encoding/json: adding a MarshalJSON method confuses JSON marshalling

[cfiderer]

Go version

go version go1.22.2 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/user/Library/Caches/go-build'
GOENV='/Users/user/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/user/go/pkg/mod'
GONOPROXY='*.sap.corp'
GONOSUMDB='*.sap.corp'
GOOS='darwin'
GOPATH='/Users/user/go'
GOPRIVATE='*.sap.corp'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.22.2'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/user/SAPDevelop/issues/MarshalJson/go.mod'
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/lc/5666d_g13j92hx9n3jdh5g2w0000gn/T/go-build1386616878=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I'm working with Kubernetes and custom resources. For logging and log filtering, I added a MarshalJSON() method to a struct field.

package main

import (
	"encoding/json"
	"fmt"
	"time"

	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
)

// Object1Meta wraps metav1.Object1Meta to control logging.
type Object1Meta struct {
	metav1.ObjectMeta
}


type MyObjectSpec struct {
	Value1 string
	Value2 string
	Secret string
}

type MyObjectStatus struct {
	Status string
	ErrMsg string
}

type MyObject1 struct {
	metav1.TypeMeta `json:",inline"`
	Object1Meta     `json:"metadata,omitempty"`
	Spec            MyObjectSpec   `json:"spec,omitempty"`
	Status          MyObjectStatus `json:"status,omitempty"`
}

// Object2Meta wraps metav1.Object1Meta to control logging like Object1Meta, but implements a MarshalJSON method.

type Object2Meta struct {
	metav1.ObjectMeta
}

func (om Object2Meta) MarshalJSON() ([]byte, error) {
	fields := make(map[string]interface{}, 4)

	fields["name"] = om.Name
	fields["namespace"] = om.Namespace
	fields["generation"] = om.Generation
	fields["creationTimestamp"] = &om.CreationTimestamp

	return json.Marshal(fields)
}

type MyObject2 struct {
	metav1.TypeMeta `json:",inline"`
	Object2Meta     `json:"metadata,omitempty"`
	Spec            MyObjectSpec   `json:"spec,omitempty"`
	Status          MyObjectStatus `json:"status,omitempty"`
}

func main() {
	now := metav1.NewTime(time.Now())

	o1 := MyObject1{
		TypeMeta: metav1.TypeMeta{
			Kind: "object1", APIVersion: "api/v1",
		},
		Object1Meta: Object1Meta{metav1.ObjectMeta{
			Name: "o1", Namespace: "ns1", Generation: 1234, CreationTimestamp: now,
		}},
		Spec: MyObjectSpec{
			Value1: "value1", Value2: "value2", Secret: "hidden",
		},
		Status: MyObjectStatus{
			Status: "ok",
		},
	}

	b1, _ := json.Marshal(o1)

	o2 := MyObject2{
		TypeMeta: metav1.TypeMeta{
			Kind: "object1", APIVersion: "api/v1",
		},
		Object2Meta: Object2Meta{metav1.ObjectMeta{
			Name: "o1", Namespace: "ns1", Generation: 1234, CreationTimestamp: now,
		}},
		Spec: MyObjectSpec{
			Value1: "value1", Value2: "value2", Secret: "hidden",
		},
		Status: MyObjectStatus{
			Status: "ok",
		},
	}

	b2, _ := json.Marshal(&o2)

	fmt.Println("b1", string(b1))
	fmt.Println("b2", string(b2))
}

Complete (vendored) source code: MarshalJson.zip

What did you see happen?

Calling json.Marshal()onMyObject2` generates only the output from MarshalJSON(), all other entities from o2 are missing.

b1 {"kind":"object1","apiVersion":"api/v1","metadata":{"name":"o1","namespace":"ns1","generation":1234,"creationTimestamp":"2024-05-03T09:40:38Z"},"spec":{"Value1":"value1","Value2":"value2","Secret":"hidden"},"status":{"Status":"ok","ErrMsg":""}}
b2 {"creationTimestamp":"2024-05-03T09:40:38Z","generation":1234,"name":"o1","namespace":"ns1"}

What did you expect to see?

I expect the two structures in my sample program to generate the same JSON output.

[cfiderer]

PS: Marshalling o2 or &o2 doesn't make a difference.

[seankhliao]

This is working as expected due to how methods for embedded types are promoted

[cfiderer]

Why does a MarshalJSON() method for a part of a struct affect the marshalling of other parts of that struct?

Can you please provide a reference/link to the documentation that describes this behaviour?

[seankhliao]

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