fix(service): 使用 os.Root 修复 Sora 存储路径告警

- 将媒体写入和删除切换为 os.Root 沙箱 API
- 移除旧的路径拼接校验辅助函数并收敛删除逻辑
- 调整并新增相关单元测试覆盖删除行为

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/sora_media_storage.go

@@ -220,17 +220,20 @@ func (s *SoraMediaStorage) downloadOnce(ctx context.Context, root, mediaType, ra
 		return "", fmt.Errorf("download size exceeds limit: %d", resp.ContentLength)
 	}
 
-	datePath := time.Now().Format("2006/01/02")
-	destDir := filepath.Join(root, filepath.FromSlash(datePath))
-	if err := os.MkdirAll(destDir, 0o755); err != nil {
-		return "", err
-	}
-	filename := uuid.NewString() + ext
-	destPath, err := joinPathWithinDir(destDir, filename)
+	storageRoot, err := os.OpenRoot(root)
 	if err != nil {
 		return "", err
 	}
-	out, err := os.OpenFile(destPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o644)
+	defer func() { _ = storageRoot.Close() }()
+
+	datePath := time.Now().Format("2006/01/02")
+	datePathFS := filepath.FromSlash(datePath)
+	if err := storageRoot.MkdirAll(datePathFS, 0o755); err != nil {
+		return "", err
+	}
+	filename := uuid.NewString() + ext
+	filePath := filepath.Join(datePathFS, filename)
+	out, err := storageRoot.OpenFile(filePath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o644)
 	if err != nil {
 		return "", err
 	}
@@ -239,11 +242,11 @@ func (s *SoraMediaStorage) downloadOnce(ctx context.Context, root, mediaType, ra
 	limited := io.LimitReader(resp.Body, s.maxDownloadBytes+1)
 	written, err := io.Copy(out, limited)
 	if err != nil {
-		removePartialDownload(destDir, filename)
+		removePartialDownload(storageRoot, filePath)
 		return "", err
 	}
 	if s.maxDownloadBytes > 0 && written > s.maxDownloadBytes {
-		removePartialDownload(destDir, filename)
+		removePartialDownload(storageRoot, filePath)
 		return "", fmt.Errorf("download size exceeds limit: %d", written)
 	}
 
@@ -296,26 +299,9 @@ func normalizeSoraFileExt(ext string) string {
 	}
 }
 
-func joinPathWithinDir(baseDir, filename string) (string, error) {
-	baseDir = filepath.Clean(baseDir)
-	if strings.TrimSpace(filename) == "" {
-		return "", errors.New("empty filename")
-	}
-	joined := filepath.Clean(filepath.Join(baseDir, filename))
-	rel, err := filepath.Rel(baseDir, joined)
-	if err != nil {
-		return "", fmt.Errorf("resolve path rel: %w", err)
-	}
-	if rel == ".." || strings.HasPrefix(rel, ".."+string(os.PathSeparator)) {
-		return "", fmt.Errorf("path traversal detected: %s", filename)
-	}
-	return joined, nil
-}
-
-func removePartialDownload(baseDir, filename string) {
-	target, err := joinPathWithinDir(baseDir, filename)
-	if err != nil {
+func removePartialDownload(root *os.Root, filePath string) {
+	if root == nil || strings.TrimSpace(filePath) == "" {
 		return
 	}
-	_ = os.Remove(target)
+	_ = root.Remove(filePath)
 }

backend/internal/service/sora_media_storage_test.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -92,20 +93,27 @@ func TestSoraMediaStorage_MaxDownloadBytes(t *testing.T) {
 	require.Error(t, err)
 }
 
-func TestJoinPathWithinDir(t *testing.T) {
-	baseDir := t.TempDir()
-
-	path1, err := joinPathWithinDir(baseDir, "ok.png")
-	require.NoError(t, err)
-	require.Equal(t, filepath.Join(baseDir, "ok.png"), path1)
-
-	_, err = joinPathWithinDir(baseDir, "../escape.png")
-	require.Error(t, err)
-}
-
 func TestNormalizeSoraFileExt(t *testing.T) {
 	require.Equal(t, ".png", normalizeSoraFileExt(".PNG"))
 	require.Equal(t, ".mp4", normalizeSoraFileExt(".mp4"))
 	require.Equal(t, "", normalizeSoraFileExt("../../etc/passwd"))
 	require.Equal(t, "", normalizeSoraFileExt(".php"))
 }
+
+func TestRemovePartialDownload(t *testing.T) {
+	tmpDir := t.TempDir()
+	root, err := os.OpenRoot(tmpDir)
+	require.NoError(t, err)
+	defer func() { _ = root.Close() }()
+
+	filePath := "partial.bin"
+	f, err := root.OpenFile(filePath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o600)
+	require.NoError(t, err)
+	_, _ = f.WriteString("partial")
+	_ = f.Close()
+
+	removePartialDownload(root, filePath)
+	_, err = root.Stat(filePath)
+	require.Error(t, err)
+	require.True(t, os.IsNotExist(err))
+}