Golang 卡在 WaitGroup 中答案

【问题标题】：Golang stuck in WaitGroupGolang 卡在 WaitGroup 中
【发布时间】：2018-06-28 11:07:41
【问题描述】：

我陷入了自己的等待循环，不确定为什么。该函数获取输入和输出通道，然后获取通道中的每个项目，为内容执行 http.GET 并从 html 中提取标签。

GET 和抓取的过程在 go 例程中，我已经设置了一个等待组 (innerWait) 以确保在关闭输出通道之前我已经处理了所有内容。

   func (fp FeedProducer) getTitles(in <-chan feeds.Item,
    out chan<- feeds.Item,
    wg *sync.WaitGroup) {

    defer wg.Done()

    var innerWait sync.WaitGroup

    for item := range in {
        log.Infof(fp.c, "Incrementing inner WaitGroup.")
        innerWait.Add(1)
        go func(item feeds.Item) {
            defer innerWait.Done()
            defer log.Infof(fp.c, "Decriment inner wait group by defer.")
            client := urlfetch.Client(fp.c)
            resp, err := client.Get(item.Link.Href)
            log.Infof(fp.c, "Getting title for: %v", item.Link.Href)
            if err != nil {
                log.Errorf(fp.c, "Error retriving page. %v", err.Error())
                return
            }
            if strings.ToLower(resp.Header.Get("Content-Type")) == "text/html; charset=utf-8" {
                title := fp.scrapeTitle(resp)
                item.Title = title
            } else {
                log.Errorf(fp.c, "Wrong content type.  Received: %v from %v", resp.Header.Get("Content-Type"), item.Link.Href)
            }
            out <- item
        }(item)
    }
    log.Infof(fp.c, "Waiting for title pull wait group.")
    innerWait.Wait()
    log.Infof(fp.c, "Done waiting for title pull.")
    close(out)
}

func (fp FeedProducer) scrapeTitle(request *http.Response) string {
    defer request.Body.Close()
    tokenizer := html.NewTokenizer(request.Body)
    var titleIsNext bool
    for {
        token := tokenizer.Next()
        switch {
        case token == html.ErrorToken:
            log.Infof(fp.c, "Hit the end of the doc without finding title.")
            return ""
        case token == html.StartTagToken:
            tag := tokenizer.Token()
            isTitle := tag.Data == "title"

            if isTitle {
                titleIsNext = true
            }
        case titleIsNext && token == html.TextToken:
            title := tokenizer.Token().Data
            log.Infof(fp.c, "Pulled title: %v", title)
            return title
        }
    }
}

日志内容如下：

2015/08/09 22:02:10 INFO: Revived query parameter: golang
2015/08/09 22:02:10 INFO: Getting active tweets from the last 7 days.
2015/08/09 22:02:10 INFO: Incrementing inner WaitGroup.
2015/08/09 22:02:10 INFO: Incrementing inner WaitGroup.
2015/08/09 22:02:10 INFO: Incrementing inner WaitGroup.
2015/08/09 22:02:10 INFO: Incrementing inner WaitGroup.
2015/08/09 22:02:10 INFO: Incrementing inner WaitGroup.
2015/08/09 22:02:10 INFO: Incrementing inner WaitGroup.
2015/08/09 22:02:10 INFO: Waiting for title pull wait group.
2015/08/09 22:02:10 INFO: Getting title for: http://devsisters.github.io/goquic/
2015/08/09 22:02:10 INFO: Pulled title: GoQuic by devsisters
2015/08/09 22:02:10 INFO: Getting title for: http://whizdumb.me/2015/03/03/matching-a-string-and-extracting-values-using-regex/
2015/08/09 22:02:10 INFO: Pulled title: Matching a string and extracting values using regex | Whizdumb's blog
2015/08/09 22:02:10 INFO: Getting title for: https://www.reddit.com/r/golang/comments/3g7tyv/dropboxs_infrastructure_is_go_at_a_huge_scale/
2015/08/09 22:02:10 INFO: Pulled title: Dropbox's infrastructure is Go at a huge scale : golang
2015/08/09 22:02:10 INFO: Getting title for: http://dave.cheney.net/2015/08/08/performance-without-the-event-loop
2015/08/09 22:02:10 INFO: Pulled title: Performance without the event loop | Dave Cheney
2015/08/09 22:02:11 INFO: Getting title for: https://github.com/ccirello/sublime-gosnippets
2015/08/09 22:02:11 INFO: Pulled title: ccirello/sublime-gosnippets · GitHub
2015/08/09 22:02:11 INFO: Getting title for: https://medium.com/iron-io-blog/an-easier-way-to-create-tiny-golang-docker-images-7ba2893b160?mkt_tok=3RkMMJWWfF9wsRonuqTMZKXonjHpfsX57ewoWaexlMI/0ER3fOvrPUfGjI4ATsNrI%2BSLDwEYGJlv6SgFQ7LMMaZq1rgMXBk%3D&utm_content=buffer45a1c&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer
2015/08/09 22:02:11 INFO: Pulled title: An Easier Way to Create Tiny Golang Docker Images — Iron.io Blog — Medium

我可以看到我正在根据日志访问 innerWait.Wait() 命令，这也告诉我入站通道已在管道的另一侧关闭。

似乎没有调用匿名函数中的延迟语句，因为我看不到任何地方打印的延迟日志语句。但是我一辈子都无法解释为什么该块中的所有代码似乎都在执行。

感谢您的帮助。

【问题讨论】：

out 是无缓冲通道吗？如果是out <- item 将被阻塞，直到接收者（<- out）可用。
这就是我所缺少的。以答案的形式给出答案，功劳归你所有。

标签： google-app-engine go

【解决方案1】：

goroutines 在这一行发送到out 时卡住了：

        out <- item

解决方法是启动一个 goroutine 来接收 out。

调试此类问题的一个好方法是通过向进程发送 SIGQUIT 来转储 goroutine 堆栈。

【讨论】：