encoding/json: strange isEmptyValue behavior with arrays

[sorenvonsarvort]

What version of Go are you using?

$ go version
go version go1.12.1 linux/amd64

Does this issue reproduce with the latest release?

Yes, it does.

What operating system and processor architecture are you using?

go env Output

$ go env

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/sorenvonsarvort/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/sorenvonsarvort/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build582915182=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"encoding/json"
	"fmt"
)

type EncodingBreaker [12]byte

type Article struct {
	ID EncodingBreaker `json:"id,omitempty"`
}

func main() {
	articleBytes, err := json.MarshalIndent(Article{}, "", "	")
	if err != nil {
		// ...
	}

	fmt.Println("item:", string(articleBytes))
	fmt.Println("error:", err)
}

https://play.golang.org/p/JTZn-qSksgw

What did you expect to see?

item: {}
error: <nil>

What did you see instead?

item: {
        "id": [
                0,
                0,
                0,
                0,
                0,
                0,
                0,
                0,
                0,
                0,
                0,
                0
        ]
}
error: <nil>

Possible solution

Is this behavior is correct? If no:

In encoding/json the encode.go file contains:

case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
		return v.Len() == 0

Since that, any array will never be empty.

Used to fork the package and change it to:

	case reflect.Array:
		for i := 0; i < v.Len(); i++ {
			if !isEmptyValue(v.Index(i)) {
				return false
			}
		}

		return true
		
	case reflect.Map, reflect.Slice, reflect.String:
		return v.Len() == 0

[andybons]

I believe the behavior you're seeing is correct.

Since that, any array will never be empty.

A zero-length array is empty:

type EncodingBreaker [0]byte

would have the desired output you expect, however having a zero-length array is likely not what you'd want. I would use slices instead if that works for your use-case.

Changing this behavior would break people who rely on the current encoding semantics with arrays, so it's likely not going to happen.

@dsnet @mvdan in case they have any thoughts.

[sorenvonsarvort]

Thank You for the answer, since some people rely on the current implementation, now I am sure it is okay to have a separate package.