在Google App Engine中使用Go语言读取本地文件

huangapple
go评论124阅读模式
英文:

Reading a local file in Google App Engine with Go

问题

我正在尝试在Google App Engine上使用Go而不是Python来构建我的网站。但是当我在本地测试我的脚本时，我一直遇到这个错误。

  1. panic: runtime error: invalid memory address or nil pointer dereference

我很困惑，但是如果我注释掉以下代码，它将可以正常运行。

  1. channel <- buffer[0:dat]

所以我可能在使用通道时出错了，有人可以帮忙吗？

编辑：

这是可工作的代码，非常感谢Kevin Ballard帮助我解决了这个问题。

  1. package defp
  3. import (
  4.     "fmt"
  5.     "http"
  6.     "os"
  7. )
  9. func getContent(filename string, channel chan []byte) {
  10.     file, err := os.OpenFile(filename, os.O_RDONLY, 0666)
  11.     defer file.Close()
  12.     if err == nil {
  13.         fmt.Printf("FILE FOUND : " + filename + " \n")
  14.         buffer := make([]byte, 16)
  15.         dat, err := file.Read(buffer)
  16.         for err == nil {
  17.             fmt.Printf("herp")
  18.             channel <- buffer[0:dat]
  19.             buffer = make([]byte, 16)
  20.             dat, err = file.Read(buffer)
  21.         }
  22.         close(channel)
  23.         fmt.Printf("DONE READING\n")
  24.     } else {
  25.         fmt.Printf("FILE NOT FOUND : " + filename + " \n")
  26.     }
  27. }
  28. func writeContent(w http.ResponseWriter, channel chan []byte) {
  29.     fmt.Printf("ATTEMPTING TO WRITE CONTENT\n")
  30.     go func() {
  31.         for bytes := range channel {
  32.             w.Write(bytes)
  33.             fmt.Printf("BYTES RECEIVED\n")
  34.         }
  35.     }()
  36.     fmt.Printf("FINISHED WRITING\n")
  37. }
  38. func load(w http.ResponseWriter, path string) {
  39.     fmt.Printf("ATTEMPTING LOAD " + path + "\n")
  40.     channel := make(chan []byte, 50)
  41.     writeContent(w, channel)
  42.     getContent(path, channel)
  43. }
  44. func handle(w http.ResponseWriter, r *http.Request) {
  45.     fmt.Printf("HANDLING REQUEST FOR " + r.URL.Path[1:] + "\n")
  46.     load(w, r.URL.Path[1:])
  47. }
  48. func init() {
  49.     http.HandleFunc("/", handle)
  50. }
英文:

I'm trying to use go instead of python for my website on google app engine. But I keep getting this error with my script when I test locally.

  1. panic: runtime error: invalid memory address or nil pointer dereference

I'm pretty confused, however it will run without error if I comment out

  1. channel &lt;- buffer[0:dat]

So I must be using channels incorrectly, Any help?

Edit:

This is the working code, many thanks to Kevin Ballard for helping me get this one.

  1. package defp
  3. import (
  4.     &quot;fmt&quot;
  5.     &quot;http&quot;
  6.     &quot;os&quot;
  7. )
  9. func getContent(filename string, channel chan []byte) {
  10.     file, err := os.OpenFile(filename, os.O_RDONLY, 0666)
  11.     defer file.Close()
  12.     if err == nil {
  13.         fmt.Printf(&quot;FILE FOUND : &quot; + filename + &quot; \n&quot;)
  14.         buffer := make([]byte, 16)
  15.         dat, err := file.Read(buffer)
  16.         for err == nil {
  17.             fmt.Printf(&quot;herp&quot;)
  18.             channel &lt;- buffer[0:dat]
  19.             buffer = make([]byte, 16)
  20.             dat, err = file.Read(buffer)
  21.         }
  22.         close(channel)
  23.         fmt.Printf(&quot;DONE READING\n&quot;)
  24.     } else {
  25.         fmt.Printf(&quot;FILE NOT FOUND : &quot; + filename + &quot; \n&quot;)
  26.     }
  27. }
  28. func writeContent(w http.ResponseWriter, channel chan []byte) {
  29.     fmt.Printf(&quot;ATTEMPTING TO WRITE CONTENT\n&quot;)
  30.     go func() {
  31.         for bytes := range channel {
  32.             w.Write(bytes)
  33.             fmt.Printf(&quot;BYTES RECEIVED\n&quot;)
  34.         }
  35.     }()
  36.     fmt.Printf(&quot;FINISHED WRITING\n&quot;)
  37. }
  38. func load(w http.ResponseWriter, path string) {
  39.     fmt.Printf(&quot;ATTEMPTING LOAD &quot; + path + &quot;\n&quot;)
  40.     channel := make(chan []byte, 50)
  41.     writeContent(w, channel)
  42.     getContent(path, channel)
  43. }
  44. func handle(w http.ResponseWriter, r *http.Request) {
  45.     fmt.Printf(&quot;HANDLING REQUEST FOR &quot; + r.URL.Path[1:] + &quot;\n&quot;)
  46.     load(w, r.URL.Path[1:])
  47. }
  48. func init() {
  49.     http.HandleFunc(&quot;/&quot;, handle)
  50. }

答案1

得分: 5

你的程序有时会发生恐慌的原因是，在程序退出load函数后，有时会写入w http.ResponseWriter。http包在程序退出处理程序函数时会自动关闭http.ResponseWriter。在writeContent函数中，程序有时会尝试向已关闭的http.ResponseWriter写入。

顺便说一下：如果使用io.Copy函数，可以使程序源代码更小。

为了始终获得可预测的行为，请确保您希望程序在HTTP请求响应中执行的所有工作都在退出处理程序函数之前完成。例如：

  1. func writeContent(w http.ResponseWriter, channel chan []byte) {
  2.     fmt.Printf("尝试写入内容\n")
  3.     for bytes := range channel {
  4.             w.Write(bytes)
  5.             fmt.Printf("接收到字节\n")
  6.     }
  7.     fmt.Printf("写入完成\n")
  8. }
  10. func load(w http.ResponseWriter, path string) {
  11.     fmt.Printf("尝试加载 " + path + "\n")
  12.     channel := make(chan []byte)
  13.     workDone := make(chan byte)
  14.     go func() {
  15.             writeContent(w, channel)
  16.             workDone <- 1 //发送任意值
  17.     }()
  18.     go func() {
  19.             getContent(path, channel)
  20.             workDone <- 2 //发送任意值
  21.     }()
  22.     <-workDone
  23.     <-workDone
  24. }
  26. func handle(w http.ResponseWriter, r *http.Request) {
  27.     fmt.Printf("处理请求 " + r.URL.Path[1:] + "\n")
  28.     load(w, r.URL.Path[1:])
  29. }
英文:

The reason why your program sometimes panics is that it is sometimes writing to <code>w http.ResponseWriter</code> after the program exits the <code>load</code> function. The <code>http</code> package automatically closes the <code>http.ResponseWriter</code> when the program exits the handler function. In function <code>writeContent</code>, the program will sometimes attempt to write to a closed <code>http.ResponseWriter</code>.

BTW: You can make the program source code much smaller if you use the <code>io.Copy</code> function.

To always get predictable behavior, make sure that all work that you want the program to perform in response to a HTTP request is done before you exit the handler function. For example:

  1. func writeContent(w http.ResponseWriter, channel chan []byte) {
  2.     fmt.Printf(&quot;ATTEMPTING TO WRITE CONTENT\n&quot;)
  3.     for bytes := range channel {
  4.             w.Write(bytes)
  5.             fmt.Printf(&quot;BYTES RECEIVED\n&quot;)
  6.     }
  7.     fmt.Printf(&quot;FINISHED WRITING\n&quot;)
  8. }
  10. func load(w http.ResponseWriter, path string) {
  11.     fmt.Printf(&quot;ATTEMPTING LOAD &quot; + path + &quot;\n&quot;)
  12.     channel := make(chan []byte)
  13.     workDone := make(chan byte)
  14.     go func() {
  15.             writeContent(w, channel)
  16.             workDone &lt;- 1 //Send an arbitrary value
  17.     }()
  18.     go func() {
  19.             getContent(path, channel)
  20.             workDone &lt;- 2 //Send an arbitrary value
  21.     }()
  22.     &lt;-workDone
  23.     &lt;-workDone
  24. }
  26. func handle(w http.ResponseWriter, r *http.Request) {
  27.     fmt.Printf(&quot;HANDLING REQUEST FOR &quot; + r.URL.Path[1:] + &quot;\n&quot;)
  28.     load(w, r.URL.Path[1:])
  29. }

huangapple
  • 本文由 发表于 2011年8月5日 07:41:34
  • 转载请务必保留本文链接：https://go.coder-hub.com/6949899.html
  • go
  • google-app-engine
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