Golang Web应用中按F5重新提交文件时遇到的问题

这个问题的根本原因是当用户按F5刷新页面时，浏览器会重新提交之前的POST请求，但由于此时没有实际的文件数据，导致r.FormFile("image")返回错误，进而引发空指针解引用。

以下是修复后的代码：

package main

import (
    "fmt"
    "html/template"
    "io"
    "log"
    "net/http"
    "os"
    "strings"

    "github.com/satori/go.uuid"
)

var tpl *template.Template

// Page struct which describe page structure
type Page struct {
    Title       string
    Keywords    string
    Description string
    Message     string
}

func init() {
    tpl = template.Must(template.ParseGlob("templates/*.html"))
}

func homehandler(w http.ResponseWriter, r *http.Request) {
    page := Page{Title: "Home Page", Keywords: "This is home page keywords", Description: "This is description of page"}
    err := tpl.ExecuteTemplate(w, "index.html", page)
    checkWebError(w, err)
}

func uploadhandler(w http.ResponseWriter, r *http.Request) {
    var message string
    
    if r.Method == "POST" {
        // 检查是否是multipart表单
        if strings.Contains(r.Header.Get("Content-Type"), "multipart/form-data") {
            err := r.ParseMultipartForm(32 << 20)
            if err != nil {
                log.Printf("Error parsing multipart form: %v", err)
                message = "Error parsing form data"
            } else {
                file, handle, err := r.FormFile("image")
                if err != nil {
                    log.Printf("Error getting form file: %v", err)
                    message = "No file uploaded or file error"
                } else {
                    defer file.Close()
                    
                    // 验证文件大小
                    if handle.Size == 0 {
                        message = "Empty file uploaded"
                    } else {
                        // we will use this id for new unique name
                        id, err := uuid.NewV4()
                        if err != nil {
                            log.Printf("Error generating UUID: %v", err)
                            message = "Error generating file name"
                        } else {
                            // extension will come with . (dot) extension
                            ext := strings.Index(handle.Filename, ".")
                            if ext == -1 {
                                message = "Invalid file name"
                            } else {
                                newname := fmt.Sprintf("%s", id) + handle.Filename[ext:]
                                
                                f, err := os.OpenFile("./images/"+newname, os.O_WRONLY|os.O_CREATE, 0777)
                                if err != nil {
                                    log.Printf("Error creating file: %v", err)
                                    message = "Error saving file"
                                } else {
                                    defer f.Close()
                                    _, err = io.Copy(f, file)
                                    if err != nil {
                                        log.Printf("Error copying file: %v", err)
                                        message = "Error saving file"
                                    } else {
                                        message = "File Uploaded Successfully"
                                    }
                                }
                            }
                        }
                    }
                }
            }
        } else {
            message = "Invalid form content type"
        }
    } else {
        message = "Please upload a file"
    }
    
    page := Page{
        Title:       "File Upload", 
        Keywords:    "This is home page keywords", 
        Description: "This is description of page", 
        Message:     message,
    }
    err := tpl.ExecuteTemplate(w, "index.html", page)
    checkWebError(w, err)
}

func checkWebError(w http.ResponseWriter, error error) {
    if error != nil {
        log.Println(error)
        w.WriteHeader(http.StatusInternalServerError)
        return
    }
}

func checkError(error error) {
    if error != nil {
        log.Println(error)
        return
    }
}

func main() {
    http.HandleFunc("/", homehandler)
    http.HandleFunc("/upload", uploadhandler)
    http.ListenAndServe(":8080", nil)
}

主要修改包括：

1. 添加了对Content-Type的检查，确保是有效的multipart表单
2. 对r.ParseMultipartForm()和r.FormFile()的错误进行了单独处理
3. 添加了文件大小验证
4. 对所有可能出错的步骤都进行了错误处理
5. 使用变量message来存储操作结果信息，避免在出错时直接panic

这样当用户按F5刷新时，代码会优雅地处理重新提交的请求，显示相应的错误信息而不是panic。