+		return "", fmt.Errorf("failed to create config.json: %w", err)
+	}
+	defer configFile.Close()
+
+	encoder := json.NewEncoder(configFile)
+	encoder.SetIndent("", "  ")
+	if err := encoder.Encode(spec); err != nil {
+		return "", fmt.Errorf("failed to encode config.json: %w", err)
+	}
+
+	return bundleDir, nil
+}