proposal: net/http: return error for proxy CONNECT errors

[logandavies181]

What version of Go are you using (go version)?

$ go version
go version go1.15.8 linux/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

(actually using golang:latest eba5d0436b0b from dockerhub to compile)

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build988829932=/tmp/go-build -gno-record-gcc-switches"

What did you do?

main.go

package main

import (
        "fmt"
        "net/http"
)

func main() {
        url := "https://google.com"
        req, err := http.NewRequest("GET", url, nil)
        if err != nil {
                fmt.Println(err)
                return
        }

        resp, err := http.DefaultTransport.RoundTrip(req)
        if err != nil {
                fmt.Println(err)
        }
        if resp == nil {
                fmt.Println("Response is nil")
        }
}

# Proxy server returns 407 to unauthenticated requests
export https_proxy=http://proxy.example.com:3128
go run main.go

What did you expect to see?

I would like to make a contribution to update the error returned on non 200 response on CONNECT request from https proxy to be a new error type such that I can interrogate the response from the proxy, instead of having to handle the error based on the text of the error message.
So I'm able to do something like:

 switch e := err.(type) {
 case ProxyConnectError:
         handleProxyError(e.Response)
 default:
 return err
 }

Currently a basic error type is returned:

go/src/net/http/transport.go

Line 1719 in 8869086

return nil, errors.New(f[1])

I'm happy to raise a PR if this is something that would be accepted

What did you see instead?

Proxy Authentication Required
Response is nil

[seankhliao]

I don't think RoundTrip should be returning an error for successfully getting a valid http response

[logandavies181]

Hi Sean, just to confirm, are you saying you think RoundTrip should return the non 200 reponse instead of the error it's currently returning?

[logandavies181]

I was going to make an alternative suggestion in this issue that RoundTrip return the error and the response;
seeing as it would be returning an error for a request you didn't explicitly make (i.e the CONNECT request)

[elagergren-spideroak]

Dupe of #38143

[seankhliao]

let's keep it there