Golang写入HTTP响应会中断输入读取吗?

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英文:

Golang writing to http response breaks input reading?

问题

我正在尝试使用Go编写一个小型的Web应用程序,用户可以在多部分表单中上传一个gzip压缩文件。该应用程序会解压缩并解析该文件,并将一些输出写入响应中。然而,当我开始向响应中写入内容时,我不断遇到输入流损坏的错误。如果不向响应中写入内容,问题就会解决,或者如果从非gzip压缩的输入流中读取也可以解决。下面是一个示例的HTTP处理程序:

func(w http.ResponseWriter, req *http.Request) {
    // 从多部分读取器中获取输入流并使用扫描器读取
    multiReader, _ := req.MultipartReader()
    part, _ := multiReader.NextPart()
    gzipReader, _ := gzip.NewReader(part)
    scanner := bufio.NewScanner(gzipReader)
    
    // 将从输入流中读取的字符串发送到该通道
    inputChan := make(chan string, 1000)
    
    // 在该通道上发送完成信号
    donechan := make(chan bool, 1)
    
    // 这个goroutine只是从输入扫描器中读取文本并发送到通道中
    go func() {
        for scanner.Scan() {
            inputChan <- scanner.Text()
        }
        close(inputChan)
    }()
    
    // 从输入通道中读取行。它们要么以#开头,要么有十个以制表符分隔的列
    go func() {
        for line := range inputChan {
            toks := strings.Split(line, "\t")
            if len(toks) != 10 && line[0] != '#' {
                panic("Dang.")
            }
        }
        donechan <- true
    }()
    
    // 定期向响应中写入一些随机文本
    go func() {
        for {
            time.Sleep(10*time.Millisecond)
            w.Write([]byte("write\n some \n output\n"))
        }
    }()
    
    // 等待直到完成后再返回
    <-donechan
}

奇怪的是,这段代码每次都会引发panic,因为它总是遇到少于10个标记的行,尽管每次出错的位置都不同。注释掉写入响应的那行代码可以解决这个问题,或者从非gzip压缩的输入流中读取也可以解决。我是否遗漏了一些明显的东西?为什么从gzip文件中读取时向响应写入内容会出错,而从纯文本格式的文件中读取则不会出错?为什么会出错呢?

英文:

I'm attempting to write a small webapp in Go where the user uploads a gzipped file in a multipart form. The app unzips and parses the file and writes some output to the response. However, I keep running into an error where the input stream looks corrupted when I begin writing to the response. Not writing to the response fixes the problem, as does reading from a non-gzipped input stream. Here's an example http handler:

func(w http.ResponseWriter, req *http.Request) {
	
//Get an input stream from the multipart reader
//and read it using a scanner
multiReader, _ := req.MultipartReader()
part, _ := multiReader.NextPart()
gzipReader, _ := gzip.NewReader(part)
scanner := bufio.NewScanner(gzipReader)

//Strings read from the input stream go to this channel 	
inputChan := make(chan string, 1000)

//Signal completion on this channel 
donechan := make(chan bool, 1)

//This goroutine just reads text from the input scanner
//and sends it into the channel 
go func() {
	for scanner.Scan() {
		inputChan &lt;- scanner.Text()
	}		
    close(inputChan)
}()

//Read lines from input channel. They all either start with #
//or have ten tab-separated columns
go func() {
	for line := range inputChan {
		toks := strings.Split(line, &quot;\t&quot;)
		if len(toks) != 10 &amp;&amp; line[0] != &#39;#&#39; {
			panic(&quot;Dang.&quot;)
		}
	}
	donechan &lt;- true 
}()

//periodically write some random text to the response
go func() {
	for {
		time.Sleep(10*time.Millisecond)		
        w.Write([]byte(&quot;write\n some \n output\n&quot;))
	}
}()

//wait until we&#39;re done to return
&lt;-donechan
}

Weirdly, this code panics every time because it always encounters a line with fewer than 10 tokens, although at different spots every time. Commenting out the line that writes to the response fixes the issue, as does reading from a non-gzipped input stream. Am I missing something obvious? Why would writing to the response break if reading from a gzip file, but not a plain text formatted file? Why would it break at all?

答案1

得分: 1

HTTP协议不是全双工的,它是基于请求-响应的。只有在读取输入完成后,才应该发送输出。

在你的代码中,你在一个通道上使用了forrange。这将尝试读取通道,直到它被关闭,但你从未关闭inputChan

如果你从未关闭inputChan,则下面的代码将永远不会执行:

donechan <- true

因此,从donechan接收数据会被阻塞:

<-donechan

当到达EOF时,你必须关闭inputChan

go func() {
    for scanner.Scan() {
        inputChan <- scanner.Text()
    }       
    close(inputChan) // 这是必需的
}()
英文:

The HTTP protocol is not full-duplex: it is request-response based. You should only send output once you're done with reading the input.

In your code you use a for with range on a channel. This will try to read the channel until it is closed, but you never close the inputChan.

If you never close inputChan, the following line is never reached:

donechan &lt;- true 

And therefore receiving from donechan blocks:

&lt;-donechan

You have to close the inputChan when EOF is reached:

go func() {
    for scanner.Scan() {
        inputChan &lt;- scanner.Text()
    }       
    close(inputChan) // THIS IS NEEDED
}()

huangapple
  • 本文由 发表于 2015年2月5日 12:36:38
  • 转载请务必保留本文链接:https://go.coder-hub.com/28336313.html
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