缩略图上传时生成策略重构

// generateAndSaveImageThumbnail 从原始数据生成并保存图片缩略图
// 统一入口，复用于上传时生成和按需生成
func (s *Server) generateAndSaveImageThumbnail(ctx context.Context, fileID string, sourceData []byte) error {
    thumbData, err := s.generateThumbnail(sourceData)
    if err != nil {
        return err
    }
 
    // 加密并保存缩略图
    thumbKey := crypto.DeriveFileKey(s.getMasterKey(), fileID+"#thumb")
    var encryptedThumb bytes.Buffer
    if err := s.encryptor.EncryptStream(bytes.NewReader(thumbData), &encryptedThumb, thumbKey, int64(len(thumbData))); err != nil {
        return fmt.Errorf("thumbnail encryption failed: %w", err)
    }
 
    s3Key := fmt.Sprintf("thumbs/%s.enc", fileID)
    if err := s.s3.UploadBytes(ctx, s3Key, encryptedThumb.Bytes(), "application/octet-stream"); err != nil {
        return fmt.Errorf("upload thumbnail failed: %w", err)
    }
 
    log.Printf("[THUMB] Generated and saved thumbnail for %s (%d bytes)", fileID, len(thumbData))
    return nil
}

// generateImageThumbnailFromS3 从 S3 下载源文件并生成缩略图
// 用于 HTTP 上传完成后异步生成
func (s *Server) generateImageThumbnailFromS3(ctx context.Context, fileID string) error {
    fileKey := s.getFileKey(fileID)
    chunkMeta, err := s.loadChunkMeta(ctx, fileID, fileKey)
    if err != nil {
        return fmt.Errorf("load chunk meta failed: %w", err)
    }
 
    // 下载并解密所有分块
    var decryptedSource bytes.Buffer
    for i := 0; i < chunkMeta.TotalChunks; i++ {
        chunkKey := fmt.Sprintf("files/%s/chunks/%d.enc", fileID, i)
        chunkData, err := s.s3.DownloadBytes(ctx, chunkKey)
        if err != nil {
            return fmt.Errorf("download chunk %d failed: %w", i, err)
        }
        decrypted, err := s.encryptor.DecryptChunkSimple(chunkData, fileKey)
        if err != nil {
            return fmt.Errorf("decrypt chunk %d failed: %w", i, err)
        }
        decryptedSource.Write(decrypted)
    }
 
    return s.generateAndSaveImageThumbnail(ctx, fileID, decryptedSource.Bytes())
}

// generateImageThumbnailFromLocalFile 从本地文件生成缩略图
// 用于路径上传完成后，零 S3 流量
func (s *Server) generateImageThumbnailFromLocalFile(ctx context.Context, fileID, filePath string) error {
    data, err := os.ReadFile(filePath)
    if err != nil {
        return fmt.Errorf("read local file failed: %w", err)
    }
    return s.generateAndSaveImageThumbnail(ctx, fileID, data)
}

// 图片文件上传完成后，异步生成缩略图
if isImageFile(header.Filename) {
    go func() {
        if err := s.generateImageThumbnailFromS3(context.Background(), fileID); err != nil {
            log.Printf("[UPLOAD] Failed to generate thumbnail for %s: %v", header.Filename, err)
        }
    }()
}

// 图片文件上传完成后，从本地文件生成缩略图（零 S3 流量）
if isImageFile(fileName) {
    go func() {
        if err := s.generateImageThumbnailFromLocalFile(context.Background(), fileID, filePath); err != nil {
            log.Printf("[UPLOAD-PATH] Failed to generate thumbnail for %s: %v", fileName, err)
        }
    }()
}