Problems with unmarshalling of structs with interface fields

[giggsoff]

Hi! I try to use Msgpack to marshal structs with interface fields. During the unmarshal, the library panics. Here is an example: https://gist.github.com/giggsoff/b261040fb97f9ff8f81145a5ed440ec3
And I got:

2020/05/23 20:15:20 Before marshalling:  &{123}
panic: reflect.Set: value of type map[string]interface {} is not assignable to type main.interfaceAlias

goroutine 1 [running]:
reflect.Value.assignTo(0x4df9a0, 0xc000098420, 0x15, 0x4fe6dc, 0xb, 0x4e1ac0, 0xc000096530, 0xc000098420, 0x0, 0x0)
        /snap/go/5759/src/reflect/value.go:2403 +0x426
reflect.Value.Set(0x4e1ac0, 0xc000096530, 0x194, 0x4df9a0, 0xc000098420, 0x15)
        /snap/go/5759/src/reflect/value.go:1532 +0xbd
github.com/vmihailenco/msgpack/v4.(*Decoder).interfaceValue(0xc0000ce000, 0x4e1ac0, 0xc000096530, 0x194, 0x4e1ac0, 0xc000096530)
        /home/giggsoff/go/pkg/mod/github.com/vmihailenco/msgpack/v4@v4.3.11/decode_value.go:247 +0x150
github.com/vmihailenco/msgpack/v4.decodeInterfaceValue(0xc0000ce000, 0x4e1ac0, 0xc000096530, 0x194, 0x1, 0x1)
        /home/giggsoff/go/pkg/mod/github.com/vmihailenco/msgpack/v4@v4.3.11/decode_value.go:219 +0x1ef
github.com/vmihailenco/msgpack/v4.(*field).DecodeValue(0xc0000bc0c0, 0xc0000ce000, 0x4e2b60, 0xc000096530, 0x199, 0xc000096430, 0x4e7680)
        /home/giggsoff/go/pkg/mod/github.com/vmihailenco/msgpack/v4@v4.3.11/types.go:110 +0x9b
github.com/vmihailenco/msgpack/v4.decodeStructValue(0xc0000ce000, 0x4e2b60, 0xc000096530, 0x199, 0x4e2b60, 0x7f27962e50c8)
        /home/giggsoff/go/pkg/mod/github.com/vmihailenco/msgpack/v4@v4.3.11/decode_map.go:333 +0x462
github.com/vmihailenco/msgpack/v4.(*Decoder).DecodeValue(0xc0000ce000, 0x4e2b60, 0xc000096530, 0x199, 0xc000096530, 0x199)
        /home/giggsoff/go/pkg/mod/github.com/vmihailenco/msgpack/v4@v4.3.11/decode.go:280 +0x8c
github.com/vmihailenco/msgpack/v4.(*Decoder).Decode(0xc0000ce000, 0x4d2ce0, 0xc000096530, 0xc000093ed8, 0x40beb8)
        /home/giggsoff/go/pkg/mod/github.com/vmihailenco/msgpack/v4@v4.3.11/decode.go:259 +0x171
github.com/vmihailenco/msgpack/v4.Unmarshal(0xc0000cc040, 0xe, 0x40, 0x4d2ce0, 0xc000096530, 0x0, 0x0)
        /home/giggsoff/go/pkg/mod/github.com/vmihailenco/msgpack/v4@v4.3.11/decode.go:51 +0xcb
main.main()
        /home/giggsoff/go/src/msgpackTest/customIf/main.go:40 +0x1c4
exit status 2

When I initialize struct before unmarshalling to it here https://gist.github.com/giggsoff/65606a03d84288000f0cd01684eb1b48
I got wrong result (expected result is the same lines):

2020/05/23 20:43:32 Before marshalling:  &{123}
2020/05/23 20:43:32 After marshalling:  &{0 456}

And I cannot use library in app lf-edge/adam#32

[dolmen]

Here is the code on the Go playground: https://play.golang.org/p/2RpoOtWkhYi

[dolmen]

This is not an issue about msgpack.

This is an issue about your expectation that it would be possible to deserialize into an interface.
No serialization library can do that. Libraries need concrete types as serialization targets. They can't magically find which concrete type they must instantiate that implement that interface.

I have converted your example to use encoding/json to show you that it can't do better. See on the Go playground: https://play.golang.org/p/JlvyOINsi5e

[vmihailenco]

I believe this is not working in v5

[tmm1]

No serialization library can do that. Libraries need concrete types as serialization targets. They can't magically find which concrete type they must instantiate that implement that interface.

This is not strictly true. The gob serializer can encode the concrete type underlying an interface and unmarshal on the other side.

https://github.com/golang/go/blob/b6def6a34e049d5d2cc9225d991c4b84427467ec/src/encoding/gob/encode.go#L380-L384

[tmm1]

I believe this is now working in v5

@vmihailenco Can you reopen this issue because the behavior is still the same with v5

[dolmen]

This is not strictly true. The gob serializer can encode the concrete type underlying an interface and unmarshal on the other side.

The difference is that gob is a pure go serialization format. Instead msgpack is designed for interoperability and implementations for multiple programming languages exist. You are asking for a change in the serialization format that would break many existing use cases.

[tmm1]

Here's an example of how to use CustomDecoder to solve this issue: https://play.golang.org/p/lbG2ys92Oik