Decoding into interface bug

[GRbit]

When I tried to unmarshal bytes into a struct hidden under a specific interface, "go-json" failed to unmarshal it. At the same time, go standard lib or json-iterator feels just fine with this approach. Since you claim that your library is 100% compatible with the standard library, it makes sense to fix this bug or to change your README and add this exact case.

Code to reproduce the bug:

package main

import (
	"fmt"
	"github.com/goccy/go-json"
)

type EsDocument interface {
	IndexName() string
}

type Property struct {
	Name string `json:"name"`
}

func (p Property) IndexName() string {
	return "properties"
}

func main() {
	b := []byte(`{"name":"Test Property to remove"}`)
	p := Property{}

	unmarshallInterface(b, &p)

	fmt.Println(p)
}

func unmarshallInterface(body []byte, doc EsDocument) {
	if err := json.Unmarshal(body, &doc); err != nil {
		panic(err)
	}
}

Output

$ go run main.go
panic: json: cannot unmarshal main.EsDocument into Go value of type main.EsDocument

goroutine 1 [running]:
main.unmarshallInterface(...)
	/home/grbit/ssd/go/src/tst/main.go:31
main.main()
	/home/grbit/ssd/go/src/tst/main.go:24 +0x16d
exit status 2

If you change doc type name from EsDocument to just interface{} it'll work just fine.

package main

import (
	"fmt"
	"github.com/goccy/go-json"
)

type Property struct {
	Name string `json:"name"`
}

func main() {
	b := []byte(`{"name":"Test Property to remove"}`)
	p := Property{}

	unmarshallInterface(b, &p)

	fmt.Println(p)
}

func unmarshallInterface(body []byte, doc interface{}) {
	if err := json.Unmarshal(body, &doc); err != nil {
		panic(err)
	}
}

Output

$ go run main.go
{Test Property to remove}

[cmetallo42]

Solved!

[mymmrac]

Hello @goccy, in #406 you said that you would fix this issue, the fix seems to be pretty small by itself, is there any chance you will do that any time soon? Thanks!

[GRbit]

@mymmrac are you sure that the bug persists with the latest version? For me it works now. Maybe your case is a little bit different? If so, it would be great if you could provide a test case reproducing the problem.

[mymmrac]

@GRbit yes, it reproduces, here is an example:

package main

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

	"github.com/goccy/go-json"
)

func main() {
	s := Something{}
	s.Thing = &ThingA{}
	err := json.Unmarshal([]byte(`{"thing": {"a": 1}}`), &s)
	if err != nil {
		panic(err)
	}
	fmt.Println(s.Thing)
}

type Something struct {
	Thing Thing `json:"thing"`
}

type Thing interface {
	F()
}

type ThingA struct {
	A int `json:"a"`
}

func (t *ThingA) F() {}

With github.com/goccy/go-json - current master df897ae

panic: json: cannot unmarshal main.Thing into Go struct field Something.Thing of type main.Thing

With encoding/json - no issues, parsed as expected (s.Thing.A == 1)

[mymmrac]

With this changes it works (from PR that was closed, #406):

replace github.com/goccy/go-json => github.com/cmetallo42/go-json v0.0.0-20221114111108-7fed434e8044

[GRbit]

@mymmrac Indeed, you are right.

For some reason I thought that at some point it was fixed in goccy repository. Thank you for bringing this up again.