jsonpb: emit null value for oneof field with 'nil-interface' value

[marusama]

example proto:

message test {
    oneof roadIdNil {
        int32 roadId = 12;
    }
}

go code:

var roadId *proto.Test_RoadIdNil
test.RoadIdNil = roadId                // now RoadIdNil is not nil, but contains "nil" Interface  

will produce json:

{
    "roadId": null
}

instead of panic:
2018/04/04 18:07:28 http: panic serving [::1]:62625: reflect: call of reflect.Value.Field on zero Value goroutine 6 [running]: net/http.(*conn).serve.func1(0xc42009e8c0) /usr/local/Cellar/go/1.10.1/libexec/src/net/http/server.go:1726 +0xd0 panic(0x13e9e60, 0xc4202ab420) /usr/local/Cellar/go/1.10.1/libexec/src/runtime/panic.go:502 +0x229 reflect.Value.Field(0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x14c9c40) /usr/local/Cellar/go/1.10.1/libexec/src/reflect/value.go:782 +0x11b soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalObject(0xc420095bc0, 0xc420171910, 0x14c6b20, 0xc4200ac480, 0x0, 0x0, 0x0, 0x0, 0xc420170870, 0x8) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:297 +0x58d soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalValue(0xc420095bc0, 0xc420171910, 0xc4202e5cc0, 0x146a3c0, 0xc420184130, 0x196, 0x0, 0x0, 0x0, 0x8) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:525 +0xbe5 soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalValue(0xc420095bc0, 0xc420171910, 0xc4202e5cc0, 0x13bb940, 0xc4202743c0, 0x197, 0x0, 0x0, 0xc420170b60, 0xc4202e5cc0) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:476 +0x1125 soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalField(0xc420095bc0, 0xc420171910, 0xc4202e5cc0, 0x13bb940, 0xc4202743c0, 0x197, 0x0, 0x0, 0x0, 0xc420189c38) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:446 +0x10b soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalObject(0xc420095bc0, 0xc420171910, 0x14c6ae0, 0xc4202743c0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x8) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:304 +0x494 soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalValue(0xc420095bc0, 0xc420171910, 0xc4202ce280, 0x1420960, 0xc42000e1e8, 0x196, 0x0, 0x0, 0xc420171300, 0xc4202ce280) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:525 +0xbe5 soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalField(0xc420095bc0, 0xc420171910, 0xc4202ce280, 0x1420960, 0xc42000e1e8, 0x196, 0x0, 0x0, 0x0, 0xc420189358) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:446 +0x10b soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).marshalObject(0xc420095bc0, 0xc420171910, 0x14c6aa0, 0xc42000e1e8, 0x0, 0x0, 0x0, 0x0, 0xc420171930, 0x1012478) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:304 +0x494 soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).Marshal(0xc420095bc0, 0x14c0a20, 0xc4201bea80, 0x14c6aa0, 0xc42000e1e8, 0x14c6aa0, 0x1430c80) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:122 +0x9d soap/vendor/github.com/golang/protobuf/jsonpb.(*Marshaler).MarshalToString(0xc420095bc0, 0x14c6aa0, 0xc42000e1e8, 0x14c6aa0, 0xc42000e1e8, 0x14208c0, 0xc42000e1e8) /Users/btstsyrendylykov/go/src/soap/vendor/github.com/golang/protobuf/jsonpb/jsonpb.go:128 +0x6e

[marusama]

@dsnet I recreated PR from dev branch, and yes, it looks like #478

[marusama]

With my PR we will have three options:
if oneof field is

• nil -> no field in resulting JSON
• nil-interface -> "field": null in JSON (now it panics)
• filled value -> "field": "value"

I think it is more convenient

[marusama]

I've added second commit 04e8c55 for the same behavior on unmarchal json:
if json field is

• absent -> oneof field will be nil
• "field": null -> oneof field = nil-interface (now it has default value, that's seemd wrong for me)
• "field": "value" -> oneof field with "value"

so it's pretty simple to recognize if field absent or null

[dsnet]

The v2 re-implementation has logic for handling the situation when a nil wrapper type is set. Closing this as no longer relevant.