如何使用 Golang 实现 HTTP 文件上传的重试机制？

import (
    "context"
    "errors"
    "fmt"
    "io"
    "io/ioutil"
    "net/http"
    "strconv"
    "time"
)

// 重试前等待的时间，单位秒
var retryWaitSeconds = 5

// 最大重试次数
var maxRetries = 3

// UploadFileWithRetry 发送文件并重试失败的请求
func UploadFileWithRetry(ctx context.Context, client *http.Client, url string, file io.Reader) (string, error) {
    var err error

    for i := 0; i <= maxRetries; i++ {
        // 发送请求
        req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, file)
        if err != nil {
            return "", fmt.Errorf("create request: %w", err)
        }

        resp, err := client.Do(req)
        if err != nil {
            if i == maxRetries {
                return "", fmt.Errorf("client do: %w", err)
            }
            time.Sleep(time.Second * time.Duration(retryWaitSeconds))
            continue
        }

        defer resp.Body.Close()

        body, err := ioutil.ReadAll(resp.Body)
        if err != nil {
            return "", fmt.Errorf("read response: %w", err)
        }

        if resp.StatusCode != http.StatusOK {
            if i == maxRetries {
                return "", fmt.Errorf("unexpected response: %s %s", resp.Status, string(body))
            }
            time.Sleep(time.Second * time.Duration(retryWaitSeconds))
            continue
        }

        return string(body), nil
    }

    return "", errors.New("maximum retries exceeded")
}

func main() {
    ctx := context.Background()
    client := &http.Client{}
    url := "https://example.com/upload"
    file, err := os.Open("test.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer file.Close()

    body, err := UploadFileWithRetry(ctx, client, url, file)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println("File uploaded successfully:", body)
}