encoding/base64: shows wrong output

[akshaybabloo]

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

$ go version
go version go1.20.2 windows/amd64

Does this issue reproduce with the latest release?

Yes, and works fine with 1.16

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

go env Output

$ go env
set GO111MODULE=

set GOARCH=amd64

set GOBIN=

set GOCACHE=C:\Users\gollaha\AppData\Local\go-build

set GOENV=C:\Users\gollaha\AppData\Roaming\go\env

set GOEXE=.exe

set GOEXPERIMENT=

set GOFLAGS=

set GOHOSTARCH=amd64

set GOHOSTOS=windows

set GOINSECURE=

set GOMODCACHE=C:\Users\gollaha\go\pkg\mod

set GONOPROXY=

set GONOSUMDB=

set GOOS=windows

set GOPATH=C:\Users\gollaha\go

set GOPRIVATE=

set GOPROXY=https://proxy.golang.org,direct

set GOROOT=C:\Program Files\Go

set GOSUMDB=sum.golang.org

set GOTMPDIR=

set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64

set GOVCS=

set GOVERSION=go1.20.2

set GCCGO=gccgo

set GOAMD64=v1

set AR=ar

set CC=gcc

set CXX=g++

set CGO_ENABLED=0

set GOMOD=C:\Users\gollaha\Code\personal\rex\rex-cli\go.mod

set GOWORK=

set CGO_CFLAGS=-O2 -g

set CGO_CPPFLAGS=

set CGO_CXXFLAGS=-O2 -g

set CGO_FFLAGS=-O2 -g

set CGO_LDFLAGS=-O2 -g

set PKG_CONFIG=pkg-config

set GOGCCFLAGS=-m64 -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=C:\Users\gollaha\AppData\Local\Temp\go-build1860088289=/tmp/go-build -gno-record-gcc-switches

What did you do?

I am trying to convert a struct to base64 string with the following code:

package main

import (
	"bytes"
	"encoding/base64"
	"encoding/json"
	"fmt"
)

type testVariable struct {
	Out string `json:"out"`
}

func EncodeBase64(v interface{}) (string, error) {
	var buf bytes.Buffer

	encoder := base64.NewEncoder(base64.URLEncoding, &buf)
	defer encoder.Close()
	err := json.NewEncoder(encoder).Encode(v)
	if err != nil {
		return "", err
	}
	return buf.String(), nil
}

func main() {
	example := testVariable{
		Out: "Hello World!",
	}
	out, err := EncodeBase64(example)
	if err != nil {
		panic(err)
	}
	fmt.Println(out)
}

Go Play link - https://go.dev/play/p/3liPKR7H0DG <- this shows wrong output
https://go.dev/play/p/r9sXiRSXrwp <- This shows the correct output

What did you expect to see?

The output should be eyJvdXQiOiJIZWxsbyBXb3JsZCEifQo=

What did you see instead?

Instead, I get eyJvdXQiOiJIZWxsbyBXb3JsZCEi

The reason this is happening is because of the placement of defer encoder.Close(), if you move encoder.Close() after err block in EncodeBase64 it outputs the correct string.

[seankhliao]

working as intended
encoder is documented as needing to be closed to flush data
and defers go after the the return

[akshaybabloo]

So what is the right way to do it?

[ianlancetaylor]

@akshaybabloo Call encoder.Close() after calling Encode but before calling buf.String.

If you have any further questions, please see https://go.dev/wiki/Questions. Thanks.