net/http: Response.Request doesn't contain updated URL after following redirects on js/wasm

[andreademasi]

Go version

go version go1.23.5 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/andrea/Library/Caches/go-build'
GOENV='/Users/andrea/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/andrea/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/andrea/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.23.5/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.23.5/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.5'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/andrea/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/sq/sd718h_s6lq1hgfdnt_qq3d00000gn/T/go-build2937344254=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I'm trying to get the url i've been redirected to.

package main

import (
	"log"
	"net/http"
)

func main() {
	resp, err := http.Get("https://mock.httpstatus.io/301")
	if err != nil {
		log.Fatalln(err)
	}
	log.Printf("%s", resp.Request.URL.String())
}

build with GOARCH=wasm GOOS=js go build -o test.wasm

Repro: https://github.com/andreademasi/go-redirect-repro

What did you see happen?

resp.Request.URL.String() is equal to "https://mock.httpstatus.io/301"

What did you expect to see?

resp.Request.URL.String() is equal to "https://mock.httpstatus.io/200"

I understand that this happens because the browser automatically follows redirects and it is not possible to alter this behaviour under normal circumstances.
I also tried using http.Client.CheckRedirect but the function never got called while running in wasm environment.

The fetch API does expose the redirected property on the Response object to detect if a request got redirected or not (https://developer.mozilla.org/en-US/docs/Web/API/Response/redirected) and the url property contains the url you're redirected to.

As a workaround this issue, I patched my own version of go to reflect the behaviour i'd like to see:

// net/http/response.go

type Response struct {

        ...

	// Redirected indicates whether or not the response is the result of a redirect
	Redirected bool
}

// net/http/roundtrip_js.go

               ...

		redirected := result.Get("redirected").Bool()

		if redirected {
			if url, err := url.ParseRequestURI(result.Get("url").String()); err != nil {
				errCh <- fmt.Errorf("net/http: failed to parse response url: %v", err)
				return nil
			} else {
				req.URL = url
			}
		}

		respCh <- &Response{
			Status:        fmt.Sprintf("%d %s", code, StatusText(code)),
			StatusCode:    code,
			Header:        header,
			ContentLength: contentLength,
			Uncompressed:  uncompressed,
			Body:          body,
			Request:       req,
			Redirected:    redirected,
		}

                ...

I'll be willing to open a PR to patch this issue (if needed) and/or discuss a different approach