Go 语言中如何正确使用 context 控制 goroutine 超时与取消
在 Go 服务端开发里，经常遇到一个 HTTP 请求触发多个下游调用（数据库、RPC、缓存）的情况。如果某个下游迟迟不返回，goroutine 就会一直挂在那里，连接池、内存和文件描述符都会被慢慢耗尽。context 包就是为解决这类问题设计的：它让调用链上的所有 goroutine 共享同一个取消信号和截止时间。很多人第一次用 context 时会直接写 `ctx, _ := context.WithTimeout(context.Background(), time.Second)`，然后忽略 cancel 函数，导致定时器泄漏；或者把 context 存进 struct 字段，而不是作为函数第一个参数传递。这篇文章从实际场景出发，讲清楚 WithCancel、WithTimeout、WithDeadline 的区别，如何在 HTTP handler 和数据库查询中正确传播 context，以及 select 监听 ctx.Done() 时容易踩的坑。 ## 从一个下游调用超时说起 假设有一个订单查询接口，需要并发调用用户服务和商品服务： ```go func GetOrder(ctx context.Context, orderID string) (*Order, error) { var wg sync.WaitGroup var user *User var product *Product var err1, err2 error wg.Add(2) go func() { defer wg.Done() user, err1 = fetchUser(ctx, orderID) }() go func() { defer wg.Done() product, err2 = fetchProduct(ctx, orderID) }() wg.Wait() if err1 != nil { return nil, err1 } if err2 != nil { return nil, err2 } return &Order{User: user, Product: product}, nil } ``` 这段代码看起来没问题，但如果 fetchUser 内部没有接收 ctx，或者接收了却不去监听 ctx.Done()，那么即使外层设置了超时，这两个 goroutine 也不会提前退出。context 的取消信号是协作式的，必须由被调用的代码主动检查。 正确的做法是把 ctx 一路传到最底层的网络调用： ```go func fetchUser(ctx context.Context, orderID string) (*User, error) { req, err := http.NewRequestWithContext(ctx, http.MethodGet, userServiceURL+orderID, nil) if err != nil { return nil, err } resp, err := http.DefaultClient.Do(req) if err != nil { return nil, err } defer resp.Body.Close() var u User if err := json.NewDecoder(resp.Body).Decode(&u); err != nil { return nil, err } return &u, nil } ``` http.NewRequestWithContext 会把取消信号绑定到 TCP 连接上，一旦 ctx 被取消，底层的读操作会立刻返回错误。 ## WithTimeout 和 WithDeadline 到底选哪个 WithTimeout 本质上是 WithDeadline(parent, time.Now().Add(timeout)) 的语法糖。如果业务语义是“这个操作最多允许跑 800 毫秒”，用 WithTimeout 更直观；如果语义是“必须在今天 23:59:59 之前完成对账”，那就用 WithDeadline。 无论用哪个，都必须调用 cancel： ```go func handler(w http.ResponseWriter, r *http.Request) { ctx, cancel := context.WithTimeout(r.Context(), 800*time.Millisecond) defer cancel() order, err := GetOrder(ctx, r.URL.Query().Get("id")) if err != nil { if errors.Is(err, context.DeadlineExceeded) { http.Error(w, "request timeout", http.StatusGatewayTimeout) return } http.Error(w, err.Error(), http.StatusInternalServerError) return } json.NewEncoder(w).Encode(order) } ``` defer cancel() 不只是为了提前释放资源，更重要的是让 ctx 关联的定时器被回收。如果不调用 cancel，即使请求已经正常结束，time.Timer 仍然会等到超时时间才触发，长期运行的服务会积累大量无效定时器。 ## 在 select 里监听取消信号 自己写后台任务时，常见模式是 for-select 循环： ```go func worker(ctx context.Context, jobs 推荐查看天气：https://www.40tianqi.com/qilian/40tian.html 推荐查看天气：https://www.40tianqi.com/haiyan/ 推荐查看天气：https://www.40tianqi.com/haiyan/zuotian.html 推荐查看天气：https://www.40tianqi.com/haiyan/7tian.html 推荐查看天气：https://www.40tianqi.com/haiyan/10tian.html 推荐查看天气：https://www.40tianqi.com/haiyan/15tian.html 推荐查看天气：https://www.40tianqi.com/haiyan/40tian.html 推荐查看天气：https://www.40tianqi.com/gangcha/ 推荐查看天气：https://www.40tianqi.com/gangcha/zuotian.html 推荐查看天气：https://www.40tianqi.com/gangcha/7tian.html 推荐查看天气：https://www.40tianqi.com/gangcha/10tian.html 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