为什么不填充此HTTP请求的“响应”字段？

Question

The comment for the field Response in the type http.Request is as follows.

// Response is the redirect response which caused this request
// to be created. This field is only populated during client
// redirects.
Response *Response

However, it seems to me that this field is not being populated during requests, as it is implied that it is. Consider the following example:

package main

import (
  "net/http"
  "log"
  "fmt"
)

func handleA(writer http.ResponseWriter, request *http.Request) {
  http.Redirect(writer, request, "/b", http.StatusSeeOther)
}

func handleB(writer http.ResponseWriter, request *http.Request) {
  fmt.Println(request.Response)
}

func main() {
  http.HandleFunc("/a", handleA)
  http.HandleFunc("/b", handleB)
  log.Fatal(http.ListenAndServe(":8080", nil))
}

If I compile and run this code and navigate to localhost:8080/a, then I get redirected to localhost:8080/b and the server prints <nil> to the console. But shouldn't it be printing out a non-nil value, since the request is coming as the result of a redirect?

Answer 1

In your sample, the redirect is happening in the browser; the server doesn't get to know what response generated the redirect. That field is populated when making HTTP requests from a Go app; for example, if you use http.Client to request a URL, and the response is a redirect, it generates a new Request for the redirect URL, and in that Request, the Response field will be populated with the response that triggered that redirect.

This is evidenced in the source for http.Client: https://golang.org/src/net/http/client.go#L669