视频流式解密播放：从技术挑战到完整解决方案

func (s *Server) streamFile(c *gin.Context) {
    // 1. 加载分块元数据
    chunkMeta, _ := s.loadChunkMeta(ctx, fileID, fileKey)
    
    originalSize := chunkMeta.OriginalSize  // 原始文件大小
    chunkSize := int64(chunkMeta.ChunkSize) // 分块大小 (5MB)
    totalChunks := int64(chunkMeta.TotalChunks)
    
    // 2. 解析 Range 请求
    // 例如: "Range: bytes=50000000-60000000"
    rangeHeader := c.GetHeader("Range") // "bytes=50000000-60000000"
    rangeStart, rangeEnd := parseRange(rangeHeader)
    
    // 3. 计算需要的分块范围
    // 假设 chunkSize = 5MB = 5242880
    // rangeStart = 50000000
    // startChunk = 50000000 / 5242880 = 9
    // endChunk = 60000000 / 5242880 = 11
    startChunk := rangeStart / chunkSize  // 第 9 个分块
    endChunk := rangeEnd / chunkSize      // 第 11 个分块
    
    // 4. 只下载需要的分块（不是整个文件！）
    var decryptedData bytes.Buffer
    for i := startChunk; i <= endChunk; i++ {
        chunkKey := fmt.Sprintf("files/%s/chunks/%d.enc", fileID, i)
        chunkData, _ := s.s3.DownloadBytes(ctx, chunkKey)
        
        decrypted, _ := s.encryptor.DecryptChunkSimple(chunkData, fileKey)
        decryptedData.Write(decrypted)
    }
    
    // 5. 从解密数据中截取精确的请求范围
    fullData := decryptedData.Bytes()
    offsetInFirstChunk := rangeStart - startChunk*chunkSize
    responseLen := rangeEnd - rangeStart + 1
    responseData := fullData[offsetInFirstChunk : offsetInFirstChunk+responseLen]
    
    // 6. 返回 206 Partial Content
    c.Header("Content-Range", fmt.Sprintf("bytes %d-%d/%d", rangeStart, rangeEnd, originalSize))
    c.Header("Accept-Ranges", "bytes")
    c.Data(http.StatusPartialContent, contentType, responseData)
}

const maxResponseSize = 10 * 1024 * 1024 // 10MB
 
// Range 请求没有指定结束位置时，限制响应大小
if len(parts) > 1 && parts[1] != "" {
    rangeEnd, _ = strconv.ParseInt(parts[1], 10, 64)
} else {
    // bytes=0- 这种请求，限制最多返回 10MB
    rangeEnd = rangeStart + maxResponseSize - 1
    if rangeEnd >= originalSize {
        rangeEnd = originalSize - 1
    }
}
 
// 非 Range 请求也限制响应大小
if !isRangeRequest && originalSize > maxResponseSize {
    rangeEnd = maxResponseSize - 1
}

void _startUploadJob(_UploadJob job) {
  // ... 上传逻辑 ...
  
  if (result.isSuccess && result.data != null) {
    _files.add(result.data!);
    
    // 视频文件上传成功后，生成并上传缩略图
    if (isVideoFile(result.data!.mimeType, result.data!.name)) {
      _generateAndUploadVideoThumbnail(
        job.filePath,    // 本地原始文件路径（完整文件）
        result.data!.id,
      );
    }
  }
}
 
Future<void> _generateAndUploadVideoThumbnail(
  String videoPath,
  String fileId,
) async {
  try {
    final tempDir = await getTemporaryDirectory();
    final thumbPath = path.join(tempDir.path, 'thumb_$fileId.jpg');
    
    // 使用原生 API 生成缩略图（Android: MediaMetadataRetriever）
    final plugin = FcNativeVideoThumbnail();
    final success = await plugin.getVideoThumbnail(
      srcFile: videoPath,  // 完整的本地文件，100% 成功
      destFile: thumbPath,
      width: 256,
      height: 256,
      format: 'jpeg',
      quality: 85,
    );
    
    if (success) {
      final thumbFile = io.File(thumbPath);
      if (await thumbFile.exists()) {
        final thumbData = await thumbFile.readAsBytes();
        await api.uploadThumbnail(fileId, thumbData);  // 上传到后端
        await thumbFile.delete();
      }
    }
  } catch (e) {
    // 缩略图生成失败不影响上传流程
    debugPrint('[VideoThumb] Failed: $e');
  }
}

func (s *Server) uploadThumbnail(c *gin.Context) {
    fileID := c.Param("id")
    data, _ := io.ReadAll(c.Request.Body)
    
    // 加密缩略图
    thumbKey := crypto.DeriveFileKey(s.masterKey, fileID+"-thumb")
    encrypted, _ := s.encryptor.EncryptChunkSimple(data, thumbKey)
    
    // 上传到 S3
    s3Key := fmt.Sprintf("thumbs/%s.enc", fileID)
    s.s3.UploadBytes(ctx, s3Key, encrypted, "application/octet-stream")
}

enum TransferStatus { 
  queued,     // 排队中
  running,    // 传输中
  finishing,  // 处理中 ← 新增
  success,    // 完成
  failed      // 失败
}

onProgress: (sent, total) {
  final prog = total > 0 ? sent / total : 0.0;
  final isFinishing = prog >= 1.0;
  
  _transfers[idx] = _transfers[idx].copyWith(
    progress: prog,
    // 进度到 100% 时切换到 finishing 状态
    status: isFinishing ? TransferStatus.finishing : TransferStatus.running,
  );
}

if (item.status == TransferStatus.running)
  LinearProgressIndicator(value: item.progress), // 确定进度
  
if (item.status == TransferStatus.finishing)
  LinearProgressIndicator(), // 不确定进度（动画）
  Text('处理中...'),