用golang gin框架 写一个分片上传阿里云OSS功能，注意 上传的文件很大 5G左右

好的，下面是一个示例代码，使用 `gin` 框架实现分片上传阿里云OSS功能：

package main

import (
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"path/filepath"
	"strconv"

	"github.com/aliyun/aliyun-oss-go-sdk/oss"
	"github.com/gin-gonic/gin"
)

const (
	MaxUploadSize = 5 * 1024 * 1024 * 1024 // 5 GB
	ChunkSize     = 10 * 1024 * 1024       // 10 MB
)

func main() {
	r := gin.Default()
	r.POST("/upload", UploadHandler)
	if err := r.Run(":8080"); err != nil {
		log.Fatal(err)
	}
}

func UploadHandler(c *gin.Context) {
	file, header, err := c.Request.FormFile("file")
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}
	defer file.Close()

	fileSize, err := strconv.Atoi(c.Request.FormValue("size"))
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	fileName := header.Filename
	filePath := filepath.Join(os.TempDir(), fileName)

	dst, err := os.Create(filePath)
	if err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}
	defer dst.Close()

	if _, err := io.Copy(dst, file); err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}

	if int64(fileSize) > MaxUploadSize {
		c.JSON(http.StatusBadRequest, gin.H{"error": "file size exceeds the maximum limit"})
		return
	}

	client, err := oss.New("oss-cn-hangzhou.aliyuncs.com", "<access_key>", "<access_secret>")
	if err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}

	bucket, err := client.Bucket("<bucket_name>")
	if err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}

	chunkSize := ChunkSize
	totalPartsNum := (fileSize + chunkSize - 1) / chunkSize
	objectName := fileName

	// Initiate the multipart upload
	imur, err := bucket.InitiateMultipartUpload(objectName)
	if err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}

	var uploadedParts []oss.UploadPart
	var partNumber int

	// Upload each part
	for i := 0; i < totalPartsNum; i++ {
		partSize := chunkSize
		if i == totalPartsNum-1 {
			partSize = fileSize - i*chunkSize
		}

		partNumber = i + 1

		// Open the file and read the bytes
		file, err := os.Open(filePath)
		if err != nil {
			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
			return
		}
		defer file.Close()

		// Seek to the start of the part
		offset := int64(i * chunkSize)
		if _, err := file.Seek(offset, io.SeekStart); err != nil {
			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
			return
		}

		// Read the part into memory
		partBuffer := make([]byte, partSize)
		if _, err := file.Read(partBuffer); err != nil {
			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
			return
		}

		// Upload the part to OSS
		uploadPart, err := bucket.UploadPart(imur, partBuffer, partNumber)
		if err != nil {
			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
			return
		}

		uploadedParts = append(uploadedParts, uploadPart)
	}

	// Complete the multipart upload
	cmur, err := bucket.CompleteMultipartUpload(imur, uploadedParts)
	if err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
		return
	}

	fmt.Println(cmur)
	c.JSON(http.StatusOK, gin.H{"message": "upload success"})
}

以上代码中，实现了以下功能：

1. 从请求中获取文件和大小
2. 将文件存储到本地磁盘中
3. 初始化分片上传，并上传每个分片
4. 完成分片上传，将分片合并成一个对象

需要注意的是，代码中使用了阿里云OSS的 SDK 进行操作，因此需要先在阿里云控制台上创建 OSS Bucket，并在代码中填写正确的 access_key、access_secret 和 bucket_name。

另外，由于上传的文件可能很大，因此需要设置分片大小，本例中设置为每个分片的大小为10MB。同时，为了避免服务器崩溃，还需要设置文件大小的最大限制，本例中设置为5GB。