返回多个错误或相应处理它们的惯用方式

Question

我有这段代码，我不喜欢这种感觉，更不用说golint 不喜欢error should be the last type when returning multiple items (golint)。解释这段代码：

• 我想让用户决定他们是否关心返回的任何错误
• 特别是在此代码中，有时不需要或不需要音频文件，可以忽略它
• 无论用户在做什么，都可能总是需要视频和输出文件

我愿意以任何方式重构它（无论是将其拆分、移动等）。Go 中有没有更惯用的方式来完成这样的事情？

// Download will download the audio and video files to a particular path
func (r *RedditVideo) Download() (outputVideoFileName string, outputAudioFileName string, errVideo error, errAudio error) {
    outputVideoFile, errVideo := downloadTemporaryFile(
        r.VideoURL,
        TemporaryVideoFilePrefix,
    )
    if errVideo == nil {
        outputVideoFileName = outputVideoFile.Name()
    }

    outputAudioFile, errAudio := downloadTemporaryFile(
        r.AudioURL,
        TemporaryAudioFilePrefix,
    )
    if errAudio == nil {
        outputAudioFileName = outputAudioFile.Name()
    }

    return outputVideoFileName, outputAudioFileName, errVideo, errAudio
}

同样，我发现自己后来在代码中再次使用了同样的模式：

// SetFiles sets up all of the input files and the output file
func (l *localVideo) SetFiles(inputVideoFilePath string, inputAudioFilePath string, outputVideoFilePath string) (errVideo error, errAudio error, errOutputVideo error) {
	// Set input video file
	l.inputVideoFile, errVideo = os.Open(inputVideoFilePath)
	if errVideo != nil {
		return
	}
	if errVideo = l.inputVideoFile.Close(); errVideo != nil {
		return
	}

	// Set output video file
	l.outputVideoFile, errOutputVideo = os.Create(outputVideoFilePath)
	if errOutputVideo != nil {
		return
	}
	if errOutputVideo = l.outputVideoFile.Close(); errOutputVideo != nil {
		return
	}

	// IMPORTANT! Set input audio file LAST incase it is nil
	l.inputAudioFile, errAudio = os.Open(inputAudioFilePath)
	if errAudio != nil {
		return
	}
	if errAudio = l.inputAudioFile.Close(); errVideo != nil {
		return
	}

	return
}

这次在这段代码中再次出现了一些与上面相同的情况：

• 我们关心视频和输出的设置，可能关心也可能不关心音频
• 有多个错误需要由用户处理

Answer 1

您可以在标准库中看到相当多的特定函数，它们包装了更通用的非导出函数。请参阅下面的注释代码。

// download is a rather generic function
// and probably you can refactor downloadTemporaryFile
// so that even this function is not needed any more.
func (r *RedditVideo) download(prefix, url string) (filename string, error error) {
    outputFile, err := downloadTemporaryFile(
        r.VideoURL,
        prefix,
    )

    if err == nil {
        return "", fmt.Errorf("Error while download: %s", err)
    }

    return outputFile.Name(), nil
}

// DownloadVideo wraps download, handing over the values specific
// to the video download
func (r *RedditVideo) DownloadVideo() (filename string, averror error) {
    return r.download(TemporaryVideoFilePrefix, r.VideoURL)
}

// DownloadAudio wraps download, handing over the values specific
// to the audio download
func (r *RedditVideo) DownloadAudio() (filename string, averror error) {
    return r.download(TemporaryAudioFilePrefix, r.AudioURL)
}

func main() {

    r := RedditVideo{
        VideoURL: os.Args[1],
        AudioURL: os.Args[2],
    }

    var videoerror, audioerror error
    var videofileName, audiofileName string

    if videofileName, videoerror = r.DownloadVideo(); videoerror != nil {
        fmt.Println("Got an error downloading the video")
    }

    if audiofileName, audioerror = r.DownloadAudio(); audioerror != nil {
        fmt.Println("Got an error downloading the audio")
    }

    fmt.Printf("Video:\t%s\nAudio:\t%s", videofileName, audiofileName)
}

这样一来，返回的错误来自哪个下载就很明显了。

Answer 2

来自https://blog.golang.org/error-handling-and-go：

Go 代码使用错误值来指示异常状态。

在您的情况下，音频是可选的，而视频是必需的。因此只有视频下载应该返回错误：

// Download will download the audio and video files to a particular path
// empty outputAudioFileName indicates inability to download Audio File
func (r *RedditVideo) Download() (outputVideoFileName string, outputAudioFileName string, err error) {
    outputVideoFile, err := downloadTemporaryFile(
        r.VideoURL,
        TemporaryVideoFilePrefix,
    )
    if err != nil {
        return "", "", err 
    }
    outputVideoFileName := outputVideoFile.Name()
    outputAudioFile, errAudio := downloadTemporaryFile(
        r.AudioURL,
        TemporaryAudioFilePrefix,
    )
    outputAudioFileName := ""
    if errAudio == nil {
        outputAudioFileName = outputAudioFile.Name()
    } else {
        // Log error, it is still important to fix it
    }

    return outputVideoFileName, outputAudioFileName, nil
} 

经验法则 - 任何可以产生异常状态的代码都应该在下一行：

    if err != nil {
        return funcResult, err
    }