net/http: Error adds newline to message

[YetAnotherRussian]

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

go1.17.5 windows/amd64

Does this issue reproduce with the latest release?

Yes

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

Microsoft Windows [Version 6.3.9600]

set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\DMusin\AppData\Local\go-build
set GOENV=C:\Users\DMusin\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Go\bin\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Go\bin
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.17.5
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=NUL
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Temp\go-b
uild1689227927=/tmp/go-build -gno-record-gcc-switches

What did you do?

I use a small program to test GET requests to API:

package main
import (

"fmt"

"log"

"net"

"net/http"

"strings"

//"encoding/json"

)
func process(w http.ResponseWriter, r *http.Request) {
switch r.Method {
case "GET":
	key := r.Header.Get("API-key")
		if key != "123-345-678" {
		fmt.Println("The request was missing a valid API key!")
		http.Error(w, "", http.StatusForbidden)
		return
		}
    http.ServeFile(w, r, "list.html")
case "POST":
    fmt.Fprintln(w, "Method not implemented yet.")
	return
default:
    fmt.Fprintln(w, "Method not supported!")
}

}
func GetLocalIP() string {

conn, err := net.Dial("udp", "8.8.8.8:80")

if err != nil {

log.Fatal(err)

}

defer conn.Close()
return strings.Split(conn.LocalAddr().String(), ":")[0]

}
func main() {

fmt.Println("Server is starting...")

http.HandleFunc("/", process)

localIp := GetLocalIP() + ":12345"

fmt.Println("Bound to " + localIp)

log.Fatal(http.ListenAndServe(localIp, nil))

}

What did you expect to see?

http.Error(w, "", http.StatusForbidden) - expected an empty message here
http.Error(w, "Wrong API key!", http.StatusForbidden) - expected "Wrong API key!" message

What did you see instead?

http.Error(w, "", http.StatusForbidden) - "\n" message here
http.Error(w, "Wrong API key!", http.StatusForbidden) - "Wrong API key!\n" message here

From Chrome:

From Visual Studio:

[davecheney]

The new line is added by Fprintln https://cs.opensource.google/go/go/+/refs/tags/go1.17.5:src/net/http/server.go;drc=3c50f2739136bb4a6ff529c6ec975723a047a741;l=2060

[YetAnotherRussian]

Yep, but I'm unsure that's a correct way. If it is, then I'll have to fix that @ client side apps.

[davecheney]

What is the problem that the new line causes?

[YetAnotherRussian]

Line break increases height of any UI element which is used to show that message (user side). Even if it's you who form these messages, you have to forcibly use Trim() (or any of it's equivalent from any lang) @ client side. Then you do the same for logging. If you concat these strings, you go even further. My vision is that if the string is empty, it should be empty, but not a line break/whitespace(s) etc.

[seankhliao]

Duplicate of #24530