当向通道写入数据时，永远不会调用”defer”。

huangapple
go评论106阅读模式
英文:

Defer never called when writing to channel

问题

我正在尝试在goroutine函数的最后一个操作中向通道写入数据。

不幸的是，这并没有起作用，而且等待组永远不会完成。

  1. import (
  2.     "sync"
  3.     "github.com/SlyMarbo/rss"
  4.     "fmt"
  5. )
  7. func main() {
  8.     urls := []string{"http://rss.cnn.com/rss/edition.rss", "http://rss.time.com/web/time/rss/top/index.xml"}
  10.     var c = make(chan string)
  11.     var wg sync.WaitGroup
  12.     for _, url := range urls {
  13.         wg.Add(1)
  14.         go receiveRss(url, &wg, c)
  15.     }
  16.     wg.Wait()
  17.     fmt.Println("==============DONE=================")
  18. }
  20. func receiveRss(url string, wg *sync.WaitGroup, c chan string) {
  21.     defer wg.Done()
  22.     feed, err := rss.Fetch(url)
  23.     if err != nil {
  24.         fmt.Println("Failed to retrieve RSS feed", err)
  25.     }
  27.     items := feed.Items
  28.     for _, item := range items {
  29.         c <- item.Title
  30.     }
  31. }

当将c <- item.Title替换为fmt.Println(item.Title)时，延迟函数被调用并打印出"DONE"。

英文:

I'm trying to write to a channel as last action in a goroutine function.

Unfortunately this is not working. And the waitGroup is never done.

  1. import (
  2.     &quot;sync&quot;
  3. 	&quot;github.com/SlyMarbo/rss&quot;
  4.     &quot;fmt&quot;
  5. )
  7. func main() {
  8. 	urls := []string{&quot;http://rss.cnn.com/rss/edition.rss&quot;, &quot;http://rss.time.com/web/time/rss/top/index.xml&quot;}
  10. 	var c = make(chan string)
  11.     var wg sync.WaitGroup
  12. 	for _, url := range urls {
  13. 		wg.Add(1)
  14.     	go receiveRss(url, &amp;wg, c)
  15. 	}
  16.     wg.Wait()
  17. 	fmt.Println(&quot;==============DONE=================&quot;)
  18. }
  20. func receiveRss(url string, wg *sync.WaitGroup, c chan string) {
  21. 	defer wg.Done()
  22.     feed, err := rss.Fetch(url)
  23. 	if err != nil {
  24.     	fmt.Println(&quot;Failed to retrieve RSS feed&quot;, err)
  25. 	}
  27.     items := feed.Items
  28. 	for _, item := range items {
  29.     	c &lt;- item.Title
  30. 	}
  31. }

When replacing c &lt;- item.Title with fmt.Println(item.Title) the deferred function is called and DONE is printed.

答案1

得分: 1

问题是我只是向通道写入数据，从未从中读取数据。
如果不这样做，通道将是无用的。

解决方法是：

在启动goroutine的循环之后从通道中读取数据：

  1. for title := range c {
  2. 	fmt.Println(title)
  3. }

这会导致无限循环，如果通道从未关闭。
所以我在写入数据后关闭通道：

  1. close(c)

以下是完整的代码：

  1. func main() {
  3. 	urls := []string{"http://rss.cnn.com/rss/edition.rss", "http://rss.time.com/web/time/rss/top/index.xml"}
  5. 	var c = make(chan []string)
  6. 	var wg sync.WaitGroup
  8. 	for _, url := range urls {
  9. 		wg.Add(1)
  10. 		go receiveRss(url, &wg, c)
  11. 	}
  12. 	for title := range c {
  13. 		fmt.Println(title)
  14. 	}
  15. 	wg.Wait()
  16. 	fmt.Println("==============DONE=================")
  17. }
  19. func receiveRss(url string, wg *sync.WaitGroup, c chan []string) {
  20. 	defer wg.Done()
  21. 	feed, err := rss.Fetch(url)
  22. 	if err != nil {
  23. 		fmt.Println("Failed to retrieve RSS feed", err)
  24. 	}
  26. 	items := feed.Items
  27. 	var titles []string
  28. 	for _, item := range items {
  29. 		titles = append(titles, item.Title)
  30. 	}
  31. 	c <- titles
  32. 	close(c)
  33. }
英文:

The problem is that I was only writing to the channel. Never reading from it.
Without doing this the channel would be useless.

The solution to this is:

Reading from the channel after the loop which starts the goroutines:

  1. for title := range c {
  2. 	fmt.Println(title)
  3. }

This then causes an endless loop if the channel is never closed.
So I just close the channel after writing to it:

  1. close(c)

Here is the whole code:

  1. func main() {
  3. 	urls := []string{&quot;http://rss.cnn.com/rss/edition.rss&quot;, &quot;http://rss.time.com/web/time/rss/top/index.xml&quot;}
  5. 	var c = make(chan []string)
  6. 	var wg sync.WaitGroup
  8. 	for _, url := range urls {
  9. 		wg.Add(1)
  10. 		go receiveRss(url, &amp;wg, c)
  11. 	}
  12. 	for title := range c {
  13. 		fmt.Println(title)
  14. 	}
  15. 	wg.Wait()
  16. 	fmt.Println(&quot;==============DONE=================&quot;)
  17. }
  19. func receiveRss(url string, wg *sync.WaitGroup, c chan []string) {
  20. 	defer wg.Done()
  21. 	feed, err := rss.Fetch(url)
  22. 	if err != nil {
  23. 		fmt.Println(&quot;Failed to retrieve RSS feed&quot;, err)
  24. 	}
  26. 	items := feed.Items
  27. 	var titles []string
  28. 	for _, item := range items {
  29. 		titles = append(titles, item.Title)
  30. 	}
  31. 	c &lt;- titles
  32. 	close(c)
  33. }

huangapple
  • 本文由 发表于 2017年5月3日 03:16:55
  • 转载请务必保留本文链接：https://go.coder-hub.com/43745595.html
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