优化 Django 投票系统：避免支付后票数重复增加及竞态条件

from django.db.models import F
# ...
profile_obj.totalVote = F("totalVote") + payment.vote
profile_obj.save(update_fields=['totalVote'])

from django.db import transaction
from django.db.models import F
from django.shortcuts import render, get_object_or_404
from django.conf import settings
# 假设 Paystack, Payment, Contestant 等模型和类已正确导入

@transaction.atomic
def verify_payment(request, ref):
    print(f"verify_payment called for reference: {ref}")
    try:
        payment = Payment.objects.get(ref=ref)
    except Payment.DoesNotExist:
        print("Invalid payment reference:", ref)
        return render(
            request,
            "competitions/error.html",
            {"error_message": "Invalid payment reference."},
        )

    # 关键的幂等性检查：如果支付已经验证，则直接返回成功，不再重复增加票数
    if payment.verified:
        print(f"Payment {ref} already verified and votes processed. Skipping duplicate update.")
        # 可以记录一个警告日志，表示收到重复的回调/请求
        return render(request, "competitions/success.html")

    # 调用 Payment 模型的 verify_payment 方法进行外部支付网关验证
    # 此方法成功时会设置 payment.verified = True 并保存
    verified_with_paystack = payment.verify_payment()

    if verified_with_paystack:
        contestant = payment.contestant
        profile_obj = contestant.user.profile

        # 使用 F() 表达式进行原子性更新，避免竞态条件
        # 注意：这里我们只更新 totalVote 字段，以避免不必要的副作用
        profile_obj.totalVote = F("totalVote") + payment.vote
        profile_obj.save(update_fields=['totalVote'])

        # 刷新 profile_obj 以获取数据库中最新的 totalVote 值，用于日志输出
        profile_obj.refresh_from_db()
        print(f"Payment verified for {contestant.user.first_name}. New total votes: {profile_obj.totalVote}.")

        return render(request, "competitions/success.html")
    else:
        print(f"Payment verification failed for reference: {ref}")
        return render(
            request,
            "competitions/error.html",
            {"error_message": "Payment verification failed."},
        )

class Payment(models.Model):
    # ... 其他字段

    def save(self, *args, **kwargs):
        while not self.ref:
            ref = secrets.token_urlsafe(50)
            object_with_similar_ref = Payment.objects.filter(ref=ref)
            if not object_with_similar_ref:
                self.ref = ref
        super().save(*args, **kwargs)

    def amount_value(self):
        return int(self.amount) * 100

    def verify_payment(self):
        # 确保只在未验证时才尝试与支付网关交互
        if not self.verified:
            paystack = Paystack() # 假设 Paystack 类已正确定义和导入
            status, result = paystack.verify_payment(self.ref, self.amount)
            if status:
                # 再次确认金额匹配，防止篡改
                if result["amount"] / 100 == self.amount:
                    self.verified = True
                    self.save() # 保存验证状态
        return self.verified