Go语言在GAE上处理PayPal IPN：保持参数顺序的实践指南

import (
    "net/http"
    "google.golang.org/appengine"
    "google.golang.org/appengine/urlfetch"
)

func ipnHandler(w http.ResponseWriter, r *http.Request) {
    c := appengine.NewContext(r)
    client := urlfetch.Client(c)

    // r.Form 是 url.Values 类型，其Encode()方法会按键排序
    // 这将破坏PayPal IPN要求的原始参数顺序
    resp, err := client.PostForm("https://www.sandbox.paypal.com/cgi-bin/webscr", r.Form)

    if err != nil {
        // 处理错误
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    defer resp.Body.Close()

    // 读取并处理PayPal的验证响应
    // ...
}

package main

import (
    "bytes"
    "io"
    "log"
    "net/http"

    "google.golang.org/appengine"
    "google.golang.org/appengine/urlfetch"
)

// ipnHandler 处理PayPal IPN请求
func ipnHandler(w http.ResponseWriter, r *http.Request) {
    // 确保是POST请求
    if r.Method != http.MethodPost {
        http.Error(w, "Method Not Allowed", http.StatusMethodNotAllowed)
        return
    }

    c := appengine.NewContext(r)
    client := urlfetch.Client(c)

    // 1. 创建一个bytes.Buffer来构建验证请求体
    var verificationBody bytes.Buffer

    // 2. 首先写入PayPal要求的验证参数
    _, err := verificationBody.WriteString("cmd=_notify-validate&")
    if err != nil {
        log.Errorf(c, "Error writing cmd parameter: %v", err)
        http.Error(w, "Internal Server Error", http.StatusInternalServerError)
        return
    }

    // 3. 将原始请求体（r.Body）的内容完整地复制到verificationBody中
    // 这确保了所有参数及其原始顺序被保留
    _, err = io.Copy(&verificationBody, r.Body)
    if err != nil {
        log.Errorf(c, "Error copying request body: %v", err)
        http.Error(w, "Internal Server Error", http.StatusInternalServerError)
        return
    }

    // 4. 定义PayPal的验证端点
    // 注意：在生产环境中，请使用 https://www.paypal.com/cgi-bin/webscr
    // 对于沙盒测试，使用 https://www.sandbox.paypal.com/cgi-bin/webscr
    paypalVerifyURL := "https://www.sandbox.paypal.com/cgi-bin/webscr" 

    // 5. 使用 client.Post 发送手动构建的请求体
    // Content-Type 必须与原始IPN请求的类型一致，通常是 application/x-www-form-urlencoded
    resp, err := client.Post(paypalVerifyURL, "application/x-www-form-urlencoded", &verificationBody)
    if err != nil {
        log.Errorf(c, "Error posting to PayPal for verification: %v", err)
        http.Error(w, "Internal Server Error", http.StatusInternalServerError)
        return
    }
    defer resp.Body.Close()

    // 6. 读取PayPal的验证响应
    responseBytes, err := io.ReadAll(resp.Body)
    if err != nil {
        log.Errorf(c, "Error reading PayPal verification response: %v", err)
        http.Error(w, "Internal Server Error", http.StatusInternalServerError)
        return
    }

    paypalResponse := string(responseBytes)
    log.Infof(c, "PayPal verification response: %s", paypalResponse)

    // 7. 根据PayPal的响应进行处理
    // 如果响应是 "VERIFIED"，则IPN有效
    // 如果响应是 "INVALID"，则IPN无效
    if paypalResponse == "VERIFIED" {
        // IPN已验证，可以安全地处理交易数据
        // 例如：更新数据库、发送确认邮件等
        log.Infof(c, "PayPal IPN VERIFIED. Processing transaction...")
        w.WriteHeader(http.StatusOK)
        w.Write([]byte("IPN VERIFIED"))
    } else if paypalResponse == "INVALID" {
        // IPN无效，可能是一个欺诈尝试或配置错误
        log.Warningf(c, "PayPal IPN INVALID. Potential fraud or misconfiguration.")
        w.WriteHeader(http.StatusBadRequest)
        w.Write([]byte("IPN INVALID"))
    } else {
        // 未知响应
        log.Errorf(c, "Unexpected PayPal verification response: %s", paypalResponse)
        w.WriteHeader(http.StatusInternalServerError)
        w.Write([]byte("UNEXPECTED PAYPAL RESPONSE"))
    }
}

func main() {
    http.HandleFunc("/paypal-ipn-listener", ipnHandler)
    appengine.Main() // GAE 标准入口
}