HTTP 重定向链

概念定义

重定向链是一串 3xx 响应：每一跳再指向另一个重定向，直到 200。除非连接已预热且命中缓存，每跳都要付出 DNS、建连与请求往返。状态码的缓存语义与方法保持各有差异，链长同时劣化延迟与爬虫权重。

工作机制

301 与 308 表示永久并鼓励长期缓存；302、303、307 是临时性、默认缓存更保守。方法保持也不同：308 与 307 保留方法与请求体，301 与 302 历史上允许把 POST 改写为 GET。浏览器会缓存链条，爬虫会封顶跳转深度。

使用场景

每次迁移或域名合并后都应审计。修复通常是把中间跳直接指向最终 URL、把内链更新到规范目标，并确保跟踪参数不再强制额外跳转。

常见误区

瀑布图里每一跳是独立请求，链长容易被忽略——五跳链条在 100 毫秒 RTT 的移动网络能烧掉一秒耐心。部分中间设备会剥除或改写重定向，行为因网络而异。循环往复是退化形态：应按入口 URL 监控重定向计数。

代码示例

package main

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"time"
)

type Shipment struct {
	ID     string `json:"id"`
	Status string `json:"status"`
}

func queryShipment(ctx context.Context, baseURL, id string) (*Shipment, error) {
	ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
	defer cancel() // 必须释放计时器资源

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, baseURL+"/shipments/"+id, nil)
	if err != nil {
		return nil, fmt.Errorf("build request: %w", err)
	}
	req.Header.Set("Accept", "application/json")

	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		if errors.Is(err, context.DeadlineExceeded) {
			return nil, fmt.Errorf("shipment %s timed out after 3s: %w", id, err)
		}
		return nil, fmt.Errorf("request failed: %w", err)
	}
	defer resp.Body.Close()

	var shipment Shipment
	if err := json.NewDecoder(resp.Body).Decode(&shipment); err != nil {
		return nil, fmt.Errorf("decode body: %w", err)
	}
	return &shipment, nil
}

func main() {
	s, err := queryShipment(context.Background(), "https://api.example.com", "sh-1")
	fmt.Println(s, err)
}

package main

import (
	"context"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/http/httptest"
	"time"
)

type fetchResult struct {
	Status int
	Body   string
}

func fetchWithDeadline(ctx context.Context, client *http.Client, url string, timeout time.Duration) (fetchResult, error) {
	ctx, cancel := context.WithTimeout(ctx, timeout)
	defer cancel()

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return fetchResult{}, fmt.Errorf("build request: %w", err)
	}

	resp, err := client.Do(req)
	if err != nil {
		if errors.Is(ctx.Err(), context.DeadlineExceeded) {
			return fetchResult{}, fmt.Errorf("fetch %s: deadline exceeded after %s: %w", url, timeout, context.DeadlineExceeded)
		}
		return fetchResult{}, fmt.Errorf("fetch %s: %w", url, err)
	}
	defer resp.Body.Close()

	body, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return fetchResult{}, fmt.Errorf("read body: %w", err)
	}
	return fetchResult{Status: resp.StatusCode, Body: string(body)}, nil
}

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("/fast", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprint(w, "ok")
	})
	mux.HandleFunc("/slow", func(w http.ResponseWriter, r *http.Request) {
		select {
		case <-time.After(200 * time.Millisecond):
			fmt.Fprint(w, "late")
		case <-r.Context().Done():
		}
	})
	server := httptest.NewServer(mux)
	defer server.Close()

	client := &http.Client{Timeout: 5 * time.Second}
	ctx := context.Background()

	result, err := fetchWithDeadline(ctx, client, server.URL+"/fast", 500*time.Millisecond)
	fmt.Printf("fast: status=%d body=%q err=%v\n", result.Status, result.Body, err)

	result, err = fetchWithDeadline(ctx, client, server.URL+"/slow", 50*time.Millisecond)
	fmt.Printf("slow: status=%d body=%q err=%v\n", result.Status, result.Body, err)
	if errors.Is(err, context.DeadlineExceeded) {
		fmt.Println("slow request classified as a deadline error")
	}
}