我有一个从 S3 下载文件的功能。它有效但不识别 IfModifiedSince 选项。下面的函数在本地查找同名文件，如果存在，则将 time.Time 对象设置为修改日期和时间。然后在对 S3 的请求中使用它，以便仅在文件自那时以来被修改后才下载该文件。

func DownloadS3Media(filename string, mediaDirectory string, bucket string, c *configuration.Configuration) (dest string, bytes int64, err error) {
    os.Setenv("AWS_ACCESS_KEY_ID", c.AWS_ACCESS_KEY_ID)
    os.Setenv("AWS_SECRET_ACCESS_KEY", c.AWS_SECRET_ACCESS_KEY)

    // Create the directories in the path
    finalFilePath := filepath.Join(mediaDirectory, filename)
    if err1 := os.MkdirAll(filepath.Dir(finalFilePath), 0775); err1 != nil {
        return "", 0, err1
    }

    //Create the blank file to push the data into
    var t *time.Time
    if fi, err2 := os.Stat(finalFilePath); err2 == nil {
        t1 := fi.ModTime()
        t = &t1
    }
    tempFilePath := filepath.Join(mediaDirectory, filename + ".tmp")
    tempFile, err3 := os.Create(tempFilePath)
    if err3 != nil {
        log.Println("Failed to create file", err3)
        return finalFilePath, 0, err3
    }
    defer tempFile.Close()

    logLevel := aws.LogDebug
    downloader := s3manager.NewDownloader(session.New(&aws.Config{Region: aws.String("us-east-1"), LogLevel:&logLevel}))
    params := &s3.GetObjectInput{Bucket: aws.String(bucket),
        Key:    aws.String(filename),
        IfModifiedSince: t}
    numBytes, err4 := downloader.Download(tempFile, params)
    if err4 != nil {
        log.Println("Failed to download file", err4)
        return finalFilePath, 0, err4
    }

    //if downloader pulled down the file, rename the tmp file to original.
    if _, err5 := os.Stat(tempFilePath); err5 == nil {
        os.Rename(tempFilePath, finalFilePath)
        log.Println("Renamed ", tempFilePath, "to", finalFilePath)
    }

    log.Println("Downloaded file", filename, "to", finalFilePath, numBytes, "bytes")
    return finalFilePath, numBytes, nil
}

调试时，我看到 &t 的正确值类似于 sec: 63606119179。在 AWS 发送请求时， header 已存在但设置为 1/1/0001。

---[ REQUEST POST-SIGN ]-----------------------------
GET http://*****.s3.amazonaws.com/domenic-test.png HTTP/1.1
Host: *****.s3.amazonaws.com
User-Agent: aws-sdk-go/1.3.1 (go1.6.3; darwin; amd64) S3Manager
Authorization: AWS4-HMAC-SHA256 Credential= *****/20160806/us-east-1/s3/aws4_request, SignedHeaders=host;if-modified-since;range;x-amz-content-sha256;x-amz-date, Signature= *****
If-Modified-Since: Mon, 1 Jan 0001 00:00:00 GMT
Range: bytes=0-5242879
X-Amz-Content-Sha256: *****
X-Amz-Date: 20160806T223302Z

最佳答案

试试这个简化:

var t time.Time
if fi, err2 := os.Stat(finalFilePath); err2 == nil {
    t := fi.ModTime()
}

及以后:

params := &s3.GetObjectInput{Bucket: aws.String(bucket),
    Key:    aws.String(filename),
    IfModifiedSince: aws.Time(t)}

...如示例代码所示:https://github.com/aws/aws-sdk-go/blob/f1e26250235022af782521266389e4b2ae2945e4/service/s3/examples_test.go#L975

一开始以为是删除问题，后来测试发现t指向的time.Time值逃逸了 block (老实说让我吃了一惊)。也许在两者之间发生了一些其他错误，但我认为既然他们的测试通过了，这应该可行。

关于amazon-web-services - 如何使用 Go 在 AWS S3 中设置 If-Modified-Since header ，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/38809347/