图片与对象存储：上传管线、图片处理与成本治理

// Go 后端签发预签名上传地址 (AWS S3 风格)
package upload

import (
	"context"
	"time"

	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/service/s3"
)

type UploadService struct {
	client *s3.Client
	bucket string
}

// CreatePresignedUpload 生成 15 分钟内有效的直传地址
func (s *UploadService) CreatePresignedUpload(ctx context.Context, objectKey string) (string, error) {
	psClient := s3.NewPresignClient(s.client)
	req, err := psClient.PresignPutObject(ctx, &s3.PutObjectInput{
		Bucket: aws.String(s.bucket),
		Key:    aws.String(objectKey),
	}, func(opt *s3.PresignOptions) {
		opt.Expires = 15 * time.Minute
	})
	if err != nil {
		return "", err
	}
	return req.URL, nil
}

// 前端上传流程
async function uploadImage(file) {
  // 1. 获取预签名 URL
  const { url, objectKey } = await fetch('/api/uploads', {
    method: 'POST',
    body: JSON.stringify({ filename: file.name, size: file.size, type: file.type })
  }).then(r => r.json());

  // 2. 直传对象存储
  const resp = await fetch(url, {
    method: 'PUT',
    body: file,
    headers: { 'Content-Type': file.type }
  });

  // 3. 上传完成后回传 key，写入短文
  return objectKey;
}

// Go 图片处理消费者示意 (使用 govips / libvips 绑定)
package imageproc

import (
	"bytes"
	"context"

	"github.com/davidbyttow/govips/v2/vips"
)

func init() {
	vips.Startup(nil)
}

func ProcessImage(ctx context.Context, src []byte) (map[string][]byte, error) {
	image, err := vips.NewImageFromBuffer(src)
	if err != nil {
		return nil, err
	}
	defer image.Close()

	results := make(map[string][]byte)
	sizes := map[string]int{"300w": 300, "600w": 600, "1200w": 1200}

	for name, width := range sizes {
		// 按宽度等比缩放
		if err := image.ResizeToWidth(width, vips.KernelLanczos3); err != nil {
			return nil, err
		}
		// 转为 WebP（quality 80）
		webp, err := image.ExportWebP(&vips.WebPExportParams{Quality: 80})
		if err != nil {
			return nil, err
		}
		results[name] = webp
		// 重置回原图，供下一个尺寸使用
		_ = image.LoadFromBuffer(src) // 简化示意
	}
	return results, nil
}

// <picture> 元素实现响应式图片
<picture>
  <source type="image/avif" srcset="/img/pic_1200w.avif 1200w, /img/pic_600w.avif 600w" />
  <source type="image/webp" srcset="/img/pic_1200w.webp 1200w, /img/pic_600w.webp 600w" />
  <img src="/img/pic_1200w.jpg" srcset="/img/pic_600w.jpg 600w" alt="配图" loading="lazy" />
</picture>

// 定义统一存储接口
package storage

import (
	"context"
	"io"
)

type ObjectStorage interface {
	Put(ctx context.Context, key string, r io.Reader, opts *PutOptions) error
	Get(ctx context.Context, key string) (io.ReadCloser, error)
	Delete(ctx context.Context, key string) error
	PresignPut(ctx context.Context, key string, ttlSeconds int) (string, error)
}

// AWS S3 实现
type S3Storage struct{ /* ... */ }

// R2 实现（同样走 S3 兼容 API）
type R2Storage struct{ /* ... */ }

// 本地 MinIO 实现（开发环境 / 私有化部署）
type MinIOStorage struct{ /* ... */ }

{
  "Rules": [
    {
      "Prefix": "images/raw/",
      "Status": "Enabled",
      "Transitions": [
        { "Days": 30,  "StorageClass": "STANDARD_IA" },
        { "Days": 180, "StorageClass": "GLACIER" }
      ],
      "Expiration": { "Days": 3650 }
    }
  ]
}

// CDN 处理指令（部分 CDN 支持 URL 参数动态裁图）
const thumbURL = `${origin}/${objectKey}?image/view2/2/w/600/format/webp`;